1. Inverse Square Law - Guide Numbers - Bounce flash - TTL Flash Compensation
2. Continuous vs. Instantaneous light - vs. Shutter Speed - Auto FP mode (HSS)
3. Soft light (and diffusion domes?) - What Hot Shoe Flashes Do
4. Flash photos are double exposures
5. Extras - Rear Curtain Sync - White Balance with Color Filters, Fluorescent
Menu of the other photo pages here
Flash photography is many things. There is on-camera flash and off-camera flash, manual flash and automatic TTL flash, and direct flash and bounce flash. There is multiple flash units, fill flash in bright sun, studio and portrait and table top flash in umbrellas, high speed flash, and surely even more categories. Lighting is a big subject, but before anyone can get far into it, there are a few basics more fundamental we need to know. In all of these cases, there are very major basic differences between flash and continuous existing light - they are fundamentally different, and we need to understand.
Flash is not difficult, but we must treat it quite differently than either sunlight or regular continuous room light.
In short summary, the major points, the really big deal, is:
- The intensity of any near light source (flash or continuous) falls off fast with distance (inverse square law.) Therefore, we can only achieve a correct exposure at only ONE distance (flash unit to subject.) Any distant background is necessarily underexposed, any close foreground is necessarily overexposed. Bounce flash can help, but distance is a huge issue for flash, with huge implications concerning our use. In contrast, direct sunlight is quite unique, very special because its intensity appears not to vary with distance (below.) Do NOT assume flash acts the same - it doesn't.
- Flash is typically very fast, shorter duration than the shutter duration, so flash exposure is not affected by shutter speed (Part 2.) But of course, any continuous light is still affected by shutter speed, like we always understood. Normal flash pictures usually see these two different light sources (flash and ambient) as a double exposure, involving two exposures, and two different concepts of rules. The camera meters these separately, and we can control them individually. Since flash exposure does not care about shutter speed, but shutter speed does affect any ambient light, then specifically, we can use shutter speed as a tool to adjust the ratio between flash and continuous light in our photos (Part 4).
- Flash imposes a few limits we work around, like flash power capability and flash range, maximum sync shutter speed, etc. But also, we can control more things about the light in our photos - direction, intensity, soft light from umbrellas, etc.
Regardless how we might reach the final choices, remember the exposure goal is simple: No matter how mixed and complex the scene, our photo can only set one aperture, one shutter speed, one ISO value, and one flash power level (in each flash unit anyway.) These two exposures (ambient and flash) is all there is to do. Sometimes these values are a compromise, sometimes a tool, but each paramater has just one value. Look at your results, and when the one value seems wrong for your purpose, simply just fix it. That is your job. Just do what you see you need to do. Flash Compensation is the tool we use to adjust what the automatic TTL flash is doing. I am suggesting we remember to approach it as a simple problem, instead of as incomprehensible magic. Learn a few basics, and it becomes very understandable (honest, it really does).
Flash is just a light that we can aim. In one way, it is just a light source, like the sun, or the window, or the room lights, but we can aim flash where we want it, and we can turn its power up or down, and its duration is typically quite fast. It is not rocket science - it is just a light, and we can see exactly what it does (if we just look, carefully.) But much of the problem is that beginners looking probably never see much yet, and you should be aware that the ability to see is the main thing to be learned about lighting. There is a page about that too . Flash is just additional illumination, however we can aim and modify the light to be like we want it to be, and we can learn to control the shadows it makes. This concern is not so much speaking of the shadows behind the subject, because generally we often try to eliminate those. This is speaking of the shadows and gradient tones ON the subject's face, intentionally created by off-camera lighting.
As to intensity and brightness: If Manual flash, we just adjust the flash power level to produce what we want, for the best photo exposure result. For one flash, this can be trial and error, judged in the camera's rear LCD, or aided by the histogram. Or we can use a handheld flash meter to meter and set the power level of multiple lights, to known values relative to each other. For TTL flash, exposure is automatically metered, but when we discover we need a bit more or less flash than the automation provides, then Flash Compensation is the way we control TTL flash. Which is very large part of any success, and is easily the best single tip about using flash - if you don't get the result you want, don't just bemoan your fate, that never helps. Do something - Fix it, then and there. Simply adjust it until you see what you want. However, flash does have some different basic properties (discussed here), which are good to know to use it.
Flash can be a big help. The three simplest tips for universally better snapshots are:
- Pictures of people outdoors in bright sun need fill flash. It's important, and better. Camera P mode and flash TTL BL mode take care of it automatically today.
- Pictures indoors need Bounce Flash. Aim the flash head up at the ceiling. Camera A or M mode, ISO 400 and f/5 and flash TTL mode takes care of it automatically today.
