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
2. Continuous vs. Instantaneous light - vs. Shutter Speed
Continuous light is daylight, or incandescent, or fluorescent, etc. - always on, longer than shutter speed duration (if the light is present when shutter opens, and is still present when shutter closes.)
Flash is near-instantaneous light - only on for an extremely brief pulse, shorter than shutter speed duration.
This is a huge difference, and flash exposure works very differently than continuous light. 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 (shorter 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 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. 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 finishes - the flash already finished long ago. The significance of this is that shutter speed is simply NOT any factor for normal flash exposure. Exception: It is not impossible to have a large flash so slow that is is slower than a fast shutter speed (duration), but this is far from the norm. Camera flashes are called speedlights because they are fast - their duration is shorter than maximum shutter sync speed at full power level, to perhaps 1/30000 second duration at low power level (which is used for high speed flash photography).
Camera shutters do have a Maximum Flash Sync Speed, an upper limit, which varies a little with camera model, but ballpark is typically around 1/200 second, maximum shutter speed with flash (called maximum sync speed). The focal plane shutter simply can never sync flash faster. The shutter speed limit can be a pain when using fill flash in bright sun.
However (a different subject) - See the next page about 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 totally exclude Auto FP from this current discussion of regular flash. Meanwhile, continuing here, shutter speed does not affect regular flash exposure.
Aperture is the factor affecting flash power level and exposure - and ISO and subject distance too of course, but not shutter speed. One big plus is that the very fast flash duration will stop motion extremely well. But any longer shutter duration does allow more continuous ambient room light to be seen, affecting total exposure (and which can still blur motion if the light is significant.) 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.
| Things effecting: Are: | Ambient Exposure | Flash Exposure | |
| Flash Power | - | X | |
| Shutter Speed | X | - | |
| Aperture | X | X | |
| ISO | X | X | |
| Neutral Density Filters | X | X |
Qualifications, sometimes it's different:
Distance (light source to subject) affects 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 distance from any subject here. Drect sunlight is a constant (here on Earth).
And ISO can be a special case for low ambient conditions. ISO always affects ambient and flash the same, ISO does not change the ratio between them. This assumes both ambient and flash can be adequately exposed, fill in sunlight for example. However, indoors, the ambient is typically very low level (which is why we need flash.) Often we ignore it, and proceed with the flash picture, leaving ambient underexposed with insignificant effect. If 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, so 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. The flash power level changed the ratio, but ISO was the cause. In that way, high ISO is the standard tool to balance weak ambient with the flash.
The point here is about shutter speed. 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 affects the ratio (affects continuous, does not affect flash).
Changing aperture or ISO changes flash exposure for Manual flash of course, but it does NOT affect 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 flash TTL power level, and therefore maximum range and recycle time. Shutter speed does not affect flash - shutter speed only affects the continuous ambient light, room light or daylight.
Power differences (why continuous lamps are woefully underpowered for photography)
Sunlight is intensely strong, but continuous lamps indoors are relatively weak. Flash is typically strong - it may be a struggle, but at close range (like ten feet), flash can match the sun for fill in bright sunlight. Studio flash units are rated in watt seconds, which is the total of the input electrical energy (same concept as kilowatt hours on our electric bill.) Continuous lamps are rated in watts (which is a rate), with no time period defined to determine total energy. So by definition of watt seconds, if we use a shutter speed time sample of say 1/100 second, then the total energy the shutter can use from a 500 watt continuous lamp is (500 watts x 1/100 second) = 5 watt seconds. Which is not very much, because our 1/100 second shutter speed can only use a tiny fraction of the total power being expended (only while the shutter is actually open.)
However a one second shutter speed could use (500 watts x 1 second) = 500 watt seconds, or 100 times more of the continuous energy. Which is a lot of power then, but the one second shutter is normally too slow to be useful for many of our pictures (but could be very suitable for still life photos.) Note that watt seconds is a measure of the electrical input energy. The amount of output light depends on the efficiency of the light when converting electricity to light. We tend to ignore this, but flash and fluorescent (ionized gas) are a few times more efficient (less heat loss) at making light than are incandescents (tungsten filaments.)
In contrast to continuous lights, a small studio flash unit rated 160 watt seconds will give us the full 160 watt seconds of energy instantly, faster than our shutter speed, therefore regardless of what our shutter speed may be. Note that 160 watt seconds is greatly more energy than the 5 watt seconds above. 160 watt seconds means (for example), the flash unit may store this energy into a large capacitor at a slower charge rate, (for example) 320 watt rate for 1/2 second (which is 160 watt seconds of energy stored). Then the flash tube may literally consume 320,000 watts in 1/2000 second (also 160 watt seconds.) This energy is released nearly instantaneously (the flash circuit is a direct short of the capacitor across the flash tube.) We get a pop, and the result is much brightness. If using the flash, we probably must turn its power level way down to use it for portraits (perhaps 1/8 power level at f/8 at ISO 200.) But if using the 500 watt continuous light, we must place it up as close as the heat will allow, probably at ISO 400, f/2.8, and 1/30 second (as far as we can go in every direction), and hope for the best. This is a huge difference.
