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EV - Exposure Value (photography)

An EV Calculator  is at midpage below.

The EV Chart is at bottom below.


There are concepts of relative EV adjustments, or of absolute EV values.

Relative EV

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 settings (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. One stop is a factor of 2 (2x or 1/2). One EV is a step of one stops 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.

Absolute EV

EV is named Exposure Value, which sounds like an "exposure", but the EV chart does not measure light. The EV chart (below) is simply about camera settings, combinations of numerical camera settings of shutter speed and f/stop, which would be a set of equivalent exposures, regardless if it is a proper or accurate exposure or not. But 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 in 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 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 chart row of settings contains "equivalent exposure settings", labeled EV.

From Wikipedia:

N is f/stop Number, t is shutter speed (duration Time). The idea is that settings with a constant EV ratio are the same Equivalent Exposure. And that 1 EV is a 2x exposure difference.

Is ISO a factor of EV?   Maybe at first glance, no ISO term appears in the EV formula. And therefore some do make a stand that "EV is independent of ISO" (which is far too simplistic). Yes, the formula and the chart are independent of ISO, however the concept of exposure, and the EV or settings used will absolutely depend on ISO. This all works out fine (below).

EV was invented in the 1950s, when the first light meters in cameras were just being developed, and 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 aperture, since ISO is already externally defined. In use, we set an ISO value for film speed, and we 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, but when we get into using it, we see that EV cannot work without ISO. Yes, the EV formula has no ISO term, but EV is computed from the camera settings, which are already implicitly chosen for proper exposure, using settings to match some ISO with some scene light level - so EV of course depends on the ISO used. Change ISO and EV changes (it's Exposure Value, get it?) For a proper exposure of any scene, any EV number applies to only the one ISO value metered (other ISO obviously have other EV, and other settings).

ISO matches the camera settings to the scene light level, which is pretty much our only goal. Considerations of depth of field or of stopping motion can choose other Equivalent Exposure setting choices (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 aperture 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 exposure). EV is the numerical combination of shutter speed and aperture settings in the camera, which choice was of course 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.

Hand-held incident light meters often do have a mode to meter an EV value (representing a row of 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.

The EV chart itself is Not about ISO. Or rather, it is about all ISO, any ISO. It is about camera settings for a proper exposure at whatever ISO. Different ISO values in the meter will meter different EV values (meaning, different combinations of shutter speed and aperture which match that ISO). ISO 400 will meter 2 EV higher than ISO 100 (so we move down two rows in the chart). The EV value is simply the combinations of the equivalent numerical camera settings. The meter is simply measuring luminance at some ISO, and then calling it EV. (but EV alone, without an ISO, is NOT a Light Value).

Sekonic has a conversion chart, of EV, Lux, foot candles, assuming ISO 100. We often do assume ISO 100, but EV does not necessarily mean ISO 100, except when we say that is what we mean.


Below is a meter metering EV exposure in bright sun: The incident meter meters the light (aimed at the light instead of at the subject), and then uses the LV formula to convert EV to whatever ISO that we specified. Sekonic pictures are shown for the concept, which also to make the point that EV certainly varies with ISO. Bright direct sun will be near EV 15 at ISO 100. EV 15 in bright sun is very near Sunny 16. (This is Texas, 3 PM in mid-February, very clear sky. Note that days and skies can vary slightly, a previous clear day try was 0.2 EV lower). EV mode reads in tenths.


ISO 100 EV 15

ISO 800 EV 18

An example showing how the one standard EV Chart is good for any ISO, so to speak. You only want to use or consider the one standard EV chart.

On the EV 15 row in the standard EV chart below, you can find the settings f/16 1/125, which tells you those settings are EV 15 (for the ISO 100 specified). 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 the ISO 800 specified). So for whatever ISO it may represent, just look up the EV on the standard 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 Formula computes the standard chart for any use. We don't say "independent of ISO", because ISO has already influenced the situation, either the meters EV reading, or the camera settings selected.

Bright direct sunlight is normally very 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.

Used like this:

We can meter our bright sun scene as EV 15 at ISO 100, and look up EV 15 in EV chart, to see f/16 at 1/125 second settings to be used for exposure at ISO 100.

Or we can meter EV 18 at ISO 800, and look up EV 18 in EV chart, to see 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. Anyway, ISO 800 meters 3 EV higher than ISO 100, and the camera exposure will require settings 3 EV higher too.

Sekonic calls it EV mode, but it computes exposure as the LV value from the formula below. Meters for photography don't show the "light value" of the scene, they show the EV of the camera settings we should properly use at a specified ISO (for that scene).

Or, the light meter's regular mode will indicate camera f/stop and shutter speed settings directly at the specified ISO, which combination of settings is found on only one row of the EV chart. The meter basically can 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.