- However, all TTL flash pictures should consider Flash Compensation, adjust as seen needed.
- You should try these things, the results will be self-evident.
There is a detail or two though, and if your experience level is comfortable working with aperture and shutter speed, and if you want to be able to manage your flash pictures, then the material here is what you need to know. If still a beginner, and not yet comfortable, you really do need to know, and there is an excellent book mentioned below. These first basics are the dividing line where we become knowledgeable, no longer clueless. We won't be able to go very far without fundamentals.
These flash basics here are about "light itself", and are obviously important even if you always use automatic TTL metered flash.
Some people avoid any math. Inverse square law and guide number concepts do have a little simple multiplication, easy stuff. It will not do you any harm by simply reading it. And relax, only this page and the next page have any of the simple arithmetic. So the following pages are better, but the first concern might be "how bright is my flash?"
1. Inverse Square Law
Light intensity falls off rapidly with distance from its source. This is called the Inverse Square Law, which says the intensity varies with the square of the flash-to-subject distance, this way:
Light at 2x the distance is 1/4 as bright, and light at 1/2 the distance is 4x brighter (2 stops)
Light at 3x the distance is 1/9 as bright, and light at 1/3 the distance is 9x brighter (8x is 3 stops)
Light at 4x the distance is 1/16 as bright, and light at 1/4 the distance is 16x brighter (4 stops), etc.
Inverse Square Law is just a fancy name for a rather simple concept. Think of a flashlight - as the beam travels farther away from the source, the beam spreads out to illuminate a larger area, but becoming more dim with distance. All light spreads this way, your flashlight, your table lamp, and your photo flash, all weaken in this way too. All we are saying is that the light covers a larger area as it travels and spreads out.
The drawing at right is from the Wikipedia topic. The drawing simply shows when an angle spreads in space (a beam of light), and travels twice as far (2r vs. r in drawing), the Width and Height of its beam spreads to be twice as large. That 2W x 2H makes 4x the Area A which still contains the same light, which is therefore diluted to be only 1/4 as strong at 2x distance (same answer if we compute a circular beam.) The illumination varies with the square of the distance (varies inversely - more distance is a weaker light.)
Suppose the red lines are the paths of photons.
The photons of light don't become weaker with distance - the beam just spreads out. The greater area dilutes the light intensity - the same photons in a greater area, so less light per unit of area, simply because the light is the same energy distributed over a larger increasing area. Nine photons at 1x, distance dilutes density to about two per area at 2x, which is 1/4, and one per area at 3x, which is 1/9. Impressive little drawing!
It is really that simple, that is all there is to it. Inverse square law is only about the spread of any angle, and is not about any property of light at all. Space just spreads out with distance, and any light is diluted to appear wider, and thus weaker intensity (metered at any one spot.) You already know this, a flashlight beam becomes dim with distance because it spreads out with distance, becoming more dim. We might imagine twice as far is half as bright, but the big deal is that in fact, it is only 1/4 as bright there. So the big point is, light falls off fast with distance, more so up close, but the amount varies inversely with the square of the distance. Your photo flash is a light, and it does this too. It is good to realize this.
It may not matter why, but it definitely matters to photographers that this happens. All you really need to realize is that subject distance from the flash is a huge factor, like shown in the yellow box above. However, a confusion - Sunshine seems to be a major exception - direct bright sun appears to be constant brightness no matter where we stand, independent of distance to the subject.
Sunshine is quite special (due only to our own local situation.) Sunshine does of course work exactly according to the inverse square law too, there can be no exceptions. Yet sunshine seems very different, and actually appears NOT to work that way. Sunshine seems to have a constant brightness anywhere we look, simply because we are 93 million miles from the Sun, and another few miles to yonder mountain we see here on Earth is a totally insignificant difference. Even the 240,000 miles to the Moon is insignificant (1/4 of 1%), so the astronauts could use the same Sunny 16 rule there that we use here. On Mars however (half again farther from the Sun than the Earth), they will have open up about one more stop (inverse square law.) But since we cannot vary our distance from the sun source here on Earth, sunlight does in fact appear uniquely constant to us - only because the sun is always same distance from any subject here on Earth. This can give photographers false notions about how other light ought to work, but it is the Sun's distance that is the exception. The flash is in the same room with us, only a few feet from the subject, so we WILL see the Inverse Square Law in action. It is the overwhelmingly huge and major factor for our flash use. We might work around it without knowing exact details, but we absolutely must recognize it exists.
The exposure does not depend on where the camera is, or how far the camera is from the subject (unless the flash is on the camera.) What matters is how far the flash is from the subject.
