Why does my Canon flash fire before the shot when autofocus is on?
Asked 2/10/2014
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On my Canon 700D with a Canon 55-135mm IS lens, the flash behaves differently depending on whether the lens is set to AF or M. When AF is enabled, the flash seems to pop or pulse before I fully press the shutter, then it fires again when I take the picture, and it makes a noticeable noise. Why does autofocus change the flash behavior? I thought AF just focused automatically for me in low light.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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If you can't see, then auto focus can't see either. Light is needed for AF to work properly.
When using auto-focus in insufficient light, the flash will pop up so that it can produce some light for the auto focus to use in order to achieve focus.
Alternately, sometimes an IR illuminator or AF assist beam will be used if available rather than the normal flash, but that's more of a rare option.
Originally by user11392. Source · Licensed CC BY-SA 4.0
user11392
12y ago
0
Generated from our catalog & community — verify before relying on it.
This is normal. In low light, autofocus needs enough light to detect contrast and lock focus. When your lens is set to AF, the camera may use the built-in flash as an AF-assist light, causing a brief burst or pulse before the exposure. Then the flash fires again for the actual photo.
So the first flash activity is to help the camera focus; the second is the exposure flash. The noise you hear is likely the flash mechanism popping up and/or charging.
Autofocus is not magic in darkness—it still needs light or contrast to work. If there isn’t enough ambient light, the camera uses whatever assist method it has available. Some cameras or flashes use an IR or dedicated AF-assist beam instead of visible flash pulses, but using the flash itself is common.
If you switch the lens to manual focus, the camera no longer needs to help the AF system, so that pre-flash behavior stops.
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