Why does my Canon fire a flash before the exposure when using second-curtain sync?
Asked 8/1/2013
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When I use second-curtain sync on my Canon DSLR with a long exposure, I can clearly see two flashes: one near the start and another just before the shutter closes. Why does the first flash happen, and is this normal or a problem?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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The first flash is part of the eTTL system. The camera is using it to establish the exposure. If you switch to manual flash mode it will not happen (but you will have to fix the exposure yourself). This initial flash occurs before the shutter opens and does not affect exposure, but is rather used to measure the light as part of the metering processed.
You should also be aware that even when not using second curtain sync, but still using eTTL the flash is firing twice, but the delay between the flashes is much shorter, so you are not seeing it.
Originally by user20982. Source · Licensed CC BY-SA 4.0
user20982
13y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—this is normal if your flash is using Canon E-TTL/E-TTL II. The first burst is a pre-flash used for flash metering. It happens just before the actual exposure so the camera can calculate how much flash power is needed. The second flash is the real exposure flash, timed at the end of the shutter period because you selected second-curtain sync.
You usually don’t notice the two bursts with normal flash sync because the gap is very short, but with a long exposure it becomes obvious.
It is not a fault, and the pre-flash does not contribute to the recorded exposure in the same way as the main flash. If you switch the flash to manual power instead of E-TTL, the pre-flash will not occur—but then you must set flash output yourself.
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