An Exposure calculator to compare EV value and stops difference for any two camera or meter exposures is on the previous page.
Learning exposure is an extremely important initial skill in learning photography. Exposure of a photo obviously depends on the combination of the three factors, f/stop, shutter speed, and ISO. There are several possible combination choices that could work (Equivalent Exposures), but usually there is One Best One for the current scene. I have more about learning here, or see Google at "Exposure Triangle" for more basics of learning this, which is about the first elementary concept photographers need to know about Exposure. "Triangle" is the name used, but it is figurative, cute but possibly confusing, since there is no triangle, there are just the three interacting factors controlling exposure (f/stop, shutter speed, and ISO).
BTW, in "things we need to know", if the Google search terms are actually a known phrase, like "Exposure Triangle" or "Field of View" or "Extension Tube Formulas", then add the quotes around your search phrase so Google will search for it as a combined phrase. That can at times improve search results. Then "Crop Factor" won't include the individual words to find like "rain is a factor of crop growth".
There are always several equivalent exposure combinations, and the skill (to get out of Full Auto mode and control your pictures) involves knowing when and why you might give preference to each of those three factors (when to do what?) Fully automatic cameras do not aid learning this. A cell phone won't offer more, and point&shooters won't even know or care what camera settings were used or their meaning and use. But if you are standing there looking at the scene, you could be thinking about what is most important to improve this picture? That's what a photographer does.
The trick is to stop and look and think just a second. What are you trying to do? Does it need fast shutter speed to freeze motion, or it should be slow and on a tripod? Does it need stopped down aperture for greater depth of field or open wide to blur the background, or to help the flash power? Or maybe low ISO to minimize noise, or high ISO to make the rest be possible? Is your subject in deep shade but with a bright sunny background behind them? (Auto can't work then without your intervention and help). So what is important about improving this scene?
A light meter just measures the intensity of a blob of light, but it has no human brain to recognize the scene situations to know which Equivalent Exposure combination is best. But you are standing there with the necessary brain, so use it. This is your job. Beginners don't realize these things can be considered first, but this becomes automatic and immediate and easy and obvious with a bit of experience. You see the scene when you walk up, but beginners may not yet realize they need to stop and look and think just a second. These goals can conflict in tough situations, to sometimes be a compromise when you can only do what you can (which is still best choice then), but when you know why, then you know how.
If you're having any trouble beginning, an inexpensive good book (likely already in your public library) is Understanding Exposure, How to Shoot Great Photographs with Any Camera, which is about the basics of how to use the camera settings. It's a short easy fun read (with many full page pictures), and it is popular with a huge rating (and IMO has become a classic) to convey the basic first principles so extremely important to photography. The used copy is only about $10 total including shipping at Amazon. And Google at Exposure Triangle should help too.
There are concepts of relative EV adjustments, or of absolute EV values.
Relative EV is the amount of change from the current exposure, like perhaps +1 EV more, shown by camera Exposure Compensation or TTL Flash Compensation.
One major use of EV (Exposure Value) is just to measure any change of exposure, where one EV implies a change of one stop of exposure. Like when we compensate our picture in the camera. If the picture comes out too dark, our manual exposure could correct the next one by directly adjusting one of the three exposure controls (f/stop, shutter speed, or ISO). Or if using camera automation, the camera meter is controlling it, but we might apply +1 EV exposure compensation (or +1 EV flash compensation) to make the result goal brighter, as desired. This use of 1 EV is just another way to say one stop of exposure change.
Why not just say "stop"? I don't know, I suppose only two characters is easier to mark in the camera controls. In film days, both lens f/stop and shutter speed dial had mechanical click stops, but ISO was another roll of film. But we still had to tell the camera metering system which ISO, and that ISO dial had click stops.
But regardless, one stop is an exposure factor of 2 (2x more or 1/2 less). One EV is a step of one stop compensation value (could be aperture, shutter speed, or ISO, or some combination). This +1 EV means a one stop greater exposure. I assume this basic compensation use is already known. The rest of the page is about the absolute EV numbers.
Basics are that adjustable cameras offer both Exposure Compensation and Flash Compensation to correct exposures to match goals. Any -EV compensation uses less exposure to correct a brighter light. Or specifying +EV compensation adds more exposure to correct a dimmer light. The metered EV is what we actually have, and relative to it, then more EV means brighter (requiring less exposure), or vice versa. Compensation is the correction we actually want. Compensation is a Relative EV number.
The camera meter automation first adjusts the camera setting for the metered ambient light (ignoring the flash for that). With exception for shadows, typically bright sun ambient lights all distances the same, including the background distance. Nikon manuals about also adding flash say "background" when they actually mean "ambient" (ambient does reach the background). IMO, when speaking of the mixed lighting, Nikon says "background and subject" when they discuss "ambient and flash" lighting, so they speak of result instead of the cause. The point being, Background is where the Flash cannot go (unless the background is close behind the subject). Then the TTL flash necessarily must also use those same camera ambient settings, but the TTL flash is metered too, which accordingly adjusts TTL flash power, metered to reach the near main subject (but NOT any distant background). Another major topic is that much subject distance will require much flash power, but which can be helped with a wide aperture or higher ISO, if shutter speed can also make that work for the ambient.