Even our camera speedlights (the SB-800 is 75 watt seconds equivalent if in an umbrella) will run circles around continuous lights for photography. Maybe it charges at 25 watt rate for 3 seconds, and then dumps at 75,000 watt rate in 1/1000 second. As much as f/8 ISO 100, or f/11 ISO 200, is possible for portraits with speedlights in close white umbrellas (full power with main light's reflected fabric at four feet.) Battery recycle time will be slow at full power level however.
To better illustrate the point: Studio lights have incandescent modeling lights built in. 160 watt second lights may have 150 watt incandescent modeling lights, in same reflector, at the same distance. But if we unplug the sync cord to disable the flash, and take the same 1/200 second picture at the same f/8 aperture with just the modeling lights, we get a totally black picture. There is no significant contribution from the modeling lights at f/8 and 1/200 second shutter, because continuous lights have insignificant light output, compared to flash. We adjust flash power levels to be fully effective at f/8 (nearly sunlight exposure levels). Studio use would always use maximum shutter sync speed for this purpose, to keep out all of the ambient light, and its incandescent color, to retain only the carefully planned flash lighting. We can do the same thing with hot shoe speedlights.
If shopping for studio lights, you should be aware that ISO 200 with 160 watt seconds will give the same exposure combinations that ISO 100 users see with 320 watt seconds. Both cases likely shoot f/8 portraits at about 1/8 power level. Or full power on both should allow f/8 for groups with white umbrellas at ten feet. That seems plenty of power, and excessive power can be an issue in the living room. What needs more power is greater distances (school gym maybe), or small apertures like f/22, or trying to overpower the sun outdoors.
See info possibly useful to give the idea about the minimal hardware needed to use your camera speedlights in umbrellas (scroll down lower there.)
Metering differences
Our camera light meters easily meter continuous light, but they do not meter flash. One exception is TTL flash, where we set aperture, and the automatic TTL flash responds with the correct power level for that specified aperture. Even then, the exposure settings shown to us by the camera light meter in the viewfinder are only about the continuous ambient light, and this meter reading is NOT about the TTL flash at all (which has its own system, invisible to us.) Camera exposure settings are done by camera automation according to only the ambient continuous light. Or of course, we may set the camera ourself manually. Then the TTL flash system necessarily uses that preexisting aperture setting to meter the TTL preflash, and it sets flash power level accordingly for it. The flash system never changes any camera setting, it only changes TTL power level. There are two cases (limitations, not exceptions.) The camera automation does normally force a Minimum shutter speed with flash, so in dim places, we always see 1/60 second instead of a too-slow shutter like perhaps 1/8 second shutter. And the camera does try to enforce a Maximum shutter sync speed with flash, so flash can work. Shutter speed has no effect on the flash exposure, but it does affect the continuous ambient exposure. Part 4 here has more about controlling the ambient light existing in a flash picture.
No matter what aperture you use, the automatic TTL flash system will try to match it, to automatically provide the right power level for it, if possible - if the flash has ability to provide that much power. Therefore, regardless of aperture, we always get the same TTL flash exposure (in that one metered situation - other metered situations will vary.) And shutter speed has no effect on flash exposure. So about any and all camera settings will give the same TTL flash exposure. Therefore, Flash Compensation is the way that we can control or adjust what the TTL flash is doing. This is a BIG deal, this is the tool we have to use. Aperture does affect the amount of flash power needed, so of course it has great affect on Manual flash, but TTL flash tries to deal with it automatically.
The camera systems with the fancy TTL wireless remote flash features can operate a couple of their own flash units in an automatic way. For example, Nikons CLS Commander/Remote flash system is quite awesome, a point & shoot remote wireless multiple flash system, which instantly and automatically does the equivalent of several minutes of manual setup to equalize the two lights at the subject. Here is a quick look at that. We can also specify the lighting ratio between main and fill by simply specifying it, and it does it. It has many fans, but automation always gives up some control of course, and there are associated downsides, like the preflash that makes our subjects blink in the picture (using FV Lock is one solution.)
