2. Continuous vs. Instantaneous light - vs. Shutter Speed
3. Soft light (and diffusion domes?)
4. Flash pictures are double exposures
5. Extras (Rear Curtain Sync, White Balance with Color Gels)
2. Continuous vs. Instantaneous light - vs. Shutter Speed
Continuous light is daylight, or incandescent, or fluorescent, etc. - always on, longer than shutter speed duration.
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, 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 long before the shutter, to long after the shutter, and the shutter speed simply takes a tiny 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.
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. 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.
Changing aperture or ISO changes flash exposure for Manual flash, but not for TTL automatic flash. Instead, TTL automatic flash simply changes power level to give the same metered exposure for the new situation. 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. Aperture is key for flash. The aperture affects flash TTL power level and maximum range and recycle time - and 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, if we use a shutter speed time sample of say 1/100 second, then the total energy available to us 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 in this case, 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 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 are a few times more efficient at making light than are tungsten filaments (incandescent).
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, regardless of what our shutter speed may be. Note that 160 watt seconds is greatly more energy than the 5 watt seconds above. The overwhelming result is that 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.
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 picture at the same f/8 aperture with just the modeling lights, we get a pitch black picture. There is no significant contribution from the modeling lights at f/8.
Even our camera speedlights (the SB-800 is 75 watt seconds equivalent if in an umbrella) will run circles around continuous lights for photography. As much as f/8 ISO 100, or f/11 ISO 200, is possible for portraits with speedlights in white umbrellas (full power with main light's reflected fabric at four feet). Battery recycle time will be slow at full power level however.
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 more 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.
Metering differences
Our camera lightmeters 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. However, even then, the exposure shown to us by the camera light meter in the viewfinder is only about the continuous ambient light, and this reading is NOT about the flash at all (which has its own system - More in Part 4). 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 it... 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.
See info possibly useful to give the idea about the minimal hardware needed to use your camera speedlights in umbrellas (scroll down lower there).
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.
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 necessairly 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 better mechanical focal plane shutter. Some of these may have CCD sensor too (D60, D80, D3000 for example), but they use 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 disbabled) 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.
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