Continuous light vs. Instantaneous Flash. Camera Shutter Speed does not affect flash exposure

Continuous light vs. Instantaneous Flash

There are really big differences, in shutter speed, in power of the lights, in metering methods, in motion stopping.

Continuous light is sunlight, or incandescent, or fluorescent, etc. — always on, longer than shutter speed duration. Continuous meaning the light is present when shutter opens, and is still present when shutter closes. The shutter and sensor sees the light as being Continuous, always there.

Flash is near-instantaneous light (except for HSS flash) and is only on for an extremely brief pulse, shorter than shutter speed duration. One plus is that the very fast flash duration is how high speed photography is done. It can stop extreme motion very well. The speedlight can be much faster (shorter duration) than any possible shutter speed, like perhaps up to 1/30,000 second (at low power level, up close) to stop hummingbird wings.

But this is a huge difference, and flash exposure works very differently than continuous light. Flash is unaffected by a shutter speed longer than itself. But both light sources are often present (continuous ambient and very brief flash), so we need to pay attention to both types of light, both sets of rules. Any flash picture often involves these two separate exposures (Part 4).

Continuous light works like we learned exposure works... a shutter speed two times faster (half of the duration) gives a result half as bright, requiring opening the aperture one stop to compensate. So shutter speed definitely affects continuous light, like we always understood it does. Continuous light lasts from before the shutter opens, until after the shutter closes, so that the shutter speed simply takes a brief time sample of its intensity. The amount of continuous light exposure we see depends on the shutter duration. The camera can only use the amount of light seen while the shutter is open. (I am equating ambient light and continuous light, saying all ambient is continuous, and am only differentiating instantaneous flash).

But speedlight flash is not affected by shutter speed. Flash is a near-instantaneous pulse, possibly 1/1000 second duration, or speedlights are perhaps much shorter, but much faster than our shutter speed duration. Therefore, regardless if our shutter speed is 1/200 second, 1/60 second, or 1 full second, the flash does all it can do in perhaps 1/1000 second, or probably even much less. The shutter merely must be open when the instantaneous flash pulse happens. It simply does not matter to the flash exposure how much longer the shutter might stay open after the flash stops — the flash already finished long ago. The significance of this is that shutter speed is simply NOT a factor for normal flash exposure.

The tremendous speed of the speedlight flash is very good to freeze the subject motion.

This page is about using speedlight flash. This box is about HSS flash.

High Speed Sync (HSS) is extremely different than speedlight flash. HSS is Not fast at all, no faster than sunlight, which is on continuously, and therefore cannot stop any motion. And HSS is indeed on continuously for the entire shutter travel duration. Sure, you can see HSS appear to pop a regular flash, but as seen by the shutter (totaling about 5 milliseconds at 1/200 second curtain duration), the flash is made continuous there, same as sunlight in the shutter. It is called High Speed Sync, not High Speed Flash (just meaning because continuous light has no sync issue). HSS does allow very fast shutter speeds (faster than Maximum Sync Speed), but speedlight flash can be much faster than any shutter speed. In fact, HSS requires a short shutter duration because HSS power cannot run continuously very long. When you set a fast shutter with HSS, you must open the aperture accordingly to compensate exposure, exactly like sunlight or incandescent work.

Other than allowing a wide aperture (at high shutter speed) in sunlight, the advantages do seem few with HSS, because the speedlight runs circles around HSS with faster speed, higher power, and longer range. But one advantage is that Equivalent exposures do work with HSS mode (but Not with speedlight mode, which is not affected by camera shutter speed). Meaning if you change f/stop, and change shutter speed to compensate, then that is an Equivalent Exposure with HSS too. With HSS in sunlight with an Automatic metering mode (like camera A or P mode), you can change f/stop without affecting exposure, because the automation will compensate by changing shutter speed. Unlike speedlight mode, both daylight and HSS use this automation to remain the same Equivalent Exposure.

The HSS flash cannot stop motion at all, but the faster shutter speed might help in some degree (but speedlight mode can be greatly faster than any shutter speed). And HSS mode only has about 20% power (to be able to run continuously), so HSS seems the difficult way.

See more, High Speed Sync (HSS).

Examples, regular speedlight flash: Shutter speed does not affect regular (non-HSS) flash exposure. To show this quickly and clearly, the next pictures are of two bright spots on a wall. At top left is one standard 150 watt tungsten incandescent lamp (it is a Alienbees modeling light, the 7 inch reflector has an 80 degree beam, with dummy sync cord plugged in it to prevent its slave from triggering its flash). And at lower right is a Nikon SB-800 speedlight flash at 1/32 power (24 mm zoom, 78x60 degree beam), on a working PC sync cord. Both lights are 38 inches from the wall (almost one meter). It should be clear that incandescent lights are not very bright, unless the exposure is long. This is ISO 200, which could be higher, or the shutter speed could be slower, but continuous lights are no fun for portraits of people (who tend to move during slow shutter speeds). The speedlight flash could still be turned up five more stops, and at maximum shutter sync speed too (it's like day and night). This D800 camera has a maximum shutter sync speed of 1/250 second.

