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I've bought the 580 EX flash used, it's my first external flash. And I'm playing around with it to get some experience. One of the problems I've noticed in very dark environments is that the autofocus can't find a point to focus on.

I know there is this "feel" or "probe" (don't know a better english expression for it) flash a camera can trigger to evaluate the light when the real is triggered. Most cameras do this automatic.

However, I fail to figure out how to set the 7D MK2 with the 580 EX II up like that. How can I set it up this way or is there even a better way to get the focus and exposure right without the probing flash?

  • The two functions you are describing are unrelated - FEL (Flash Exposure Lock) and AF-assist beams. Knowing the names may allow you to track down as existing answer. – dav1dsm1th Nov 20 '15 at 19:51
  • @dav1dsm1th, not sure FEL is what's being discussed here. I'm going with TTL in my answer. – inkista Nov 20 '15 at 20:03
  • I read "...flash a camera can trigger to evaluate the light..." which (to me) sounds like exposure evaluation/lock but I absolutely could be (and probably am) wrong. – dav1dsm1th Nov 20 '15 at 20:08
  • FEL and TTL aren't totally unrelated. FEL is just a special case of TTL where the preflash can be separated in time by more than a few milliseconds from the actual triggering of the shutter and main flash to expose the photo. – Michael C Nov 21 '15 at 16:48
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...in very dark environments ... the autofocus can't find a point to focus on.

The feature you're looking for here is called AF-assist (autofocus-assist). The flash can be set to send out a beam of light for the camera's AF system to use to achieve a lock. You can set AF-assist either through the camera's menus for external flash control, or on the flash itself (C.Fn-08; p. 49 of the 580EXII manual, which you can find online as a PDF file on the Canon website). This AF-assist uses a special beam from behind the red panel on the front of the flash for this.

I know there is this "feel" or "probe" (don't know a better english expression for it) flash a camera can trigger to evaluate the light when the real is triggered....

This is actually a separate function, called TTL or through-the-lens metering. The camera tells the flash to put out a "pre-flash" burst of light of a known power level, meters it, and then sets the power level on the flash's main burst. It's an automatic way to set the flash power level. It uses the main flash head to do the pre-burst. If you have the flash set to "eTTL", rather tha "M", then it's using a pre-flash.

Focus and exposure are two separate functions on most cameras.

  • Thank you for the comprehensive answer. I know what TTL does and how focus and exposure work. :) I've just never used a flash before. I've taken mostly animals (dogs, horses) and nature (macro, landscape) photos before. – burzum Nov 23 '15 at 0:10

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