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Basics of Flash Photography
Fundamentals we must know

Introduction to Flash Photography Basics

The reason we use flash is that flash is very bright (and its duration is very fast to freeze movement) for easy camera exposure when light is needed. In comparison, the brightest household light bulbs are dim for photography, not nearly sunlight bright. Without flash, even reasonably lighted rooms will suffer from needing long slow shutter speeds, or high ISO, or probably both. Light bulbs are continuous light, which can be fine for still life photography, when a one second shutter speed is no problem (on a tripod), but which is unacceptable for pictures of people, who tend to move. Flash is relatively about instantaneously fast (freezing motion). Flash also allows us to create the lighting, to add it like we want it to be, to place flash wherever we want the light, and to be made soft as we might desire, etc.

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 fill flash in bright sun, multiple flash units, studio and portrait and table top flash in umbrellas, high speed flash, and more. Lighting is a big and fun 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. Unfortunately, one downside of automation today is that many photographers don't bother to learn any basics any more. They can only get whatever their camera might do (and even the fanciest cameras are much dumber than we might imagine). The photographers brain is the most important tool. Seeing and then knowing what to expect is the biggest part of knowing how to deal with it. When we know why, then we know how, and then it's easy. Flash is not difficult. Flash may seem puzzling because it is just different than the sunlight or regular continuous room light we are used to, so we need to understand flash too.

In short summary, the major points, the basics (always needing a few IFs and BUTs) that you need to understand, the really big deal about using flash, is:

• The intensity of any nearby light source (flash or continuous) falls off fast with distance (Inverse Square Law, Part 1 next page). Therefore, flash can achieve a correct exposure AT ONLY ONE distance (flash to subject). TTL automation can determine that flash exposure for a subject distance, but any more distant background is necessarily underexposed, and any closer foreground is necessarily overexposed. Bounce flash can help to reduce this limitation, or additional flash units can help cover the distance range, but distance is a huge issue for flash, with huge implications concerning our use.

  But in drastic contrast, sunlight is quite unique, very special because the Sun is so extremely distant (93 million miles, 8.3 minutes at the speed of light) that its intensity appears not to vary with distance. Intensity does of course vary with distance, but we can't detect it here on Earth, where everything is at the same distance from the Sun. If we should move to Mars, about half again further than Earth, we would need to increase sunlight exposure about 1 EV. BUT DO NOT assume flash acts the same way — it doesn't. Flash is typically in the same room with us, and doubling its distance to subject (like 6 feet to 12 feet) reduces the flash exposure by 2 EV, which is a LOT (unless automatic flash metering makes flash power corrections). Exposure depends on the distance to the light (NOT the distance to the camera, unless the flash is on the camera).

• Speedlight flash exposure is Not affected by shutter speed (Part 2). Because flash duration is very fast, typically much shorter duration than the shutter duration (the shutter merely needs to be open when the flash occurs, which is called "sync"). The flash that the sensor sees is short and fast, less than an instant, even if the shutter was open for a much longer time. In comparison, then any dimmer indoor ambient doesn't actually register. The flash and the shutter must be in "sync", meaning the flash occurs when the shutter is fully open. The significance of sync is that if the shutter were only partially open, the very fast flash will "freeze" and capture and show the stopped shutter, blocking some of the image from the light (that blocked area normally appears totally black if in weak ambient light). For focal plane shutters, maximum sync speed depends on the literal speed of the shutter curtain movement, how fast it can move and open. The shutter might take 1/200 (0.005) second to fully open, and the flash has to wait for it, which is called "sync". The Maximum flash sync speed is the fastest shutter speed that can be used with flash, so that the shutter will be fully open for the flash. Sync speed is typically around 1/200 second maximum with flash. The "smart" systems today will not allow setting a faster shutter speed if it can detect the speedlight flash is in use.
  Nikon SB-800 duration specifications

  1/1050 sec.  at M1/1 Full output (t.5)
  1/1100 sec.   at M1/2 output
  1/2700 sec.   at M1/4 output
  1/5900 sec.   at M1/8 output
  1/10900 sec. at M1/16 output
  1/17800 sec. at M1/32 output
  1/32300 sec. at M1/64 output
  1/41600 sec. at M1/128 output