Neither light was changed in any of the pictures. The only thing that changed was that the shutter speed increased, which reduces the continuous light exposure in the usual standard way. But shutter speed does not affect the flash (the speedlight 1/32 power is spec'd to be only 1/17800 second duration, much shorter duration than any shutter).

The wall is light beige, and white balance is Flash, and the incandescent is orange (if with Flash White Balance here). The colors of the two lights are very different, and mixing colors is not a good thing. Any compromise white balance is inadequate here, it just makes both parts wrong. If intending to include the incandescent lighting, the flash should have had an orange CTO filter on it to match Incandescent white balance.

f/8 1 sec

f/8 1/2 sec

f/8 1/4 sec

f/8 1/8 sec

f/8 1/15 sec

f/8 1/30 sec

f/8 1/60 sec

f/8 1/125

That's simply how things work. Shutter speed affects ambient continuous light, but does not affect flash (flash is faster than the shutter speed, the shutter merely needs to be open to pass it). Many flash pictures do have some degree of continuous ambient also involved, so we can use shutter speed as additional control in our flash pictures, to choose to eliminate, or to emphasize, the presence of any continuous light.

Studio flash usually use maximum shutter sync speed to keep out influence of unwanted continuous light (and the modeling lights). For example, because of the unwanted orange light, and also to avoid disrupting the careful planning of the flash lighting, the standard studio practice would be to use the maximum shutter sync speed to eliminate any unwanted effect from any incandescent ambient light. Then a low ISO at f/8 1/200 second should make continuous light be no factor at all, a totally black picture if with just the modeling lights alone — but the flash is turned up as needed. Or casual speedlight snapshots might use 1/60 second shutter to include a little of the orange incandescent ambient, for a warming effect. Any normal existing continuous room ambient light is dimmer, and even easier to eliminate, but harder to emphasize (needs ISO). However, bright sun is the tough part, bright and hard to ignore or override, since sync speed can only go so high — and then in bright sun, flash is normally used for fill (Part 4).

f/8 1/50
Flash turned off

The room was normally lighted, but dim compared to these lights at this close distance, and at these exposures, the picture is totally black if neither light is on — so the flash was definitely doing its part before, doing more than may be obvious above. This picture at the right is 1/50 second, all else the same except the flash is turned off, only the incandescent light is on. Only a hint of the incandescent is visible then (150 watts at 38 inches, but still f/8). So bounce at f/4 1/60 second sees continuous ambient four stops brighter than f/8 1/200 second, and four stops would be comparable to f/8 1/15 second above. Then speedlight pictures sometimes can pick up more of the orange color indoors to warm the picture a bit. Which may not be all bad if you want it, and it is your choice, determined by shutter speed.

It's hard to see in these tiny pictures, but FWIW, in larger copies, the flash Ready LED is visible red up to 1/8 second, but is captured as out at 1/15 second and faster. That must be the recycle time for this 1/32 power level. This implies that continuous shooting at eight frames per second should be just about possible at 1/32 power, allowing time for this flash to recycle, at this low 1/32 power level.

Things affecting:	Flash Exposure	Ambient Exposure
Flash Power Level	Yes	No, but flash and ambient add together
Distance	Yes	No for sunlight, Yes for any local lights
Shutter Speed	No	Yes for HSS flash, which is continuous
Aperture	Yes	Yes
ISO	Yes	Yes
Neutral Density Filters	Yes	Yes

We often hear it said that ambient exposure is controlled with shutter speed, and flash is controlled with aperture. Indeed, those are often our specific concerns for attention, and it is an easy way of thinking. However, it is overly simplified, and Not precisely correct. Certainly aperture affects both, as does ISO. All it means is that shutter speed does not affect flash exposure (except for HSS mode), therefore shutter speed does become the tool to control their ratio, either shutting out ambient, or allowing it in.

Qualifications, sometimes it seems different:

Distance (light source to subject) affects flash exposure, because the inverse square law affects all light, both flash and continuous — so all light becomes dim at a distance. It is only direct sunlight here on Earth that seems to be an exception (remains constant), simply because the Sun is always at the same far 93,000,000 mile distance from any subject here, so direct sunlight is a constant here on Earth. So making the flash be stronger or closer can increase its ratio, flash relative to the sunlight, since the sunlight does not change.