EV Calculator: Full stops are Green. Third stops are Blue. Half stops are Red.

Calculate EV

Shutter speed

f/stop   ISO

 

I realize that at first glance, the numbers are similar, and this may look backwards to the photos above. The photos show ISO 100 metering EV 15 in bright sun, which one equivalent choice can be f/16 at 1/125 second. ISO 800 there is EV 18.

But this default calculation shows f/16 at 1/125 ISO 800 being EV 15. So this new light needing ISO 800 is obviously -3 EV from the bright sun. But if it were brighter so that we could drop ISO 800 to ISO 100, then that is +3 EV brighter, allowing ISO 100. But then, at ISO 800 it would be EV 18, needing f/16 1/1000 second. So it's not a contradiction, it's just a different situation.

There is also a calculator that will compare EV value and stops difference for any two camera or meter exposures.

Formulas

EV = log2 (fstop² / shutter time),   Exposure Value, the exposure effect of the camera settings used. The camera settings of f/1 at 1 second is EV 0, regardless of ISO. However, matching those camera settings to the scene light level makes ISO also be pretty important. So I'm saying it two ways. EV is independent of ISO in the formula, however the camera settings we choose to put into the formula definitely depend on ISO. EV is Not the light level, it is the camera settings we choose, which are influenced by ISO.

Mathematically, the EV values are exact only if computed with the actual precise settings that the camera uses, as opposed to the camera's nominal marked values. This calculator does that. Otherwise, 1/128 second is the actual number for nominal 1/125 second. So for f/16 and 1/128 second, log2(16x16x128) = EV 15.00. To show example of the nominal marked numbers vs the precise theoretical numbers, f/16 1/125 second computes EV 14.966, close, but no cigar. Or one stop less, f/11 at 1/125 computes EV 13.885, but using the actual real values of f/11.314 at 1/128 computes exactly EV 14.00 (the correct value). Techie details maybe, but that's how it works. However, it may be even easier to create the EV chart, since EV 0 is f/1 at 1 second (because log2(1) = 0), and all adjacent chart values are double or half (1 stop).

The log2 is needed because the basic concept is 2EV = N2/t (1 EV being 1 stop = 2x exposure).
If you compute this yourself, then log2(x) is log10(x) / log10(2), or which is log10(x) / 0.3.

The EV formula simplistically appears not to involve ISO. But clearly the inputs of camera f/stop and shutter speed settings obviously are chosen to reflect whatever the current ISO is. ISO is already factored into those settings. Other ISO choices give different settings and different EV. So in that sense, EV already includes effect of ISO (and adding ISO to EV a second time to compute exposure would give very incorrect results). You might hear opposing notions, but see the pictures of the light meter above. ISO obviously affects EV, big time.

But there is only one standard EV chart. It does work for any ISO, for whatever ISO is, and ISO certainly does change things. We will see a different EV number for other ISO, but there is just one standard EV chart.

The EV chart (below) is just the collection of EV values for all values of f/stop and shutter speed. Any possible combination of f/stop and shutter speed appears on only one row in the EV chart. The EV number is that one row in EV chart where our f/stop and shutter speed appear. The row is the set of Equivalent Exposure settings, and the camera settings on this EV row apply to the one specific ISO appropriate for a proper exposure. Or, 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. When using EV, ISO seems pretty much the entire point of different EV numbers. But there is only the one standard EV chart.

LV = EV + log2 (ISO / 100),   sometimes this reference to ISO 100 is called Light Value, which is just EV at ISO 100, using this additional added factor to change EV for ISO, specifically to be an EV conversion for ISO values. There are actually two uses, using + or - in this LV formula.

So Light Value as commonly seen probably means "EV at ISO 100" (in ExifTool for example). Not wrong necessarily, but ISO 100 is not a magic number, and is not necessarily the ISO or settings that the camera actually used, or maybe not even a useful exposure, so LV seems not necessarily a useful number. It is just a standard reference for comparison, and is not trying to imply the scene light level by imagining settings at ISO 100 are somehow more meaningful than the ISO the camera actually used to get the picture. To make meaning of it, you better know it represents ISO 100. The exposure calculator at this site uses EV at ISO 100 only to compare two exposures at the same ISO. Any ISO would do that, ISO 100 is not a magic number.