Exposure compensation affects both ambient and flash exposure for Nikon cameras, but affects only ambient exposure for Canon cameras. However recent Nikon models add a menu option to do it either way. There are advantages either way, so the important thing is to think of it in the same terms.
If both Exposure and Flash compensations are not compensated (meaning both compensations are still at 0 EV), then adding both a reasonable ambient light and the TTL flash will add two proper exposures, which sum will be +1 EV overexposure on the near flash subject. But in a dark ambient (or in typical indoor settings with flash too, the ambient will typically be well underexposed, so you can usually forget about ambient, and the flash is essentially the only light on subject), that probably works out just right. But in a normal bright ambient, the adding of the two proper exposures (both at 0 EV) is +1 EV overexposure, so typically the flash fill must be intentionally compensated to -2/3 EV or to -1 EV (meaning, the flash is relatively less than the metered ambient exposure). Or a Balanced Flash system will do the compensations automatically, making the flash be the weaker fill light (which is good to fill harsh dark sun shadows on a face). But for example of an IF or BUT, you might want the flash to be the main subject, standing out on a dimmer background. Just think it out and do what is necessary. Meanwhile, back to the EV Chart, which shows absolute EV values.
Several Sekonic light meters (and surely others too) have a mode to read EV directly, in tenth stops. This is extremely handy for metering portrait subjects in studio setups. (See the two photos below). You can easily subtract the EV in your head to know the lighting ratio between the Main and the Fill flash. In the other modes, the light meter converts the EV to show f/stop and shutter speed settings, for the ISO you declared to the meter.
A light meter reads a higher EV reading to represent a brighter light requiring less exposure. In the EV Chart at bottom of this page, the highest EV numbers use the least exposures. Bright light meters higher EV, and dim light meters lower EV. These are Absolute EV numbers, but which apply to whatever the ISO being considered. You can think of this as the Light Level, the light intensity on the scene. More EV is more light resulting in less camera exposure necessary.
The light meter measures the scene light brightness value, which is the EV reading, for example direct bright sun in a clear sky is often EV 15 (if ISO 100, or EV 16 if ISO 200). The EV 15 row of the EV Chart will show Equivalent exposures typical of bright sun and ISO 100. The camera settings of ISO, shutter speed and f/stop can match the exposure to that light reading. Another first basic of photography is Equivalent Exposures. There are multiple different settings that can give the same exposure, called Equivalent Exposure. For example, we can increase the shutter speed faster to be half duration to reduce the light seen (like from 1/100 second to 1/200 second, which is one stop less light, called -1 EV), and also simultaneously open the aperture one stop to increase the light seen, like from f/8 to f/5.6 (one stop more light, called +1 EV). These changes can be chosen to balance and cancel so that we still get the same exposure. Called Equivalent Exposures, these two relative changes combined both compute the same absolute numerical EV (details follow).
One first basic skill fundamentally needed in "photography" is in learning which of these equivalent exposures is better suited for our current picture, knowing which equivalent choice to choose for the picture. For example, changes in shutter speed may better stop fast motion or intentionally cause blur to indicate motion (like in a waterfall for example). Or changes of f/stop can increase the depth of field, or reduce it to blur and hide the background. If the EV result causes shutter speed too low, then you need more ISO too. So we can look at the scene to realize what it needs, and choose settings to improve the picture result, but the exposure can be the same. You choose the most important factor for your picture. Composition is important too. The light meter automates basic exposure, but actually learning "Exposure" really means learning which of these equivalents is the right one this time. The meaning of Equivalent Exposure is, change the shutter speed, but equally compensate with the f/stop, and it is still the same Equivalent Exposure. Compromises may sometimes be necessary, but still, often there is one best equivalent choice. It might be said that cell phone camera users don't know the details, and their camera does not offer any controls anyway, which makes the difficult jobs be always a problem, even if they're happy with most pictures.
Equivalent Exposures relate to any normal continuous light, generally any daylight or incandescent lighting, but Not flash, flash is not continuous light. Speedlight flash exposure is faster than, and independent of shutter speed, so flash has no same concept of equivalent exposures (except HSS flash does because it is continuous). But all Equivalents are not Equal — as always, there are ifs and buts. 😊 Yes, equivalents are the same exposure (regarding brightness of the picture), but freezing fast motion requires a fast shutter speed, a slow shutter won't do. Maximum Depth of Field requires stopping the f/stop down, a wide aperture won't do. Fluorescent lights with magnetic ballast do seriously flicker that the camera captures, causing possible color problems except at specially considered slower shutter speeds (CFL bulbs and electronic ballast are OK). Television screens also need a slow exposure (1/30 second is typically OK). There are many considerations, and there are better and worst choices of Equivalent Exposure, and there is often good reasons to choose the one best one.
EV is named Exposure Value, which sounds like an "exposure", and it is, but the EV chart does not measure light. The EV chart (below) is simply about combinations of numerical camera settings of shutter speed and f/stop. An EV number represents a set of camera settings, regardless if it is a proper or accurate exposure or not. EV calculation does NOT involve ISO directly, but in use, it involves the camera settings chosen for whatever ISO is being used. ISO determined those settings. A light meter could measure the light, and tell us EV at some ISO, then we look up the Settings in the EV chart for proper exposure at that ISO. EV basically gives a name to the group of several "equivalent exposure" choices comprising any one row of the EV chart below. Each row is a one stop step from its adjacent rows. A 1 EV step is one stop. This one stop step could be due to a light change, or a setting change, or an ISO change. When the camera compensation changes the camera settings by one stop, it calls it one EV. But the main concept is that this row of settings contains "equivalent exposure settings", identified as some EV number.
