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Originally Posted by dighost
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Since my camera has a sync speed of 250, what (if any) benefit would there be if I used a shutter speed of say 60? From what I have read, the flash is so fast, that it doesn't matter what I set the camera to.
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Thanks!
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Your maximum camera sync speed is really only applicable when using smaller flash units like a shoe-mounted flash. These small units emit all of their light very quickly, typically in the range of 1/2000 to 1/100,000 of a second, so the flash duration is extremely short.
The story changes with studio lighting gear (monolights or generator/head systems). Perusing the specs, you'll find a number for flash duration. But unless you specifically read otherwise, that number is the amount of time it takes for the flash to discharge 50% of it's full power. It's known as the "t 0.5" value. (From a standards perspective, the number followed by "t" is the amount of energy left in the capacitor banks. "0.5" means half). What this means is that the flash still has 50% of it's power left to discharge. That is quite a bit of light left that can affect your exposure. The "t 0.5" value is also known as the effective flash duration.
There is another number used in the lighting industry - "t 0.1" - which is the amount of time it takes for the unit to discharge 90% of it's full power (leaving 10% of the energy in the capacitor banks). The "t 0.1" value is also known as the total flash duration. You usually won't find this number specified by a manufacturer because it isn't a sexy. The t 0.1 duration is about 3 times longer than the t 0.5 value.
So what does this all really mean? It means that if you want consistent exposure you need to know your strobe's total flash duration and use a shutter speed slower than that value. Let's say, for example, that you own a monolight that advertises a flash duration of 1/500th. of a second. Unless the manufacturer specifically says that this is the "t 0.1" value, your total flash duration is in the neighborhood of 1/160th. of a second at full power. If you use a camera sync speed of 1/250th., your film or sensor won't receive the full amount of light emitted by the strobe. You will, in effect, under-expose the shot because your shutter curtain closes before the strobe has discharged all of its energy.
When you dial down your strobe, say to 1/2 power (reducing exposure by 1 stop), the flash duration decreases because the strobe interrupts the discharge. Now your total flash duration might be 1/300th. of a second. But your resulting exposure won't be 1 stop darker, it might only be 2/3 stops darker. This is about the time you start cursing your strobe or meter as being inaccurate.
Another side effect of using too high a sync speed is exposure falloff in the direction of shutter travel. As the second shutter curtain travels across the film (or sensor) plane, it prevents exposure to part of the sensor. Let's say that the shutter travels from top to bottom. The bottom of the sensor receives more light than the top of the sensor, causing light falloff.
In locations with low ambient lighting, you are almost always safe using a shutter speed of 1/60th. of a second with any brand of strobe equipment. That is, just about every strobe out there has a (t 0.5) value of at least 1/300th. of a second. But how much head room you have above 1/60th. depends on your strobes. Here are a few monolight examples of manufacturer and advertised flash duration. I've picked models where the number is the advertised w/s power rating.
Interfit Colorflash 500 -- 1/500
Bowens Esprit 500DX -- 1/700
Profoto CPPS 600 -- 1/850
Elinchrom Style 400BX -- 1/900
Hensel Integra Pro 500 -- 1/1600
Elinchrom Style 600RX -- 1/2050
I recently had the opportunity to play with the new Canon 1D MkIII. The Canon rep made the point of noting that the camera is being advertised as having a sync speed of 1/60th. of a second with studio strobes and 1/250th. of a second when using a speedlight. When pressed as to the reason why he said it's because of the longer flash durations exhibited by studio equipment. I also suspect they'll use it as a reason to recommend purchasing speedlights to the less well informed.
-Chip