This is yet another confusion, another classic paradox, about how flash distance greatly affects exposure, but camera distance does not. It is enough to know it is true. Frankly, this topic may better be omitted for beginners, and instruction sources always do skip it. Yet, we may be puzzled about why camera distance appears not to affect exposure? Harder to explain, and it is covered here, but that explanation seems an advanced topic, not essential, perhaps complicated, requires some thought, philosophical even. Don't let it distract the pursuit of flash basics. What we need to know is that flash intensity falls off fast with distance, according to the inverse square law.
Exposure variation due to flash-to-subject distance
Since intensity at the subject varies with distance from the light source, an implication is that any flash exposure can only be "correct" at one distance from the light source. That is an essential to know, a biggie. This Inverse Square Law (light falloff with the square of the distance) is true of all light, any light, a table lamp or a campfire at night, etc, but using flash for photos is commonly where this becomes more important to us to know. We cannot "fix" this Inverse Square Law situation, nor can we ignore it - we can only learn to work with it.
The Inverse Square law explains why the room is seen to be darker behind nearby people in a snapshot using direct flash. The distant background obviously has to be darker, it is farther from the flash (just how life is.) Using ceiling bounce flash greatly helps to minimize this difference, since most parts of the (small) room are more equal-distant from the ceiling. Or in studio situations, another light is commonly used to illuminate the background.
It is quite important to expect and plan on this distance variation for flash. Again, it does not matter to lighting where the camera is, but pay attention to distance between flash and subject. Arrange your subject, or look for a lighting angle for the flash, so that all parts of your subject are near the same distance from the flash.
I'm just saying, if your subject requires a camera angle something like this sketch, then the frontal flash will be different illumination levels at the three subject distances. Consider a flash arrangement like shown here, which seems the only reasonable solution for one flash. And of course, bounce from ceiling there, or from the left wall, comes to mind too. The three subjects will be more evenly illuminated when equal distant from the flash, regardless of where the camera is. Or if multiple distances are necessarily involved, consider more flash units to illuminate each area - for example, another light on the background for portraits. Otherwise, that is why a white background half again farther than the subject will be underexposed about one stop, and will appear gray, not white. White backgrounds pretty much require their own light, to be white.
For reference, we know that one stop of exposure is a 2x brightness difference, and two stops is 4x.
A rough guide to estimate the light falloff is this: Suppose the subject is at 8 feet from the flash, and the flash picture is setup to be correctly exposed there. Then we can be certain that background objects at 11 feet will be underexposed 1 stop, and objects at 16 feet will be underexposed 2 stops. Foreground objects at 5.6 feet will be one stop overexposed, and objects at 4 feet will be 2 stops overexposed. You recognize those example distance numbers (4, 5.6, 8, 11, 16) as being f/stop numbers, and coincidentally (due to both definitions using squares), this aperture scale we have memorized provides a good quickie guide to estimate this falloff. F/stops are not feet of course, but if the flash distance approximates any f/stop number, then half or double brightness corresponds at distances of "one stop intervals". This is merely coincidentally true (we know f/stops are not distances), but nevertheless it works, a reasonable guide. More technically and precisely, the square root of 2, or 1.414 times the distance is one stop down, and 2 times the distance is 1/4 power, or 2 stops down - which just coincidentally agrees with the f/stop numbers - each f/stop number is 1.414 times the next one (incidentally, if you pursue this, what we call f/11 is mathematically f/11.3. We simply say f/11 for round off convenience - see More.)
So this inverse square relationship also implies that if the flash is close, its illumination falls off fast at close distances behind the subject. If the flash is farther, it is already weaker of course, and since twice a far distance is farther than twice a close distance, decline requires more feet, but same percentage. Either way, close or far, if at twice that distance, the flash will be 1/4 as bright, which is two stops down. That is a big deal, we notice that.
Recommended Books
Beginners to photography: Bryan Peterson's
Understanding Exposure, 3rd Edition: How to Shoot Great Photographs with Any Camera
Neil van Niekerk's On-Camera Flash Techniques
Also, his Off-Camera Flash Techniques for Digital Photographers
Neil also provides much information on his excellent web site. If you want to know how to improve your flash pictures, see the extensive blue column menu at right on his page. Good flash lighting is greatly easier than you may think, but realized only after you actually try it, and start getting the idea about the basics.
Effective Flash power
Quick notes about the relative scale of things related to flash power. Some random facts, cute facts even, but which ought to become obvious to your understanding.
Stopping the aperture down one stop (like from f/4 to f/5.6) requires double flash power. Two stops is 4x power, and three stops is 8x power. Speedlights often don't have enough power to do low ISO bounce at much more than about f/4. ISO 400 f/4 is generally a safe try.