The EV concept was developed in the late 1950s, as a computing method to be able to add light meters into cameras for the first time (which became the common practice in the 1960s). Film cameras used the current roll of film, with its predetermined ISO, so ISO wasn't technically a camera setting yet (but it was a light meter setting). Yes, there certainly are the three variables of exposure, but light meters measured light and computed shutter speed and f/stop for that existing film ISO. And this separation of ISO is in fact how the actual exposure formula works too (next). We specify ISO, and then the meter provides the corresponding exposure settings.
You should have heard all the uproar complaining back then (early 1960s) about the concept of meters actually being built into the camera to provide automation. There was no internet yet, but monthly camera magazines were very popular back then as about the only current communication, and they exploded with articles about could we trust a meter in the camera? But that wasn't about the accuracy of the meter reading (in fact, the "through the lens" metering was a big advantage for reflective metering, it saw what the lens saw). The debate was about the control of the camera, about the necessary skill of selecting the proper Equivalent Exposure for the situation. Could a dumb computer replace the human brain to recognize the situation? Camera chips today are still not able to recognize the situation, and it's still a very good question for any difficult situation. Meters do need human help to recognize the situation, but we obviously do need the light meter to measure the light, if the photographer makes good use of their eyes and brain too.
Some math computing details of EV are on anther page here if interested in calculating — but relax, you don't need to know any math to take pictures. We don't even need to know EV number, unless you choose to use it. In fact, many of us taking pictures don't even know about details like f/stop and shutter speed, and they often do pretty well. But it is how things work, and which provides ways to control and improve your photos. A glance at the math might be interesting, but we can rely on the light being metered. The question is, what can we do then?
The EV formula: See Wikipedia EV
N is f/stop Number, t is duration Time of shutter speed — the camera settings.
EV is this value as a power of 2, and inversely, EV is the Log₂ of the value.
The reflected light meter Exposure formula:
See Wikipedia light meter calibration
where L is the scene luminance, S is the ISO sensitivity, and K is a constant typically 12.5.
The only purpose here is to just show there is a formula where ISO adjusts the scene luminance to match a proper exposure with the camera settings expressed as EV. We don't have to know it, but it is how light meters work. Meter results show one of the setting combinations, or some meters can show the EV number too.
Log₂ computes the exponent of 2, which (in the camera case) is EV (Exposure Value). The EV formula EV = log₂(N²/t) does not mention ISO directly, but Yes, EV certainly and absolutely does have ISO figured into it, because it was ISO that determined the choice of the EV = log₂(N²/t) exposure settings of f/stop and shutter speed.
Caution: Math ahead. The camera makes it unnecessary to know any math details, but you might find a detail or two here interesting, as a brief example of log₂ and other stuff photographers don't need to know (unless doing exposure calculations):
23 = 8, and log₂(8) = 3. This shows two things to know:
1. The logarithm value is just the exponent of the base (23) that gives this value (8). In the case of the photography base 2, this exponent (3) is the power of 2 of the value (8). The exponent is EV, which is a power of 2, therefore any EV increasing by 1 doubles the exposure (i.e., EV is in 2x steps of exposure).
2. Exponent functions and logarithm functions are inverse operations like multiplication and division, or addition and subtraction, are inverse operations (one can reverse the other). So logarithms are not so complicated.
Note that a special trick is that the f/stop Number in any Exposure EV calculation must always be squared (N² above). That's because the f/stop Number is in √2 steps, but Exposure is steps of 2x. (√2)² = 2. N may represent the f/stop Number, but N² represents the aperture exposure (of the circular area of aperture). But shutter speed or ISO values are NOT squared, because their full stops are steps of 2, same as exposure.
An exception that does NOT do the squaring is for Guide Number for flash, because the √2 matches how Inverse Square Law works, so we don't square f/stop for Guide Numbers. Guide Number already automatically takes Inverse Square Law into account, which is the beauty of Guide Number for flash.
f/stop Number (like f/4) = focal length / aperture diameter of the lens. The circular aperture area (the diameter is measured looking into the magnification of the front glass element) determines the exposure, and the circular area = PI r², so f/stop Numbers are steps of √2, but Exposure is in steps of 2.
EV 0 is f/1 at 1 second. This does make the zero point, but it's just math, because any number to the power of zero is 1, and 1²/1 = 1. Then log₂(1) = 0, and 20 = EV 0, so f/1 at 1 second computes EV 0.
And FWIW, 2²/4 and 4²/16 and 8²/64, etc are also all EV 0 (which are Equivalent Exposures, all on One row of the EV Chart, which math is what the EV Table shows. Any EV row of the EV Table contains Equivalent Exposures. See more about the EV math here.
The base 2 in log₂ is what causes EV to be in powers of 2, i.e., to make 1 EV be exactly a 2x exposure change.