And ISO can seem a special case for low ambient conditions. ISO always affects ambient and flash the same, ISO does not change the ratio between them. However, this assumes both ambient and flash can be adequately exposed, fill in sunlight for example. But indoors, the ambient is typically very low level (which is why we need flash). Often we ignore the dim ambient, and proceed with the flash picture, leaving ambient underexposed with insignificant effect. If instead, we did want to balance weak ambient with the flash, but the ambient is too weak, we can 1) slow the shutter speed (not always feasible past certain limits), or 2) otherwise we must increase ISO to bring the ambient up to usable levels. ISO affects both, but now we have to turn the flash power down (either by TTL automation or our manual flash efforts). This ISO increase leaves the ambient much stronger than before, and stronger relative to the reduced flash (since we turned it down). The lower flash power level changed the ratio, but higher ISO was the cause. In that way, high ISO is the standard tool to balance weak ambient with the flash.

An exception: A very large high power flash is slow, longer duration than a fast shutter sync speed but a slow flash is not the usual case. Camera flashes are called speedlights because they are fast — their duration is very short at lower power levels, literally like 1/1000 second at 1/2 power, and perhaps 1/30000 second duration at lowest power level (which is used for high speed flash photography). But when at full maximum power level, the speedlights are slower too, but probably not less than 1/300 second T.1 at full power.

Camera shutters do have a Maximum Flash Sync Speed (discussed on next page), which is an upper limit on shutter speed with flash. This is due to the shutter, not the flash. Maximum sync speed can vary with camera model and shutter type, but ballpark is typically around 1/200 second, maximum shutter speed with flash (called maximum sync speed). The focal plane shutter simply is not fully open to sync flash faster. If you do use a shutter speed faster than Maximum Sync Speed, the focal plane shutter will cause a dark unexposed band across top or bottom edge of the picture. This limit on shutter speed with flash is not much issue indoors (the flash is fast), but the shutter sync speed limit can be a pain when using fill flash in bright sun.
But see (second page after this) an alternative called Auto FP (Nikon) or HSS (Canon), which is a drastically different system, able to bypass the sync issue, when shutter speed definitely does affect flash exposure. I exclude HSS from this current discussion of regular flash because it is a very different subject.

We often hear it said: "When you are mixing ambient and flash, the aperture controls the flash, and the shutter speed controls the ambient." It's a popular saying, and useful, but technically, NOT fully complete (aperture affects ambient too, and ISO affects both). The point here is about shutter speed, which affects continuous ambient, but does not affect flash. So the difference between continuous light and flash is that we can use shutter speed (and/or flash power level) to adjust the ratio between the flash and the ambient light. Using slow shutter speed lets the dim ambient have more effect (sometimes called "dragging the shutter", a slow shutter speed for this purpose). But a fast shutter speed keeps out more (or all) of the ambient, both without affecting the flash exposure. We have choices. Low ISO or a ND filter reduces both flash and ambient light equally, and these do allow a wider aperture, but they will not change the ratio between flash and ambient — UNLESS, we turn the flash power back up, which then does. But shutter speed directly

Aperture affects depth of field, and is also the factor affecting flash power level and exposure. And ISO and subject distance does too, but not shutter speed. One big plus is that the very fast flash duration will stop motion extremely well, faster than any possible shutter speed. But any longer shutter duration does allow more continuous ambient room light to be seen, affecting total exposure (and the shutter speed can still blur motion if the continuous ambient light is significant level). We need to be aware that the rules are different for continuous light and flash, in this way. Shutter speed not affecting flash exposure is indeed something big to know. Part 4 here is more about using this fact.

Changing aperture or ISO changes flash exposure for Manual flash, but camera settings do NOT change TTL automatic flash exposure. Simply because, instead TTL automation simply changes flash power level to give the same metered exposure for the new situation. We don't see a change in our picture. Flash Compensation is how we adjust and control automatic TTL flash. The way that digital TTL flash automation works is that we first set some aperture, maybe f/5.6. Then the automation triggers a low level preflash which the camera meters to judge the requirement, and then from that metered preflash, the flash power level adjusts automatically to be appropriate for that f/5.6 that we set previously. If we are using manual flash mode, then we set the flash power level manually, for the same aperture goal. So opening aperture one stop does make a manual flash picture brighter, but it only reduces flash power for TTL flash.

Still, Aperture is key for flash. The aperture affects both flash and ambient, including the flash TTL power level required, and therefore maximum range and recycle time. However, we can turn flash power up or down (changing the ratio mix), which we normally cannot do for ambient light.

Shutter speed does not affect flash exposure — shutter speed only affects the continuous ambient light, room light or daylight. Which makes shutter speed and aperture be an optional control for flash pictures, affecting how ambient is mixed in, or not (More at Part 4).

Summary of lighting for portrait setups

	Pros	Cons
Flash	Typically plenty of light. ISO 100 f/8 1/200 second is likely easy for depth of field and stopping motion.	Setup needs an external light meter to adjust individual lights, but which allows perfect measured setup. Recycle time can be a nuisance.
Continuous	Is visible to eye to see and judge. Camera can meter it. Lower cost.	Relatively dim: Needs high ISO, and/or wide open aperture, too-slow shutter speed. Enough incandescent light power to use heats the subject seriously. Ambient room light can interfere.

Other differences, Continuous light and Flash continued on next page.

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