  The speedlight flash can be extremely fast (awesomely so when at lower power levels, but then needing closer range), much faster than any possible shutter speed, easily stopping extreme motion (like water drop splashes, hummingbird wings, or even bullets in flight). Except when at Full power level, speedlights are considerably less fast (technically if at the one full power level, the standard engineering specification t.5 is the time the intensity is at least half of the peak. The full exposure time is about 3x slower than this t.5 spec, so /3 is 1/350 second, but still faster than the shutter speed). But when at less than Full power, speedlight mode kicks in to simply chop off the duration to reduce power. Even at half power level, speedlights are typically only about 1/1000 second duration (and perhaps 1/30000 second at 1/64 power, called speedlights). Studio flash units are typically fast enough too, but camera speedlight flash units can be extremely faster at lower power (but low power range is less too). Speedlights are how high speed flash photography is done (see an example).

  However HSS Mode (High Speed Sync) is an extreme exception. HSS flash is a longer continuous duration, dependent on shutter speed, but independent of sync speed (continuous light is always present, so it always in "sync"). The HSS flash mode is intentionally made to be continuous, in this case meaning the HSS flash is present when the shutter opens, and still present when the shutter closes (like sun light is always present). So as seen by the shutter, HSS acts exactly like continuous light (it is continuous for the full shutter duration, which for focal plane shutters is at least the entire shutter curtain travel time at any shutter speed). The human eye doesn't detect the flash duration was longer, because the shutter duration still looks about instantaneous to us, but the flash is continuous all during the shutter duration.

  And since HSS is continuous light (like sunlight, incandescent, etc), the Equivalent Exposure concept works for HSS too (but NOT for speedlight flash), meaning HSS is affected by shutter speed, like we always understood for continuous light. if using HSS, that's a big concept to know. Equivalent Exposure means that a fast shutter speed must be compensated by a wider open aperture (if at same ISO to still be equivalent exposure). HSS has no shutter sync speed limit, since continuous light has no speed at all, all it has going for it is shutter speed. In contrast, speedlight mode can be much faster than any shutter, but is limited by a maximum flash sync speed, where HSS is not (in fact, HSS only works at shutter speeds faster than speedlight maximum sync limit, because it would require excessive power to keep it flashing continuously during slower shutter speeds). Fast HSS shutter speed is usually used to achieve a wide open aperture for shallow depth of field purposes with flash in bright sun (speedlight shutter speeds would typically be approaching f/16 in bright sun).

  But to be continuous duration, HSS is limited in flash power, so otherwise, normal speedlight flash mode (Not HSS) has all of the advantages (more power, range, speed), and is typically much faster than the shutter, and so does not care about shutter speed (except the shutter merely has to be open to pass it).

  But regular speedlight flash is the same flash exposure at any shutter speed (not exceeding maximum sync speed). So then specifically (except HSS), we can use shutter speed as an important tool to adjust the ratio between regular flash and continuous ambient in our photos. Shutter speed does not affect speedlight flash, but it does affect the continuous ambient. So for example, in an indoor portrait session with flash, using low ISO 100, and maximum shutter sync speed of say 1/200 second, and a stopped down aperture like f/8, the normal indoor room lighting will come out black, but we can use sufficient flash power to do our portrait job. That's a big concept to know. So we might choose to totally shut ambient out (overpowering it with flash), or we can include a bit of flash as fill to help soften any included harsh ambient shadows.