EV is very important to our light meter's exposure calculations. The EV value alone is not actually an exposure without knowing the ISO associated with it, because exposure also needs the specified ISO. It is very important to make that match. EV is a reaction of exposure to the scene light level and ISO. Then the EV value determines which other camera Equivalent settings that will match the scene light level. This EV calculation is just a scale with 2x stops, proportional to f/stop number squared, and inversely to shutter speed.
The EV formula does compute EV using only f/stop Number and shutter speed, so some "experts" claim EV is independent of ISO. It is true that there is no term for ISO there, but it's not that simple. EV is about the f/stop and shutter speed choices that were caused by ISO. Any absolute value of EV is quite meaningless without a specific associated ISO, because any camera exposure setting numbers of interest to us were determined by the ISO choice. Exposures in direct bright sun normally meter near EV 15 at ISO 100, or the same light is near EV 18 at ISO 800. EV does determine the appropriate settings for the ISO number we use (and for the existing light level).
The ISO is set, either by the roll of film, or by your digital settings. Then the light meter reading at that ISO determines the row of equivalent exposures (like the row in the EV chart below). Fully auto modes cannot recognize if the situation is special (regarding motion to be frozen for example, or that the depth of field needs to be extreme), but do try to keep shutter speed not excessively slow without a tripod. Auto ISO automation may change ISO for other numbers (is 1/2 second too slow? Is f/1.8 too wide and fuzzy? Higher ISO can be a solution then.)
Anyway, the EV number is determined by the light meter from the luminance of the scene and by the ISO film speed (ISO was called ASA until 1974). This single EV number represents the group of shutter speed and f/stop combinations that all match the proper exposure, called Equivalent Exposures. This set of Equivalent Exposures (of one EV number) were all the "same exposure", which was a huge plus for using exposure, but not quite the same as "same picture", because f/stop affects depth of field, and shutter speed affects the degree of freezing motion blur. The light meter gives us an exposure, but the basic skill of photographic exposure involves evaluating the proper combination of the Equivalent Exposures, knowing when to choose what (basically deciding the relative importance of the need to freeze action or to increase depth of field), which every photographer seriously needs to learn. This topic of control is often named Exposure Triangle, simply because there are the three interacting exposure factors involved (the actual graphic "triangle" adds nothing else to the concept).
A brighter light, or a higher ISO number, meters a greater EV number, so less camera Exposure is needed. A greater EV number is a lower row in the EV chart with faster shutter speeds, which is less Exposure. EV number is about the necessary camera settings to match the scene and ISO. EV number increases in the opposite direction from the Exposure required (one EV is same amount as one stop, both are a 2x change in exposure).
To clarify a common misunderstanding, exposure depends on luminance, which is the average light per unit area of the scene, and which is NOT about the total area of scene or sensor. Photo exposure is NOT affected by the size of the sensor. If it were otherwise, hand-held light meters would be useless for differing cameras (and they certainly are Not useless). A scene (for example containing a dark shadow area with a bright patch of sun near it) will contain several areas of different exposure, and the trick is finding the one camera exposure suitable for the mixture. Camera automation can only try for a middle or averaged value; not too bright, not too dark. Unfortunately, this is true of reflected meters regardless if the subject ought to be quite bright or quite dark. Without your help, the metering result will come out as a mid-level value (see How Camera Light Meters Work).
A reflected meter (like in a camera) reads the light reflected from the subject. Whereas an incident meter does not even see the scene. It is instead aimed from the subject towards the camera, to measure the actual incident light falling on the subject. Both reflected and incident meters center the exposure on their average reading. The incident meter has the strong advantage of Not being affected by the variable colors reflected by the subject (green reflects brighter, blue reflects dimmer, affecting reflective metering). But it means more than that.
If a reflected meter sees a mostly black or dark-colored subject area (colors that do Not reflect well), it reads a dimmer light, and puts that at mid-range, and the picture turns out gray (brighter). If it sees a mostly white or bright-colored subject area (colors that do reflect well), it reads a brighter light, which it puts that at mid-range, and the picture turns out gray (dimmer). If it sees an average subject with a mix of average colors from dark to light color, averaging middle, it puts it at mid-range, and it comes out good. Meters cannot recognize the subject to know what it is or how it should be, they can only put all exposures at mid-range. But photographers know and can see the subject colors and know how it will come out, and can make corrective actions. Which was important in film days, but with digital, we can preview it and get a second chance.
In contrast, an incident meter reads the incident light directly instead of the subject reflections, meaning brighter white things actually will be white and darker black things actually will be black. Which is great, however an incident meter reads the light at the actual subject location, instead of in the camera, which can be more awkward to use (it's Not point&shoot). Incident metering is on the third page of How Camera Light Meters Work.
These next pictures are of a handheld meter mode in EV mode, which shows the EV reading directly. These are a Sekonic L-308S metering EV exposure in brightest sun. The incident meter sees the light directly (meter aimed from subject toward the camera instead of vice versa), and then the light reading and specified ISO compute the EV. It makes the point that EV certainly varies with ISO. Bright direct sun will be near EV 15 at ISO 100, which one choice is 1/125 at f/16. This was Texas, 3 PM in mid-February, very clear sky. Note that days and skies can vary slightly, and my photo waited because the previous very similar clear day try metered 0.2 EV lower (haze, humidity, etc). EV mode reads in tenths. Tenths are especially handy for metering multiple manual flash (studio situations, when lighting ratio is a trivial subtraction in your head). Then for example, a tenth stop metering in f/stop mode might be read as f/8 plus 7/10 EV. That does NOT mean f/8.7, but instead means 7/10 of the way to f/11, which is about f/10 (0.7 EV is near 2/3 EV from the f/8).
