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When I select AF instead of M on my 55-135 canon IS Lens, mounted on my Canon 700D, the flash sort of pops before I finish pressing the button and then again after I press it and it makes this scary noise...Why is this happening?

Also, isn't AF just supposed to auto-focus for me for when I can't see the focus because of lightning or such?

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1 Answer 1

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.

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When would I use AF then? When trying to catch birds or something like that or? –  Bjarni Jóhannsson Feb 10 at 22:32
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@BjarniJóhannsson - any time you don't want to have to bother with manual focus. A modern auto-focus does a very good, very fast job. Sports, birds, shooting anything that is moving are all situations when MF really isn't an option, but AF is good enough to do portraits and the like as well. These days, manual focus is more the exception case than the rule. –  AJ Henderson Feb 10 at 22:48
    
Aside of an IR illuminator, some flashes, like the YN565EX, have a red grid pattern for AF assist. Which looks quite funny, actually, but it does not blind people with the AF preflash. –  Cornelius Feb 11 at 10:26
    
Curious. I plainly thought "I can see it, so it can't be infrared". A quick google seems to indicate that the red pattern is not an actual IR assist beam, though it might use both a visible pattern as well as IR. (Differing reflectivity of target objects?) Not really having access to my flash's manual I could be completely wrong, though. –  Cornelius Feb 11 at 13:47
    
@Cornelius - I take it back, I double checked the manual and it is described as an AF Assist beam rather than an IR assist. Sounds like they may in fact be two distinct things. –  AJ Henderson Feb 11 at 14:11

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