July 16, 2018, 12:48:46 PM

Author Topic: What is the actual time between the pre-flash and the exposure flash?  (Read 4052 times)

tiggy@mac.com

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Odd question...

I found an insect. I believe it's a soldier bug. Whenever I attempted to take a picture of it, it would anticipate the flash and jump up before the exposure would happen. I'd have a picture of an empty leaf. It happened a couple dozen times.

Thinking the E-TTL pre-flash might be tipping him off, I moved the flash to manual and was able to photograph him. Put it back on E-TTL, and he'd hop up, just over where I'd framed him, and land right back down on the leaf. I photographed a couple dozen other species that day as well, and a few more dozen in days past, and none of them exhibited this behavior.

I'm curious where I might find what the time in picoseconds/milliseconds/etc. is between the little pre-flashes used for calculating exposure versus the actual exposure flash. A pretty extensive Google search turned up nothing. I'd like to be able to calculate what this bug's reaction time would be.

Thanks for any help. -tig

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JoelMac

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Hey there,

Did you ever find the answer to your question?  I'm also interested in the preflash time.

tpatana

  • EOS 5DS R
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Interesting question. I did some google, and found couple places.

This one is using (optical) wireless on Olympus/panasonic: https://strobehacker.tumblr.com/post/94380820663/mix-wireless-ttl-and-studio-flashes-with-a-smart

Looks like 230ms between pre-flash and main.

This picture: http://www.friedmanarchives.com/DSO_Wireless_Flash_72_dpi.jpg

Shows only 140ms.

I can't find any information on that though :(

pwp

  • Canon EF 300mm f/2.8L IS II
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Long enough to induce a blink...

-pw

kphoto99

  • EOS 80D
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Check out this video:

https://www.youtube.com/watch?v=DdiYVkltlYk

He decodes the optical flash protocol. The time between master sending commands to the slave and slave firing is 55ms. Maybe ETTL is similar.

tpatana

  • EOS 5DS R
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Check out this video:

https://www.youtube.com/watch?v=DdiYVkltlYk

He decodes the optical flash protocol. The time between master sending commands to the slave and slave firing is 55ms. Maybe ETTL is similar.

I don't think they are similar. On slave the master is sending things, and then slave goes off. On preflash there's small flash burst, the camera measures light, decides how much more power needed, tells flashes and then goes off for the actual picture.

stefang

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I guess this depends on the camera. In a typical DLSR the following events happen:
1. ETT preflash fires
2. Mirror moves up
3. Diaphragm closes
4. First curtain is opened
5. Flash fires
So the faster the camera, the shorter the delay. EOS 1d camera's have a custom function the enable a shorter shutter release lag: By default, the lag is 55ms for every aperture used. By enabling the CF, using larger apertures reduces the shutter lag because there's no diaphragm to close.
Mirrorless camera's are totally different of course since they don't have all these mechanical parts to move. (Which does not guarantee they're faster BTW)

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IglooEater

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https://youtu.be/-h8YGkZnRRg

This is somewhat interesting in this more complicated scenario the entire sequence  took something like 0.034 seconds.

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