This example shows how the one standard EV Chart is good for any ISO, or rather, meaning specifically for whatever ISO that you used to measure the exposure settings, including Auto ISO (if you know that number). A different ISO will simply match a different EV number on a different row. The pictures here show that. Both pictures were taken within a few seconds in same sunlight.
The light meter does NOT select ISO. You tell the meter what ISO you are using, and it selects the metered exposure of f/stop and shutter speed to match.
The EV Chart does not select ISO. It's about the f/stop and shutter speed you chose for the ISO number you're using. The EV chart and EV number are about whatever ISO you are using.
I don't know if that was the only way to do it, but it works. When we used film, ISO was more of a fixed constant. But tell it a ISO, and the meter can match it. Either way, the light meter cannot determine issues like stopping motion or tripod use, or Depth of Field, but it or the EV Chart, or even the modern camera lets us choose Equivalent Exposures.
The EV formula EV = log₂(N²/t) does not mention ISO directly, so then we hear people telling us that ISO is not a factor of EV, or that the EV Chart is only for ISO 100 (which is blatantly Wrong). The EV number (Exposure Value) is the camera settings (f/stop and shutter speed) for exposure of the current light level at whatever the current ISO. You might be using ISO 100, but metered EV is about whatever ISO determined the other settings. ISO certainly does affect the EV number, ISO affects the camera settings that determine EV. An exposure with a different ISO is on a different row in the EV Chart. There is only the one standard EV chart (for full stops). A row of Equivalent Exposure settings are indicated by the EV number. EV is that chart row of Equivalent Exposure settings. That EV row applies to whatever ISO was used to measure it.
On the EV 15 row in the EV chart below, you can find the settings f/16 1/125, which the EV 15 row number tells you those settings are EV 15 (for the ISO being used). But if you look on the EV 18 row (on this same standard chart), you find f/16 1/1000 second, which is +3 EV, which row is correct exposures for EV 18 (for an ISO 3 EV higher). So for whatever ISO it may represent, just look up the EV on the EV chart. Or look up the settings to find the EV (for the ISO that causes those settings). That is why it is the standard chart. The EV formula computes the standard chart for any use. We can't say "EV is independent of ISO", because a specific ISO has already influenced the situation, in the meters EV reading, and the camera settings selecting it.
Bright direct sunlight is normally near EV 15 at ISO 100, or near EV 18 at ISO 800. In both cases, a light meter set to that ISO will read that EV value. The metered EV 15 row will show the correct settings for ISO 100, and the metered EV 18 row will be correct for ISO 800. On this one standard EV chart.
Any one row of the EV chart shows the Equivalent Exposures at THAT EV, for that ISO that measured the EV.
Wikipedia has a chart of typical EV values for many types of scene lighting conditions (which that chart is for ISO 100), some of which might be difficult to meter.
Use of the camera light meter:
So we generally specify two values, and the meter finds the remaining third value that matches the exposure. If Auto ISO, metering can also select ISO. If in Full Auto, the metering selects all settings.
Depending on how small the senor, the camera's "Smarts" generally tries to prevent shutter speeds less than about 1/30 second, and must keep the f/stop within the available range of the lens.
The following is about EV mode (the two photos above), where we specify ISO and the meter finds the EV value. EV mode is in some hand held meters, but not in camera meters since it finds the EV value instead of the specific settings. Or rather, camera P mode which finds both a shutter speed and f/stop for a specified ISO does find EV, but shows the settings instead of the EV value.
In EV mode, we can specify ISO 100 and meter our bright sun scene as EV 15, and look up EV 15 in EV chart, to see one Equivalent Exposure choice on that row is f/16 at 1/125 second settings to be used for exposure at ISO 100.
Or we can specify ISO 800 and meter EV 18, and look up EV 18 in EV chart, to see one choice is f/16 at 1/1000 second settings to be used for exposure at ISO 800.
Either setting would be the proper metered exposure at the ISO specified. Any of the Equivalent Exposures on the same row of the EV chart are the same exposure too. But as we would expect, ISO 800 meters 3 EV higher than ISO 100, and the camera exposure will require settings 3 EV higher too.
Sekonic meters call it EV mode, and shows the "EV reading" of the scene computed for the ISO specified. For a standard comparison, the term Light Value is converted always for ISO 100. Which is just convention, ISO 100 is Not special at all, other than it is popular, but 100 is just a number. Unless actually using ISO 100, this is Not the EV values that meters show, but it can be converted.
The light meter's EV mode will indicate one row of the EV chart, and every cell on that row is an Equivalent Exposure. Or the meter (in A, S or M modes) basically do the EV chart lookup for us. So absolute EV is not a concept we really need for photography now, but it is still valid, and the rest of this page should help to explore the numbers.
When no light meter is available, the Sunny 16 Rule on next page offers approximations for several outdoor daytime situations. It was generally included on a printed sheet with every roll of film sold. Bright sun often reads EV 15 at ISO 100, which could be f/16 at 1/125 second. The Sunny 16 Rule is based on bright sun being f/16 at shutter speed of 1/ISO seconds, for example 1/100 second at ISO 100. That is a 1/3 EV difference, but pretty close, certainly ballpark. And some days bright sun in a clear sky may read a bit less (variations in the atmosphere haze or humidity).