A change of manual flash power level to half or double the previous power level is a one stop difference. Increasing the flash power by one stop simply means to double the previous power level (like from 1/8 power to 1/4 power is double power). If you reduce your flash power to half of any previous level (like from 1/16 to 1/32 power), you can open one stop of aperture to compensate (from f/5.6 to f/4).
Increasing ISO to double value (like ISO 200 to ISO 400) requires only half of the flash power (one stop.) Doing that ISO double twice (ISO 200 to ISO 800) requires only 1/4 the power (2 stops.)
Increasing flash-to-subject distance by 1.414 (square root of 2) times more distance requires double flash power (one stop.) Two times the distance needs 4x power (inverse square law), which is two stops.
About adjusting settings for TTL flash:
For TTL flash, changing aperture, or ISO, or distance, just changes power level, NOT exposure.
For TTL flash, adding a diffuser, or bounce, or an umbrella, just increases power level, NOT exposure (the lighting may look different however.)
TTL compensates the exposure with power, tries to keep exposure right. That's what TTL does.
So - Flash Compensation is the tool we use to control what automatic TTL does, to adjust the automatic flash exposure.
However, for Manual flash mode, changing aperture or distance or ISO changes flash exposure of course, unless we manually compensate the power level ourself.
Exposure when adding multiple lights:
Using two equal flashes directed at the same subject area from same distance is double power level and twice more powerful, which is one stop. Four equal flashes doubles again, to two stops. Eight flashes is three stops. Each double is one stop.
Using two flashes of unequal power will still add, and two will be brighter than the brightest alone, but two (even if equal) are never more than 2x brighter than one (all else the same, distance, etc.) Your Main/Fill light situation for portraits will meter 1/3 or 2/3 stop more (depending on lighting ratio) than just the Main light alone. If you set camera aperture to what just the main light meters, you will overexpose a little. So meter both main and fill together, to set the camera aperture. FWIW, the math is that if we have main at f/8, and fill at f/5.6, they add to be sqrt(8² + 5.6²) = f/9.76 (but just meter them).
Fill flash in bright sunlight: This is also two lights, sun and flash. If we expose the subject in bright sunlight properly (using the camera meter reading), and add TTL mode fill flash, to be also exposed properly, that is TWO correct exposures added, and 2x overexposure (one stop) on the subject. We could back off on the background ambient exposure, which makes it darker. Or commonly, we back off on the fill flash, maybe -1.7EV flash compensation, to reduce overexposure, and to reduce flash to a proper fill level anyway (less flat and washed out). This sum is still very slightly overexposed. We just know to start there, -1.7EV flash compensation for TTL mode fill flash. You may still want to tweak it, but fill flash should not be obviously noticeable as such. Understanding this will go a long way for you.
NOTE: In contrast, Nikon TTL BL mode tries to do this flash reduction automatically for us, but TTL mode does not. Watch, and do what you see you need to do. We can compensate TTL BL flash results too, and sometimes it is variable. All of the Nikon camera manuals recommend TTL flash mode if you are doing exposure compensation (Doubters, see D7000 page 146, D90 page 265, D300 page 170, D5100 page 164 - or search PDF for "Standard i-TTL".) It is not a big problem, but otherwise compensating TTL BL mode is like two cooks salting the broth, not knowing what the other is doing.
Inverse square law with umbrellas and softboxes:
Note that for any Inverse Square Law distance computation (where half the distance is expected to give two stops more intensity), we cannot measure distance from the fabric. The distance should be measured from the real light source, which is from the flash tube to subject. For a softbox or shoot-through umbrella, the distance is from the flash tube through the fabric to the subject (distance of subject to light stand pole is close.) For a reflected umbrella, this is the distance from the flash tube to the umbrella fabric, and then back to the subject (includes two trips along the umbrella shaft.) The fabric is not the source of light, it is merely a bump in the path that looks that way. Out in front of the fabric, this bump is is just a constant power step, as if we just turned the light down, which does not affect the theory.
There are doubters who have not tried this, but my notion is that inverse square law obviously does hold true if measured from the flash tube source. Inverse square law is about distance, not about light. We can easily test it. For example, an Alienbees B400 at 1/4 power in a 40 inch AB softbox (double baffled, two nylon panels), metered at ISO 100. Flash tube is 17 inches behind front fabric. Using a makeshift plumb bob and a tape on the floor, a point 7 inches in front of fabric is 17+7" = 2 feet from flash tube, and it meters f/11 plus 0.6 stops there. Two more feet is double distance, and f/5.6 plus 0.6 stops, that's two stops. Four more feet is double again (8 feet), f/2.8 + 0.5 stops, that's two more stops. I call this real close. Still fairly close if we just measure from the light stand pole (the flash tube is nearly above it). For a reflected umbrella, measure from that stand pole, and add 2x umbrella shaft length.
