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Basics of Flash Photography
Four Fundamentals we must know

B&H Photo - Video - Pro Audio

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   -   What Hot Shoe Flashes Do

4. Flash photos are Double Exposures

    Menu of the other photo pages here


Flash Photography Basics, an Introduction

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 are multiple flash units, fill flash in bright sun, studio and portrait and table top flash in umbrellas, high speed flash, and more. Lighting is a big subject, but before anyone can get much into "lighting", there are a few more fundamental basics we need to know, about "light". In all of these cases, there are basic differences between flash and existing continuous ambient light. Flash is not difficult, it is just different than either sunlight or regular continuous room light, and we need to understand both types.

In short summary, the major points, the really big deal about flash, is:

I just have to say that this really is all easy stuff, and I suggest that it could help to try just a little before letting your eyes glaze over. :) Photographers simply need to know. Yes, there are a few details to know, and we may actually have to think a little now and then, but none of it is difficult.

Flash is just a light that we can aim. In one way, it is just another light source, but we can aim flash where we want it (lighting), and we can turn its power up or down (exposure), to deliver the lighting and exposure we want. It is not rocket science. Our picture shows everything that happens. For Exposure, we simply adjust the flash intensity to give the result we want at the subject. In manual flash modes, we simply adjust flash power level to do this. In TTL flash modes, TTL automation gets it close, and then we simply adjust Flash Compensation to adjust this level for our preference. Either way, manual or TTL, if it is too bright, then turn it down, etc.

"Lighting" is a skill to be developed (which is mostly about learning to actually "see" the results), but the flash is just a light, and we can see exactly what it does (if we just look at the result, carefully). Anything you cannot see should not matter in a picture, however, we do learn to observe more with experience. Much of the problem is that beginners probably have not yet learned to see much, 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 soft gradient tones ON the subject's face, intentionally created by off-camera lighting (including bounce flash).

Flash can be necessary, and it can be a big help. The simplest tips for universally better hot shoe speedlight snapshots (the way we ought to be using our speedlights, but sadly, many don't know yet) are:

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 a very useful book here. 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 arithmetic, easy stuff. It will not do you any harm by simply reading it. We don't actually do any math, but we need to understand the concepts, about what to expect. And relax, only this page and the guide number page have any of the simple arithmetic (multiplication and division). 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. We might imagine that if the light were twice as far away, it would be half as bright, but the correct answer is only 1/4 as bright. The drawing below explains why it falls off so fast. The Inverse Square Law is only saying that the light spreads to cover a larger area as it travels.

Inverse square law

This drawing is from the Wikipedia topic. It shows that 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 (Similar Triangles). That 2W x 2H expands to 4x times the first Area, which still contains the same light, but 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).

We can suppose the red lines are the paths of a few photons of light traveling from the source. Photons don't ever become weaker with distance - the angle of 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 really is that simple, that is all there is to it. The inverse square law is only about the spread of any angle, and is not about any property of light at all. The effect is the same on light, gravity, sound, and radio waves, because it is only about the angle and distance. Angles just spread out with distance, and any light is diluted to appear wider in that space, 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, explained above. So the 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.

We may not care why, but it definitely matters to photographers that this does happen. All you really need to realize is that subject distance from the flash is a huge factor, like shown in the yellow chart above. However, a confusion is that 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. However, it is the distance to the light source that matters, NOT the distance to the camera. Sunshine seems to have a constant brightness anywhere we look, which is only 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 full 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 with 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, if you must, but that explanation seems an advanced topic, not essential. 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. Stop and think about that a second, it 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). There are ways to help this situation. Flash pictures are double exposures, of flash and ambient. Using a slow shutter speed will aid bringing the low room light level up, at risk of motion blurring the image. Or using high ISO will aid bringing the low room light level up to match the flash. Both methods are at risk of the incandescent light causing a strong orange cast (high ISO flash pictures often will require a CTO filter on the flash, so Incandescent white balance can be used with flash). Often far best, simply using ceiling bounce flash greatly helps to minimize this distance 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 area.

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 picture and subject has a camera angle something like this sketch, then a frontal flash will be different illumination levels at the three subject distances. If using only one flash, then consider a flash arrangement like shown here, to illuminate the subjects evenly. And of course, bounce from the ceiling comes to mind too (or maybe bounce from the left wall, aimed at a spot about where this flash is shown now). 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 show as white.

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 easily 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, each set to known ratio values relative to each other. Metering is much faster for multiple lights, instead of guessing at trial and error multiple times. Each light can simply be set precisely, so we actually know what each light is doing, and then we can easily repeat the same setup exactly next time.

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. Flash Compensation is the tool to adjust what TTL automation does. However, flash does have some different basic properties (discussed here), which are good to know to use it.

This calculator computes the stops of light falloff between any two distances from direct flash. Distances can be any units (feet or meters), but the two must be the same units (it is a ratio). Inverse Square Law says double the distance is two stops less light.

Enter two distance values to compare. Numbers only   (NaN result will mean Not A Number).

to Stops of difference: -2.00 (Example, 5.2 and 10.4 feet)

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 direct 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 (simply 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 distance of course, but if the flash distance (feet or meters) 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 about these values).

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.