But for manual flash, the only good solution is a handheld flash meter, like Sekonic. In a studio situation with multiple lights which work in manual flash mode, we meter each light individually to manually set its power level so that it does in fact meter what we want it to meter. This is big plus, a huge advantage called "control". We can set them exactly like we want them. For example, maybe we meter the main light (alone) at the subject to give f/8. We meter the fill light (alone) at the subject to give our lighting ratio, maybe to be one stop less if desired, or to meter f/5.6. We meter the background light at the background to give f/8, or whatever effect we want there. We meter the hair light at the subjects head, to give maybe f/11 for black hair (more light) or f/5.6 for light hair (less light), whatever we know we want. Then we also meter the main and fill light together to get the lens aperture setting (both together will be a fraction of one stop more than the main light, depending on lighting ratio.) This is full control, consistent and repeatable, which we can easily do again when we setup next time.
The incident flash meter for manual flash has another advantage: The automatic camera TTL meter necessarily uses reflected light, which is dependent on the light reflected from the bright or dark color of the subject. For example, we get different readings if the subject's dress is black or white. But the incident manual flash meter points at the light instead of at the subject, and it measures the actual light intensity itself, which is totally independent of the subject, and frankly, is pretty awesome. Any subject will come out about right then.
Shutter Speed Sync differences
We can of course use any shutter speed with continuous light, like sunlight. Faster shutter speed does limit the amount of light seen, but we simply open the aperture for any other equivalent exposure.
Flash is different. Shutter speed may not affect flash exposure, but our camera has a maximum shutter sync speed for flash, in the ballpark of about 1/200 second for the focal plane shutters used on most DSLR. The shutter must be fully open when the flash fires, to expose the entire area of the photo frame in that instant. At faster shutter speeds, the focal plane shutter is never fully open all at once, it is only a narrow open slit moving across the frame. This means faster shutter speeds cannot be used for flash, or else we would get a dark unexposed band in our picture, where the total frame area was not open. The fastest shutter speed when the shutter is in fact 100% open all at one time to allow flash to go through it is the definition of "the maximum shutter sync speed" (see camera specs.) It is a hard limit, but see the Electronic shutter and FP HSS sections below, and next page about Auto FP flash mode.
Indoor vs Daylight Shutter Speed differences
Indoors, the continuous ambient light is usually dim and insignificant (the reason we need flash.) Shutter sync speed will be limited to ballpark of 1/200 second, but this is rarely any concern indoors, since the flash duration is faster, especially speedlights. We may set the Manual shutter speed slow to emphasize the ambient a little (camera A mode likely uses 1/60 second minimum), or we may set shutter speed fast (maximum sync speed) to keep any ambient out of the picture (to keep white balance more pure, etc.) Generally shutter speed is little issue indoors, it only affects the ambient, which is dim and insignificant.
For fill flash in bright sunlight, the continuous ambient is very significant, we must deal with it. To expose it properly, the Sunny 16 Rule says for ISO 200, typical exposure in bright sun is 1/200 at f/16. If using flash, we simply cannot use equivalent exposure of 1/400 f/11 or wider, because the shutter cannot sync the flash faster. This means with flash fill in bright sun, 1/200 at f/16 is about all that can work (due to sync speed requirements.) Camera mode P knows this, but camera mode A will allow you to set f/4, and then fuss at you about it being unusable (HI warning). Mode P is a good thing for flash in bright sun.
But in bright sun, it would be great to have a way to increase the sync speed so we can open the aperture. This is simply impossible with the focal plane shutters, but many DSLR systems do have a way (sort of a kludge) to allow this, called High Speed Sync flash (Auto FP, next page.) However, it does reduce the effective power of the flash considerably (maximum distance range will be no more than half.)
Summary:
1. Flash is greatly affected by its inverse square distance, but sunlight is not affected by distance (on Earth.)
2. Flash is not affected by shutter speed, but shutter speed only briefly samples continuous light.
3. Shutter speed with flash is limited to the maximum shutter sync speed, typically in the ballpark of about 1/200 second.
Shutter speed simply does not affect the exposure from the flash. Aperture and ISO and flash power level affects flash exposure. Aperture and ISO and shutter speed affect continuous light exposure. See Part 4 here about using this difference. Note that we can turn the flash power up or down, but we have to seek some shade to affect the sunlight.
Ifs and buts - Exceeding maximum shutter sync speed
A couple of confusion factors which do not change the basics above:
There are mechanical focal plane shutters, and there are electronic shutters in the CCD sensor chip.
If we had the faster electronic CCD chip shutter (no mechanical motion involved), then these can sync flash at the fastest shutter speed. The CCD sensor is electronically enabled and disabled, which is also used as a shutter, even like at 1/4000 second. However, the faster shutter speeds could be fast enough to truncate the longer flash duration, reducing effective light from the full power flash.