In film cameras, the cameras provide settings for shutter speed and aperture, and the choice of film can choose ISO.
Digital cameras additionally have a setting to provide a variable ISO, but the EV system itself is unchanged. So in that existing way which still works, the EV number is computed from shutter speed and aperture settings, which are of course chosen to match whatever ISO is in use, so ISO certainly does change the EV number.
A pet peeve is that you might hear someone claiming that ISO does not affect the EV number, said simply because ISO is not a factor in the EV formula. That is wrong, far too simplistic, ignorant of fact, because of course the ISO chosen obviously does strongly affect the choice of the shutter speed and aperture settings used in the EV formula. ISO is certainly a big factor of both the exposure and the resulting EV number (called Exposure Value). The light meter photos just above clearly show that, and the next calculator also shows it. Any absolute EV number is meaningless without specifying its associated ISO value.
The one standard EV Chart (below) is NOT only about ISO 100. EV does NOT involve ISO, but in use, it involves the camera settings chosen for whatever ISO is being used. ISO determined those settings, so EV is about any ISO you might use, including Auto ISO (if you know that number). A different ISO will match a different EV number on a different row. ISO simply determines the resulting EV to be whichever EV row in that same chart that has your other two exposure settings in it. Once you have the correct EV number for the exposure situation, then all other cells on that EV row are Equivalent Exposures. But no other row is equivalent, at least not without changing at least one of the three settings, which often is a frequent choice to change rows.
Frequent because the exposure subject is about more than just finding a correct basic exposure. The light meter does that when you specify ISO and one of the shutter speed or f/stop values, and the meter finds the other. But photography basics are about finding the Best Equivalent Exposure, when concerned with matters like shutter speed to freeze or show the subject motion, or aperture (and focal length) increasing or decreasing Depth of Field range, or about choosing the ISO that best facilitates those goals but still avoiding the sensor noise of excessively high ISO. The photographers thinking brain does that.
Half stops are marked with *½
EV charts normally only show only full stops for EV and settings. A third stop difference wasn't much for B&W negative film, easily adjusted in the darkroom print, and old cameras likely only did full stops. But a third stop is more critical for positive slides or digital images. EV charts for Full, Third, and Half stops have been provided below.
The EV number takes into account only the shutter speed and f/stop settings, but which settings were chosen of interest for some specific ISO value, so of course the EV number does vary with ISO. To provide exposure comparisons, "Light Value" is a conversion of the EV number to be the same exposure at ISO 100 (below). ISO 100 is a nice arbitrary standard choice, convenient maybe, and very popular but not special in any way. 100 is just a number, with no special significance (humans just see it as a nice round number). The EV Chart itself is NOT about ISO 100, it is about whatever ISO you are using at the time (which might even be ISO 100, or ISO 3200, whatever).
In bright sun, the photo above shows ISO 800 was metered as EV 18 (the settings necessary for ISO 800). And the EV chart shows f/16 at 1/1000 second is EV 18, so that works if using ISO 800. And if we compute EV of those settings, we get EV 18.
But ISO 100 metered EV 15 (the settings necessary at ISO 100), which is a different row in the EV chart, and different settings of f/16 at 1/125 there, which also work if using ISO 100. These different settings will compute that EV 15, which due to ISO, is an Equivalent Exposure. ISO determines proper exposures for the settings, so to speak, and exposure cannot use the same settings at different ISO. So it's just different situations. Different light brightness does meter different EV values, requiring different settings. Then simply look up the proper settings on the correct EV row. Only one chart EV row has Equivalent Settings for that EV.
A chart of estimated exposures for many common situations is at Wikipedia. Nothing like that can be precise, but it should be a good starting point. The EV chart below will give Equivalent Exposures for their EV value. Note that the Wikipedia chart is for ISO 100. ISO 400 for example is numerically 2 EV greater.
The EV chart shows rows of Equivalent Exposures for a given light level. Each row is a light level measured in absolute EV. If you find your metered exposure settings in the chart, that row has other Equivalent Exposures for the same light.
The EV chart shows camera shutter speeds, in seconds. It "shows" nominal stops, but it computes precise stops. The combination of a f/stop with a shutter speed is the camera settings which comprise the EV value. Any such combination can appear on only one row of the chart (row is called EV). The EV chart is about the numerical combinations of the shutter speed and f/stop values, and the resulting exposure at some ISO. Specifically, an EV row is the list of Equivalent Exposures for that EV (at that ISO). ISO is of course an exposure factor, it matches the settings to the light. Instead of computing the math, the chart could be drawn by just starting at f/1 at 1 second is EV 0, and then adjacent values differ by one step (usually steps of one stop).
