For Rik Littlefield...E-TTL Pre-Flash

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twebster
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For Rik Littlefield...E-TTL Pre-Flash

Post by twebster »

Hi ya' Rik, :D

I saved a couple of pre-flash exposures of flies so that you could see how fast the flies react to the e-ttl pre-flash...

Image

When I shot these tests the temperatures were in the high 60s°F. Normally I don't catch a long-legged fly in the frame at all. Cooler temperatures must slow down the fly's reactions.

Image

Picture wing flies can react to the e-ttl pre-flash, too. They are just not as fast as long-legged flies. :D

The flies aren't reacting quite as fast as I first thought. Here's how e-ttl flash works:

1) When you trip the shutter the flash fires a pre-flash. You can actually see this flash through the viewfinder.

2) The main flash fires when the shutter curtains open to fully expose the cmos sensor to the flash exposure. Typical x-synch for electronic flash.

3) The time lag between the pre-flash and the firing of the main flash exposure is approximately 65 milliseconds.

Long-legged flies can react fast enough to the pre-flash to be able to jump completely out of the frame in 65/1000th of a second :!: :shock: Picture wing flies barely have enough time to get their butts of the leaf :!: :D During our original discussions we forgot to add in the extra time for "shutter lag". :D

Best regards as always, :D
Tom Webster
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Think about this...maybe Murphy is an optimist!!!

rjlittlefield
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Post by rjlittlefield »

Tom, super job!

These pictures really nail down that it's the pre-flash spooking the flies, as we did conjecture in our earlier discussions at http://www.photomacrography1.net/forum/ ... hp?p=18070. The metering flash goes off, the fly starts to spook, the mirror goes up, the shutter opens, and (yawn...by some standards...) when the image-forming flash finally goes off, the fly is nowhere to be seen.

To get gone in 65 milliseconds is pretty fast, but definitely not record-breaking. (Too bad -- I always like to see records broken :D ) Changing the units may make the number seem less impressive -- 65 milliseconds is more than 1/16 of a second!

Anyway, I guess the rule for shooting these flies is pretty simple -- be real sure that pre-flashes are turned off. Easier said than done, I suppose, for many of the new cameras, particularly if all the photographer has is integrated flash.

--Rik

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