- Cameras with CCD sensors necessarily must disable the CCD sensor chip after each picture, to transfer the charge out (to access that image.) This is how CCD works. It is very easy to also use this feature as a fast shutter, instead of a more expensive mechanical shutter. These do use a lesser mechanical shutter at slow speeds (roughly 1/100 second or slower), which also protects the sensor from light when not supposed to be accumulating exposure. Almost all compact cameras use the CCD for a shutter. This may be an advantage with respect to fast flash sync and price, but it is a disadvantage in all the other technical ways.
- Many better cameras use a CMOS sensor today. Disabling the chip is not a feature of CMOS sensors. It takes too long to boot them up again. These models provide the focal plane shutter.
- Better cameras already have the mechanical focal plane shutter. Some of these may have CCD sensor too (D40X, D60, D80, D3000 for example), but they choose to provide the better mechanical focal plane shutter.
- CCD must electronically enable and disable the CCD sensor chip, which can also be used as a fast shutter. However, without a protective cover (mechanical shutter) also covering the chip, this additional light accumulation (while chip is disabled) can cause bands of flare. See this example comparing a D80 and D70, both use the same CCD sensor, but the D80 uses a focal plane shutter, and the D70 uses the CCD electric shutter. The fast electronic shutter speed exposes the picture, but the flare is caused by the longer duration of the slower mechanical shutter, while the chip is not covered, protected from the sun. The better focal plane shutter is considered a very big plus, for several reasons, however the fastest sync speed with flash is not one of its features.
The Nikon D40, D50, D70 cameras do use the CCD shutter, but these models also limit the maximum shutter sync speed to 1/500 second. However, if you use a PC sync cord (which is Manual flash mode only) to break communication so the camera does not know the flash is present (so firmware does not limit shutter speed), you can use any faster shutter second (on these models with this type of electronic shutter, indicated with 1/500 flash sync speed), which will still sync flash, which in some cases might truncate the flash duration and reduce the useful flash exposure. So things do change then - this case of flash is affected by shutter speed. It is just NOT a normal case.
It is very difficult to define how to measure actual flash duration. Flash is a fast pulse which then decays relatively slowly to zero, and it is very hard to agree when it effectively finishes. When does the gradual trail-off stop being effective? (10%? 5%? 1%?) The standard method for published flash duration is called t.5, which measures the time that the flash is stronger than 50% of its peak intensity. This is an engineering convention, convenient for engineers, but is not a photographer's convention. Because 50% intensity is only one stop down, and is still rather bright, so photographically, this means the t.5 specification number is actually about three times faster than we realistically see in our pictures - the useful 10% limit of the flash (called t.1, measuring 90% of the power) is about 3 times longer than the conventional 50% spec number for duration - BUT this limit of usability is really difficult to specify.
Camera speedlight units are a big exception, and are "different", being extremely fast at lower power levels (the name "speedlight".) Their 1/32 power level may have an actual duration of 1/20,000 second, because speedights reduce power by truncating the flash duration. This truncated pulse has very steep sides going up and down, and so t.5 really hardly applies, EXCEPT when at their full power level when t.5 very much does apply (just saying, the truncated lower power speed specs are more unequivocally precise regarding what actually happens.) But otherwise, speaking of the speedlight full power level, then in general, a t.1 time is more meaningful for photographers, which duration is mathematically about three times slower than the t.5 stated by the spec, and so the full power duration may approach normal maximum shutter sync speeds closer than we may realize. If that may be not be clear, please see the the actual description at High Speed Flash.
Most cameras with interchangeable lenses, like film SLR, and more expensive DSLR models have a mechanical focal plane shutter, which is like an open slit moving across the sensor frame (to expose the frame.) Not all of the frame is open to be exposed at any one instant, except at shutter speeds not exceeding maximum sync shutter speed, which by definition, is when the frame is fully open, to be able to be exposed by the instantaneous flash. If the camera incorrectly allowed using a shutter faster than maximum sync speed, we would see a dark unexposed band, where the moving slit was not fully open at the time. A shutter speed faster than its maximum sync speed simply cannot work right with flash. The realistic focal plane maximum shutter sync speed is typically around 1/200 second (a few are 1/250 second.) But technically, the flash is even faster duration, with more motion-stopping ability than the shutter has, so long as there is little continuous ambient light to let the slower shutter blur the motion.
These focal plane cameras often offer a FP High Speed Sync mode (often named Auto FP), which in fact does allow any shutter speed faster than the maximum sync speed possible.
Auto FP flash is a big subject, Continued on next page.
Copyright © 2008-2012 by Wayne Fulton - All rights are reserved.