So there is an additional rough guide about "depth" of the light field. Direct flash light falloff has a small range around the subject which might be usable. TTL automation might follow a moving subject, but manual flash will need to keep the subject in this small range. Or the subject might have this much shape extent itself. We saw from the rough guide that if the light is at 4 feet, then it is a stop wrong at 2.8 or 5.6 feet. In that case (4 feet), one foot difference varies the light nearly a stop. But if that light is at 11 feet, then the one stop difference is at 8 or 16 feet. One stop wrong is still a large problem, but the 1/3 stop values either side of f/11 are f/10.1 and f/12.7 - so we know in this case (f/11), the range is around plus or minus one foot around f/11, for a 1/3 stop difference. If your subject has to move (dancing or kids), then this gives a good clue how much range you might tolerate around the median distance. But better, TTL flash is very good for such moving targets, since it keeps remetering the current situation.

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.

Changing manual flash power level to half or double the previous power level is a one stop difference. The marked manual power levels of Full, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64 are full stop steps. So 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). One implication when buying lights is that this means that a 160 watt second flash at ISO 200 is exactly the same lighting situation as 320 watt seconds at ISO 100 - both at same power level setting will use the same f/stop for same exposure at same distance.

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.

The Guide Number (next page) of multiple equal flashes used in combination as one, is
  (GN of one) times sqrt(number of flashes).
Two equal flashes are one stop more power than one flash. Four flashes is two stops. Eight flashes is three stops, etc.

About adjusting settings for TTL flash:

Note that when using automatic TTL flash, the automatic flash metering will simply respond to any changes with a different level of flash power, trying to keep the same correct exposure.

For TTL flash, changing aperture, or ISO, or distance, or adding a diffuser, or bounce, or an umbrella, just changes power level, NOT exposure.
TTL compensates the exposure with power, tries to keep exposure right. That's what TTL does.

So - Flash Compensation is instead 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 or modifier changes flash exposure of course, unless we manually compensate the power level ourself.

White Balance with Flash:

Flash White Balance is very nearly the same as Daylight White Balance, however the color temperature of the flash varies a little with flash power level - Color is NOT a constant. Ionization spectrum in flash tubes depends on the level of electrical current through the flash tube (power). Speedlights adjust power by truncating the duration, to be shorter, but necessarily more reddish at high power level (full low level trailing discharge tail is retained), and more bluish at low power level (red tail chopped off). Here is obvious evidence of that, which you can repeat yourself. Most studio lights are the opposite, most adjusting power with voltage level, becoming reddish at low power levels. Flash tube color simply varies with adjusted power level (just how it is). However, the Paul Buff Einstein lights are an exception, which combine these two methods in a calibrated way, so that one trait offsets the other, for a more constant color at all power levels (but you still have to match that one color to your cameras White Balance). It is extremely convenient in studio sessions to include a White Balance Card in the first test pictures, to easily and trivially correct the color of the pictures later (also suggesting Raw is very helpful).

Exposure when adding multiple lights:

Combining two equal flashes directed at the same subject area from same distance is double power level, and twice more powerful is one stop. Four equal flashes doubles again, to two stops. Eight flashes is three stops. Each double is one stop.

Combining 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 (so 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(8 + 5.6) = f/9.76 (but just meter them together).

See Part 4 for more about fill flash in bright sun.

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 just a constant power step, as if we just turned the light down.

And yes, I do know this is not what is taught in school (they feel a compulsion to measure from the fabric panel, perhaps convenient - if it worked - but which is not the source of a softbox). So there are plenty of doubters (who apparently never actually tried this). Yes, I do understand that when up close, all the points out towards the edge of the large panel also contribute added light inwards to the center line. Yes, measuring from the fabric certainly is a problem up fairly close. It is also true the center path is attenuated by the fabric, so there seems to be a compensation. Yes, there are ifs and buts and exceptions. But this is not a luminous panel - the light from behind is attenuated by the fabric. There are no great new scientific principles revealed here, just that if you insist on using inverse square law from a wide source, you'll do much better measuring from the actual flash tube than from the fabric. Anyway, my notion is that inverse square law obviously does hold pretty close if simply measured from the (actual) flash tube source. Try it.

We can easily verify the truth of it. For example, an Alienbees B400 flash at 1/2 power in a 40x32 inch AB softbox (double baffled, internal nylon panel), metered at ISO 100 with a Sekonic L308S meter. Flash tube is 17 inches behind front fabric. Metered using a makeshift plumb bob string held in metering hand, over a long measuring tape on the floor (with its zero end directly under the flash tube).

Softbox Evidence: ISL at doubled distance should be 2 stops down. The closest reading (2 feet from flash tube) was seven inches in front of fabric.

Feet,   Inches
from Tube
Meter
f/ + tenths
Stops
32384f1 + 0.72.1
16192f2 + 0.81.9
896f4 + 0.72
448f8 + 0.72
224f16 + 0.70

The one 16 foot reading was 0.1 stop high, which was repeatable. The chart sure would look beautiful if that one were 0.1 less. But which is only 0.1 stop, and 2, 4, 8, and 32 feet were right on, within 0.1 stop, and 16 feet is extremely close, no more than 0.1 stop. There was an obstacle at the side, in front of that point, probably it created a little added reflection? The stated accuracy of this Sekonic meter is ± 0.1 stop, and I was careful, but I'd suppose my procedure might cause another bit of error now and then. Switching to less precise third-stop metering might help hide or smooth tiny variations.

It sure does appear that metering from the flash tube obviously does follow inverse square law (from the actual light source). I've checked this several times, it always works for me, certainly close enough. There is no noticeable discrepancy. Try it yourself.

Of course, it is much easier to simply meter the lights, than to worry with this.

Continued

Copyright © 2008-2014 by Wayne Fulton - All rights are reserved.

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