• Flash pictures are concerned with Two exposures, of flash and of ambient, a double exposure so to speak, with two different concepts of rules (Part 4). This mixed lighting usually involves two white balance requirements, but we can only set one white balance (any compromise WB might be a bit better, but is still simply wrong for both). The camera meters these two sources separately (flash and ambient), and we can control them individually. Balanced flash is a mode that limits the flash power so that the sum of the two exposures do not overexpose the near subject. And there are also other metering differences. The camera metering modes Matrix, Center Weighted, or Spot, only are about metering Ambient. They are not used by TTL flash, which has its own central metering zone. And the initial automated camera settings for the scene are only about the ambient... the TTL flash wakes up and has to use whatever settings that the ambient metering has already set up (unless we override to give the flash more consideration). Flash is not difficult, but is just different.
• Flash has different modes. Flash can be automatic flash exposure, called TTL (TTL meaning, metered Through The Lens). If the automatic flash does not come out as the proper exposure, for TTL, you can use Flash Compensation to add or subtract a bit more, and try again, as desired. Or flash exposure can be set to Manual, meaning we set it ourself to whatever power level that we think will be the correct exposure. Flash Compensation won't affect Manual flash, but you instead can simply change the flash level manually until exposure is as desired. Cameras do easily measure sunlight or artificial room lighting (continuous light), but cameras cannot meter flashes other than their own systems TTL flash, however we can use most handheld meters to meter the manual flash (most commonly done for studio lighting). None of this is hard. The flash is just a light that we can turn up or down. Then we must take a test picture to see the result of our flash adjustments (which today's digital camera immediately shows to us).
• Flash is convenient to modify the light itself, and placed as needed, as we desire. For example, we can add large close lights (umbrellas for example) to be very soft light with pleasant vague diffused shadow gradient tones, instead of the harsh sharp dark shadows from a small light source (Part 3). Photography of course has other important factors (composition, etc), but flash is about adding light, and flash exposure is simply about adjusting the flash power level to deliver the right amount of light to the specific distance of your subject, and creating the light with the appearance we desire it to be.

• Flash does impose a few limits that we work around, like the flash power capability and the flash range are limited. Waiting for flash recycle time between pictures can be a issue. Also cameras impose both a Maximum and Minimum shutter speed limit with flash:
  • Maximum Flash Sync Speed, which varies with camera model, but usually in the ballpark around 1/200 second. This is actually a camera shutter limitation when used with flash. This limit is not much concern indoors (the flash is still faster), but with flash in bright ambient, this 1/200 second limit means that ISO 100 probably requires stopping down to f/11 in bright sun (Sunny 16). Which works, but some users object to that limit, and the purpose of HSS is to bypass it (at cost of power and speed).
  • Minimum shutter speed with Flash (around 1/60 second, only in camera A or P or Auto modes, but there is no Minimum limit in camera M or S mode) in dim places with flash, only because we are using flash instead, and we normally don't want the very slow shutter speed that the dim ambient otherwise typically would meter. So all of your indoor flash pictures taken with automation are all probably at 1/60 second shutter speed. This limit is NOT a metered value of proper exposure, but instead is probable underexposure of ambient unless automation can compensate with aperture or ISO, but it is instead of the actual metered slower shutter speed for the dim ambient (however, we are using flash instead). There can be better choices for specific purposes (Part 4). Camera mode M or S will allow any slow shutter speed with flash, and if in camera A and P mode, the Slow Sync setting will also ignore this minimum limit, and will use the actual metered slower shutter speed.
  These two shutter speed limits with flash, typically around 1/200 second Maximum, and (in A or P or Auto automation) typically 1/60 second Minimum, only affect the ambient exposure. Neither of Maximum or Minimum shutter speed affect flash exposure. Neither are about measuring a proper ambient exposure, both are simply limits, regardless, be what may. Shutter speed does affect ambient, and indoors, the ambient is often negligible level unless the shutter speed is made slow and long. But shutter speed does Not affect the flash exposure (except for HSS mode). Therefore, use of shutter speed is an important tool to affect the ratio of ambient and flash, fast to block out ambient, or slow to let it in, as considered best.

  Also rapid shooting has to wait up to a second or two for Recycle time between shots, to recharge to be Ready for the next shot. Very much rapid shooting also has dangerous overheating possibilities which can damage the flash. Studio lights are normally fan cooled.

• But there are huge advantages of flash too, it can make all the difference in getting the best picture. 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. But flash allows us control over more things about the light in our photos, direction, intensity, soft light from umbrellas, etc. Some people may like "available light" photography, but others find that their flash is always available too. 😊 It is not rocket science. Our picture shows everything that happens. For Exposure, we 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, to be the level we want. In TTL flash modes, TTL automation often gets it close, and then in some cases, we might 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.

