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12/31/2007 01:35:22 PM · #1 |
Hi. Hopefully someone can offer some technical information on the Canon 580EX flash.
1) I know what curtain sync is. The sync speed on my camera is 1/200. What I don't understand is this:
a. When the flash is in ETTL mode setting the high speed sync (and a high shutter such as 1/4000) doesn't show a partial frame from the shutter. How does it do this?
b. When the flash is connected through the CENTER PIN ONLY (i.e. off camera hotshoe/PC cord) and I use a high shutter speed (greater than sync) I see the curtain (which is supposed to happen). What makes this so different than in ETTL mode.
2) When the flash is in ETTL mode, it fires a pre-flash. From what I understand, this is for white balancing. But, it doesn't ALWAYS fire the pre-flash. Why? What caused me to ask this question was because I noticed that using optical slave flashes, the preflash sets off the flash. But if I take another photo right after, the optical slave flashes in sync with the shutter.
Thanks,
Paul |
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12/31/2007 02:33:52 PM · #2 |
Normally the flash is much faster (shorter duration) than your shutter. To achieve high speed sync, the flash actually has to SLOW DOWN. You need the flash to last as long as it takes to get both curtains across the plane (i.e. 1/200th of a second if that is your sync speed). So, regardless of your shutter speed, the flash must stay on for 1/200th of a second. (it'll actually be pulsed to make it last that long)
The pre-flash always fires. Your slave is probably missing it sometimes (or is set to ignore one of the flashes?). And the pre-flash does more than set white balance ... eTTL means it is determining the flash exposure using the pre-flash. It figures out, from that first flash, how much power will be required for the real flash to get a proper exposure.
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12/31/2007 02:46:40 PM · #3 |
If you have some extra time, there's some interesting information to read about flash speeds and shutter speeds in this thread:
Splash Pictures/Sync Speed?
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12/31/2007 03:36:46 PM · #4 |
Lets see If I can put some insight into this.
Originally posted by PGerst:
1) I know what curtain sync is. The sync speed on my camera is 1/200. What I don't understand is this:
a. When the flash is in ETTL mode setting the high speed sync (and a high shutter such as 1/4000) doesn't show a partial frame from the shutter. How does it do this?
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For high speed shots, your shutter actually doesn't close all the way, it moves like a slit over the film (sensor). That's what the sync speed is. Under the sync speed, the whole shutter opens and closes. Above the sync speed, the shutter becames a slit instead. When your flash is in high spped sync, instead of one blast of flash, it does multiple blasts of flashes. That's why the Guide number goes down also with high speed sync.
Originally posted by PGerst:
b. When the flash is connected through the CENTER PIN ONLY (i.e. off camera hotshoe/PC cord) and I use a high shutter speed (greater than sync) I see the curtain (which is supposed to happen). What makes this so different than in ETTL mode.
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I'm not certain what's going on. I didn't think you could use High Shutter speed through a PC cord. When I do, I also see the shutter, you can't go above the sync speed.
Originally posted by PGerst:
2) When the flash is in ETTL mode, it fires a pre-flash. From what I understand, this is for white balancing. But, it doesn't ALWAYS fire the pre-flash. Why? What caused me to ask this question was because I noticed that using optical slave flashes, the preflash sets off the flash. But if I take another photo right after, the optical slave flashes in sync with the shutter.
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It's not for white balancing, it for metering the shot. It fires a pre-flash of a standard amount of light and takes a meter reading to figure out how much flash to give. You can turn this off with the custom setting to TTL. I forgot which one it is.
Check out photonotes.org for everything you need to know (especially if I didn't answer correctly). |
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12/31/2007 03:39:28 PM · #5 |
Thanks alot, this information really helps.
As for the PC cord. The flash is firing as it is supposed to. My guess, from all the information, is that with the center pin only, there is no pulse flash, hence, I'm seeing the 2nd curtain. |
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