Again, the EV formula and the EV chart are NOT about any specific ISO value (and is certainly NOT only about ISO 100). When ISO is changed, the related shutter speed or f/stop is also changed, and those appropriate settings are moved to another chart row, appropriate to the ISO being used. The chart values are for any ISO that was metered giving the current exposure. The chart is appropriate for the camera exposure settings due to the ISO being used. Our common usage may condition our brain to favor thinking of ISO 100, but the EV number in the chart is determined by the settings we choose for whatever ISO we are using. Not necessarily ISO 100. ISO 100 is simply just another number, one of many, familiar but special only because our memory may be conditioned to think of it first. The EV number is technically computed only from the f/stop and shutter speed, but any proper exposure is then due to that ISO used. Higher ISO increases the EV number for the same scene light value (needing different camera settings). All I am saying here is the EV Chart is NOT about ISO 100. It is good for whatever ISO you are using, which does include ISO 100. 😊
In normal usage, if your light meter meters bright sun to be EV 18 at ISO 800, then you look up EV 18 in the chart, which row gives the Equivalent Exposure settings for use to give a correct exposure at ISO 800. f/16 at 1/1000 second for example. The scene light is NOT EV 18, that number depends on the ISO. The camera settings are EV 18 to match the scene at ISO 800.
If using ISO 100 and the light meters as EV 15, the correct camera settings are on the EV 15 row.
If using ISO 800 and that same light meters now as EV 18, the correct camera settings are on the EV 18 row.
If metering ISO 800 and then converting to use ISO 100, that's the same -3 EV that we would always expect.
Any one setting combination (like f/11 at 1/250 second) is found on only one row of the EV Chart. That one row of the EV number represents several equivalent exposures (like f/8 and 1/500 second or f/16 and 1/125 second are Equivalent Exposures), all the same EV.
The row EV number applies to whatever ISO you are currently using that produced those settings. Exposure Value (EV) represents the light level corresponding to the selected ISO. If a different ISO were used, it would compute a different EV which would be different camera settings on a different row. This really is too simple to dwell this much on it, but too many don't get it. ½ ISO is 1 EV row above, and 2x ISO is 1 EV row below.
We mostly use the light meter in the camera, so I doubt anyone ever actually stops while photographing to look up something in the EV Chart, but the EV concept is important, and the EV chart is used to as the map to show how it works.
This chart computes with the precise design goal values, but shows the camera's nominal values, except there are no nominals longer than 30 seconds, so then the precise actual values in seconds are shown instead (up to 512 seconds which is 8 minutes and 32 seconds). If curious about the nominal full 2x stops being uneven shutter speed progressions of 1/8, 1/15, 1/30, 1/60, 1/125 seconds, then see the explanation of camera precise values. The camera actually always targets the more precise goal values, as best as it can implement them.
There is only ONE definition of EV and EV charts, used for any and all ISO. For any ISO value, your f/stop and shutter combination (determined by the ISO in use) would appear on only ONE EV row. If your exposure in direct bright sun was ISO 100 f/16 1/125 second, f/16 1/125 is on only one row which is EV 15. If it is ISO 800 f/16 1/1000 second, f/16 1/1000 is on only one row, which is EV 18. The EV is the row where you find your actual used camera settings. The correct way to verbalize these is EV 15 at ISO 100, or EV 18 at ISO 800.
A regular EV Chart normally only shows full stops, starting from f/1 (f/1 at one second is EV 0). Our cameras do use third stops, but EV Charts for third stops are never shown because that chart becomes quite excessively wide on the screen. Nevertheless, three EV charts are provided here, the standard EV chart for Full stops, and also EV charts for third or half stops are offered.
Third stops from f/1.4 to f/32 will fill much of a 1920 pixel full screen width, but you might limit the range to show only maybe f/2.8 to f/16 (which is then half width). You can also specify a smaller font size in the chart too.
The "Show Chart" checkboxes toggle Third and Half stop charts to be On or Off (Half stop is Off by default).
These charts are for any and all ISO, specifically the one you are currently using. There may be three charts here showing Full, Half, and Third stops (of the same information), but regarding ISO, there is ONLY ONE EV Chart (which shows f/1 at 1 second is EV 0). If your metered settings at ISO 400 are found on the EV 14 row, then your Equivalent Exposures are those on the EV 14 row. If doubling ISO, it will then be the settings one more EV row lower, then at EV 15 in the same chart. Or if instead a dark cloud covers the sun, exposure at the same ISO will move to a higher row.
Is the EV chart just for ISO 100? Absolutely NOT. The correct idea is that the EV chart is for any and all ISO values, specifically for whatever ISO we're currently using. It wouldn't be of much use if it was only for one ISO. A different ISO of course changes the exposure choice of f/stop and shutter speed used in the formula which then computes EV, which is then just a different row of the same EV chart. The clearly obvious fact is that any meaningful numerical EV (Exposure Value) obviously does depend on our ISO that created the camera settings computing EV. It also depends on the current light level.
Is ISO a factor of EV? Of course it is. To have any meaning, certainly it is. The question only arises because, maybe at first glance, no ISO term appears in the EV formula. However a photographer's usage needs more understanding. The EV number chosen for a real photo is computed from the applicable shutter speed and f/stop settings, the choices of which were already influenced by ISO. Different ISO choices would certainly require different settings computing a different EV. You might hear other notions, but see the pictures of the light meter above. It does not get much plainer. Still some do choose the stand that "EV is independent of ISO" (which is far too simplistic, ignorant of fact).