This really is pretty easy stuff, but it can be new ideas too. So I suggest that it could obviously help to try just a little before letting your eyes glaze over. Photographers simply need to know. Many of us do just "want the picture" without any bother. But we are either interested in the photography or we're not (and there's much hope for those who are interested). It's about like learning to drive — we did have to learn to operate the car, and we did have to learn the rules of the road, and maybe a little about maps, and ice and snow too. So we learned a few easy things, and now we've got it for life, and can use it at will to great advantage, in any situation, with great results. So yes, there are a few extra details to know about flash too, and photographers may actually have to think a little about creating their picture now and then, but none of it is difficult. The rewards are great.

If you don't bother to learn anything about flash, then flash pictures may be among your worst results, and enthusiasm is surely diminished. But the opposite view is if you try just a little to understand how to use it, your very best pictures may be due to flash. Flash is how we make the light be great, to improve the scene. Being willing to give it a little thought will certainly always help photography.

"Lighting" is a skill to be developed (which is mostly about learning to actually "see" the results, to look, to notice, to actually realize what you see, to think, to actually be aware of what happened). But the flash is just a light, and we can see exactly what it does (if we just bother to look at the result, carefully enough to actually "see" it). Anything you cannot see should not matter in a picture, however, we do learn to observe much 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 try to eliminate those. This is speaking of learning to notice and see 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/or 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:

• Pictures indoors often need flash, and specifically, Bounce Flash routinely offers much better lighting. Aiming the flash head up at the ceiling is simple, not always possible, but better lighting is almost automatic when it is possible. Direct flash is flat uninteresting light, but bounce is from an off-camera angle, causing soft graduated tonal shading that shows shapes, and is greatly improved. Turn off Auto ISO, camera mode A or M, ISO 400 and f/5, and flash TTL mode mostly takes care of bounce automatically today (under white ceilings, 8 or 10 feet, or maybe 12 feet high). Pull out the white bounce card (card should be "as small as is usable", definitely NOT "large as possible". Too much card obliterates the bounce, and just becomes flat direct flash again). The large cards (for example umbrellas) are used to be a larger source, in lieu of bounce, or when there is no bounce opportunity. For proper portrait perspective, don't stand too close (at least 6 or maybe 8 feet, then zoom in all that is desired). Bounce is all good, but bounce does need more flash power.
• Pictures of people outdoors in bright sun need fill flash, to lighten the dark harsh shadows. Balanced flash mode is designed to do this, and even the little popup flash will help, if the distance is not too great. More detail here. Or finding some shade is always good (softer light), but a little fill flash is still needed. In bright sun, camera P mode with flash TTL BL mode can take care of fill flash automatically today, it can be point&shoot.
• The control of automatic TTL exposure is done with Flash Compensation. Simply adjust Flash Compensation as seen needed. If TTL automation gives too much flash, turn it down a little with -EV Flash Compensation, or vice versa. Adjusting aperture or ISO will not be effective, the TTL automation just resets power level to bring the flash exposure back, same as before.
• Manual flash is adjusted with flash power level, or aperture or ISO (or subject distance). Flash Compensation has no effect on Manual flash, it does its power level setting regardless. Auto ISO cannot be used, because the manual flash cannot react to Auto ISO changes.

Which flash? The camera internal flash is a very small flash, extremely minimal, perhaps sometimes better than nothing, and convenient to carry, but it is very limited in use. A regular size hot shoe speedlight will have at least 4x power, at least 2x more Guide Number, and at least 2 stops more range. Other than power limitations, the internal flash won't zoom for longer lenses, it won't tilt for bounce flash, and it can't be used off-camera. Today, there are Chinese import flashes for less than $100 US, some of which are excellent, for example, Yongnuo flashes are very popular. See a Beginners Guide to Speedlights.

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 arithmetic that a user might follow, easy stuff. It will not do you any harm by simply reading it. We do need to understand the concepts, about what to expect. There are numbers in photography, so there are several little calculators scattered around here to help any deeper questions. Most of the following pages involve less calculation, but still, a concern might be "how bright is my flash?"

Inverse Square Law, Continued