Is the EV chart for ISO 100? It is certainly not wrong at ISO 100, but the EV chart has infinitely wider use, at any ISO. It is common practice for camera manufacturers to specify an EV that refers to a certain luminance at ISO 100, as when specifying metering range or auto focus sensitivity. It is a way for them to specify luminance. When they say "EV at ISO 100", then what they mean is these camera settings if at this ISO. For example, the Nikon DSLR cameras specify their metering exposure range as 0-20 EV, qualifying that these numbers apply to a f/1.4 lens at ISO 100. If it is said that some EV statement applies to ISO 100, then the statement is about ISO 100. Light Value is computed to be EV at ISO 100. EV should not be confused with Light Value, but the combination of EV and any ISO becomes a light value.

For example, Sunny 16 in bright direct sun is near EV 15 only if we are also referring to ISO 100. Technically, EV 15 is 1/125 second at f/16 at ISO 100 .. and Texas bright sun within a few of hours from midday does meter near EV 15 at ISO 100. The same bright sun will meter EV 18 if we choose ISO 800. The proper exposure at ISO 800 will require different camera settings, and those different settings are the different EV value. So any exposure will have other EV values at other ISO.

An interesting point is that LV is the same number as EV if at ISO 100, which can make the EV chart seem to be for ISO 100 (because the EV chart agrees with LV at ISO 100). But of course, this is Only because we arbitrarily referenced LV to ISO 100. LV formula could have used any number (but 100 does seem convenient). The EV chart is in fact instead about whatever ISO we may be using. EV is computed from the camera settings, which are choosen for the ISO we actually are using.

The EV Chart

The EV chart shows camera shutter speeds, in seconds or minutes. The combination of a f/stop with a shutter speed comprises the EV value. Any such combination can appear on only one row of the chart. The EV chart is about the numerical combinations of the shutter speed and f/stop values, and the resulting exposure at some ISO. The EV charts show ISO 100. ISO is an exposure factor, it matches the settings to the light. Each EV row is the group of Equivalent Exposure settings for that EV.

Again, the formula and the chart are NOT about any specific ISO value, until we insist our use of it is about some specific ISO. Then of course it is, at least for that specific use. :) However, yes, the number in the EV column is affected by ISO. The chart settings we select are about whatever ISO we are currently using. But higher ISO definitely increases the EV number for the scene light value.

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. The camera settings are EV 18 to match the scene at ISO 800.

The term Light Value arbitrarily uses EV at ISO 100 to specify the scene light level, but ISO 100 might not always allow usable (selectable) camera settings for that exposure.

Specifically, an EV chart row is the set of numerical camera Equivalent Exposure settings, and some ISO can make the exposure match it. The normal standard is that EV charts are simply printed for ISO 100. Most EV charts only show ISO 100 values.

Note: You only want to use or consider the one standard EV chart. All other notions about shifting the EV column up or down for an ISO value can only create nonsense. The way it works is that the one EV Formula computes these values, for this one chart. We could debate the words: "independent of ISO" or "taking ISO into account". ISO may not be in the EV formula, but the chosen camera settings are due to ISO. Read the EV15/EV18 example above again.

EV
f/stop
1.01.42.02.84.05.68.0111622324564
-5301 m2 m4 m8 m16 m32 m64 m128 m256 m
-415301 m2 m4 m8 m16 m32 m64 m128 m256 m
-381530602 m4 m8 m16 m32 m64 m128 m256 m
-2481530602 m4 m8 m16 m32 m64 m128 m256 m
-12481530602 m4 m8 m16 m32 m64 m128 m
012481530602 m4 m8 m16 m32 m64 m
11/212481530602 m4 m8 m16 m32 m
21/41/212481530602 m4 m8 m16 m
31/81/41/212481530602 m4 m8 m
41/151/81/41/212481530602 m4 m
51/301/151/81/41/212481530602 m
61/601/301/151/81/41/21248153060
71/1251/601/301/151/81/41/212481530
81/2501/1251/601/301/151/81/41/2124815
91/5001/2501/1251/601/301/151/81/41/21248
101/10001/5001/2501/1251/601/301/151/81/41/2124
111/20001/10001/5001/2501/1251/601/301/151/81/41/212
121/40001/20001/10001/5001/2501/1251/601/301/151/81/41/21
131/80001/40001/20001/10001/5001/2501/1251/601/301/151/81/41/2
141/80001/40001/20001/10001/5001/2501/1251/601/301/151/81/4
151/80001/40001/20001/10001/5001/2501/1251/601/301/151/8
161/80001/40001/20001/10001/5001/2501/1251/601/301/15
171/80001/40001/20001/10001/5001/2501/1251/601/30
181/80001/40001/20001/10001/5001/2501/1251/60
191/80001/40001/20001/10001/5001/2501/125
201/80001/40001/20001/10001/5001/250
211/80001/40001/20001/10001/500
221/80001/40001/20001/1000
EV
f/stop
1.01.42.02.84.05.68.0111622324564


Menu of the other Photo and Flash pages here. Sunny 16 on next page.


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