The EV formula and the chart may seemingly not mention ISO, however the concept of exposure is definitely about the ISO that you are using. The light meter can read the EV value only AFTER we set an ISO value into the meter (and that exposure reading will vary with ISO, and the settings will be appropriate for that ISO). The camera settings being computed for EV were selected or metered for some specific ISO value. If it is to serve any useful purpose, obviously EV represents the camera settings to use at some specific ISO (the ISO we're using when we meter a correct exposure.) A different ISO will require different camera settings. The light meter shows the settings of exposure for the ISO you are using, and those settings will be found on only one row in the EV chart.... the row for the ISO you are using.
This all works out fine. Except, for the people who then assume that row is always for their current ISO. And the row does change with the lighting and/or the ISO, so the problem is, your next picture of a different light level (maybe in shade) will (by this incorrect thinking) then wonder how a different row shows up for that same ISO. Forget that, it's wrong. The different EV row is instead for the different exposure settings on that row, affected by a different light level at whatever ISO, or by just ISO if still at the same light level.
Any possible combination of f/stop and shutter speed (for example f/8 and 1/250 second) appears on only one row in the EV chart, regardless of what ISO caused it. The EV number is the name of that one row (of Equivalent Exposures). If we meter EV xx at some ISO, then EV xx is the one EV chart row which contains the correct exposure settings for that ISO.
EV is named Exposure Value. It is the exposure of the camera settings at some ISO, and is assumed to be a proper exposure. But in the absolute sense (the EV chart), EV technically does just compute f/stop and shutter speed, but it computes with those values that were selected for the specific ISO you are using. EV is certainly about some specific ISO situation. ISO is a factor that will obviously affect which f/stop and shutter speed values you select to properly expose your photo. Just saying "EV 16" has no meaning (other than to describe shutter and aperture equivalent settings). We instead might say "EV 16 at ISO 200", which then defines a useful exposure setting, probably for full direct sunlight in that case.
In the absolute sense, EV needs a defined ISO situation.
In the relative sense, EV is just a relative change in any of these three variables, one EV is one "stop" of exposure.
The EV nomenclature was invented to aid development and control of the first light meters placed into cameras in the late 1950s, back when film speed depended only on the roll of film loaded in the camera. EV camera "settings" are in that context, meaning shutter speed and f/stop, for the film speed that was already defined and in place (for whatever ISO we are using). The old mechanical cameras only had settings for shutter speed and f/stop. However, in use, we still had to set a film ISO value for the meter, so we can meter an EV value at that ISO, and the camera settings are computed from EV (or the EV chart does this too). EV computes only the shutter and aperture settings (for that ISO), and when we use it, we see that EV cannot work without ISO. In use today, we set our light meter for ISO, and it tells us EV (or f/stop and shutter speed directly).
The EV formula may have no ISO term, but EV is computed from the camera settings which are already explicitly chosen to match some ISO to some scene light level for proper exposure — so EV depends on the ISO used. Digital cameras make it easier to change the media sensitivity, but concept is still the same. Change ISO and the EV changes (different settings are required). For a proper exposure of any scenes light level, any EV number applies to only the one ISO value metered (other ISO obviously have other EV, and other settings).
The camera settings (shutter speed and f/stop) determine the cameras exposure of the light. ISO matches the camera settings to the scene light level, which is our goal. Considerations of depth of field or of stopping motion can choose other Equivalent Exposure setting choices (those on same row of EV chart), but the EV number still needs to come out right, for the ISO used. See the pictures below, the EV metered absolutely will depend on the ISO. Set the shutter and f/stop to that EV, and the exposure will be correct.
This is not semantics, it is impossible to ignore that a different ISO will result in a different EV value (speaking of any real correct exposure). EV is the numerical combination of shutter speed and f/stop settings in the camera, which choice was selected because of the ISO used. The EV chart is about the f/stop and shutter setting numbers for proper exposure, which certainly includes consideration of ISO. ISO has been accounted for in the settings that compute EV. Doubling ISO certainly will move us down one row in the EV chart, which is different settings, and a higher EV. EV involves whatever ISO we are using. ISO makes the EV settings correct for some specific level of light. And we certainly do get wrong exposure calculations if we try to factor in ISO a second time.
If in bright sun at ISO 100, the camera shutter speed and f/stop settings are any combination of Equivalent Exposure of 1/125 second f/16 (the row of the EV chart, see below), then these camera settings are EV 15 (that chart row where 1/125 sec at f/16 appears).
In the same bright sun, if a light meter at ISO 800 meters EV 18, then your correct exposure at ISO 800 is on the EV 18 row of the EV chart (Equivalent Exposures are on that row). EV has a value at any ISO, specifically the ISO we are using, in bright sun or in a dark room. In these cases, then both EV 15 (at ISO 100) and EV 18 (at ISO 800) are the correct exposure (of Equivalent exposures).
Hand-held incident light meters often do have a mode to meter an EV value (representing a row of equivalent settings in the chart), as an exposure for the ISO value you select. This could also be used for comparing brightness for non-photographic purposes, like lighting in work areas or plant growing areas. We have come to assume bright sun will meter EV 15 at ISO 100, if we set our light meter and camera to ISO 100.
But today, it is more usual for the light meter to have a mode to directly report the f/stop and shutter speed values to us, instead of a EV value. Those setting will be in the EV chart on only ONE row, so that EV value of that row agrees with metered value at the ISO value used. The meter looks up those settings for us, from the EV value.
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