A Flash Question I Couldn't Answer

[Doug_Kerr]

Originally Posted by Jerry Skrocki

This will get the faster (but weaker) pulse flash duration to effectively stop motion.

Not exactly.

In the HSS mode, the overall flash "burst" is much longer than normal (actually now a rapid sequence of multiple short pulses).

This makes the use of the faster shutter speed practical, since now there will be flash output during the entire period required for the small shutter slit - smaller than the entire frame height at such high speed settings - to traverse the entire frame.

It is of course the faster shutter speed itself that stops the action (with regard to exposure by way of ambient light as well as flash illumination).

[ChuckWestfall]

Hi, Folks:

Most of the observations here have been correct, but one additional item of information may be helpful: Even in normal flash mode (i.e., not FP mode, also known as high-speed sync), flash duration at full power with an MR-14EX can get as long as 1/750 sec. That might not be fast enough to freeze the motion of a bee's wing.

So, David Buzzard is on the right track when he suggests raising the ISO to shorten the flash duration. Another method for doing that is to open the aperture. Making the same shot that Noel described at f/16 and ISO 400 instead of f/22 and ISO 200 would have cut the flash duration significantly with very little difference in noise levels or depth of field. Every little bit helps!

[Noel_Carboni]

One thing I didn't mention was that it looks as though the flash was the most significant light source in the photo by far.

Thanks everyone, and especially Chuck for that last tidbit of info. I believe that explains things handily.

flash duration at full power with an MR-14EX can get as long as 1/750 sec

Very interesting indeed! Somehow I always thought "normal" (non-high-speed mode) flash durations were always in the realm of one ten-thousandth, but now I see it can be an order of magnitude longer than that. That's a real eye opener.

Thanks again.

-Noel

[Doug_Kerr]

Hi, Noel,

Originally Posted by Noel_Carboni

Very interesting indeed! Somehow I always thought "normal" (non-high-speed mode) flash durations were always in the realm of one ten-thousandth, but now I see it can be an order of magnitude longer than that. That's a real eye opener.

Indeed. I'm sure I really "knew" that, but somehow never thought in those terms.

Best regards,

Doug

[Will_Mass]

Chuck,

Thanks for the input on this thread.

I'm curious; Is flash duration information for various Canon Flashes is available anywhere?

I ask b/c this subject goes to a long running discussion I've had regarding flash, and camera settings specifically for indoor horse shows, when using 550EX's

Thanks.

[ChuckWestfall]

Hi, Will:

Canon does not publish detailed specifications on flash duration for its Speedlites, but the information I quoted for full-power flash duration on the MR-14EX is also applicable, generally speaking, for the MT-24EX and the 540EZ, 550EX, 580EX and 580EX II flash units.

There are several reasons why full-power flash duration is designed to be approximately 1/750 second. Here are the top two:

1. Maximum X-sync shutter speed on high-end EOS SLRs like the 1D/1Ds series and the 40D/30D/20D cameras is either 1/250 or 1/300 second depending on the camera model, but the actual amount of time that the shutter blades are completely retracted during that time is only about 1/750 second. Makes sense if you think about it; It takes a certain amount of time for the first shutter curtain to move out of the way so that the entire image sensor is fully exposed, and it takes an equal amount of time for the second shutter curtain to cover the sensor and end the exposure. Keep in mind that except during high-speed sync mode, the flash can only fire when the shutter is completely open.

2. For any given Speedlite, the longer the flash duration, the greater the light output. Considering the relatively low-capacity flashtubes and capacitors used in these Speedlites, a 1/750 second flash duration provides the greatest possible output at all shutter speeds up to maximum X-sync. That's usually a good thing, except in the case of a very rapidly moving subject like the wings of a hummingbird or a bee.

The MR-14EX and MT-24EX each have two flashtubes, whereas the 540/550/580 series has only one. If both flashtubes are fired at full power simultaneously, their individual durations will be half that of the single tube Speedlites. However, it is also possible to fire only one flashtube at a time with the Macro Speedlites, which will bring the full power duration back to approximately 1/750 second.

I could go on, but hopefully this information provides some additional useful perspective on the topic.

[Will_Mass]

Thanks Chuck,

This explains the results I've seen.

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