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The "Rear Sync" setting available in some cameras simply causes the flash to fire at the end of the exposure, rather than the beginning. I thought its only purpose is to cause the blur to occur in the appropriate direction.

For example, from here:

Front-sync flash
Rear-sync flash
Both images show the same object moving in the same direction

However, in his book The Digital Photography Book, Part 2, Scott Kelby makes the following claim:

Changing to Rear Sync makes the flash fire at the end of the exposure (rather than the beginning), which lets the camera expose for the natural background light in the room first, and then at the very last second, it fires the flash to freeze your subject.

Sorry for the distortion :3

Why would the flash firing at the end, rather than the beginning, cause the background to expose more? Would the background have the same amount of total exposure regardless of when the flash is fired?

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As a note, do you have permission from Wayne Fulton to use his images? He specifies all rights reserved on the page for copyright. If you don't, please remove the images. –  John Cavan Apr 26 '13 at 14:29
    
@JohnCavan: This should qualify as fair use. See also here and here. –  BlueRaja - Danny Pflughoeft Apr 26 '13 at 14:45
    
Read again, Scott talked about the ORDER of the exposure. He never said rear sync exposes more. –  Gapton Apr 26 '13 at 14:53
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Including enough of the paragraph to establish the relevant context for the purposes of education and criticism is probably also fair use, although blah blah blah I'm not a lawyer etc. –  mattdm Apr 26 '13 at 15:02
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You can't relicense images to which someone else uses the copyright, but fair use is an assertion that in a given context a portion of copyrighted material may be used without license. I think our community standards are generally that we discourage use of unlicensed images when the purpose isn't clearly to comment and criticize those images directly. Just as it's not OK to lift an entire explanation from a blog, we shouldn't take example photos and diagrams to explain a topic. But, if the Q&A is about a quote or an image, including enough to make the whole point might be considered OK. –  mattdm Apr 27 '13 at 2:00

3 Answers 3

up vote 2 down vote accepted

I'm pretty sure that Scott Kelby means slow sync in general as opposed to the normal flash modes, not first curtain vs. trailing curtain slow sync. From the context (see excerpt in google books), although he talks about the timing of the shutter, he really only contrasts to normal operation.

I searched the rest of the book too, and he never talks about first curtain slow sync as a possible option at all. Possibly the thinking is that rear curtain is likely to be the only slow sync flash mode available on some cameras – although in my much more limited experience with point & shoots it's generally the other way. (And I've never seen a DSLR that doesn't have both.) Or, perhaps Scott thinks that the other reasons you mention are good strong enough that people should get in the habit of using second curtain sync instead of first curtain sync, and doesn't want to confuse people by contrasting the two at this point. That's one of the risks of "no nonsense" beginner photography books — sometimes they unintentionally mislead through oversimplification.

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Hmm, that makes sense, though it's unfortunate. Thanks! –  BlueRaja - Danny Pflughoeft Apr 26 '13 at 14:52

There is no reason that a rear (second curtain) flash should reveal more of the background than a front (first curtain) flash should. The second image you post looks much more like a higher ISO that didn't use a flash. You can see that the highlights on the face in the right come from behind the bar, not the on camera flash (which is where they come from in the left image.)

It also may be that he is combining techniques and moving the flash off camera and lowering the flash power to achieve a more natural look that still exposes more of the background, but again, this is by increasing the exposure of the image in general through the typical exposure triangle (shutter, aperture, ISO).

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I think he's effectively over-simplified the situation a bit, probably intentionally...

The slow-sync mode is enabled by Nikon in rear-curtain sync mode (Kelby is a Nikon shooter if I recall) which results in the camera metering for ambient light. That may result in a longer shutter speed, and so more of the ambient light is exposed to the camera before the flash fires. In front curtain sync, the shutter speed is usually at or close to max sync speed and so the ambient is generally excluded.

All of that applies to automatic modes, in manual mode you can control the ambient, through manual shutter speed settings, regardless of the sync and so can get leading or trailing blur as appropriate to your creative need.

Not sure if other cameras entirely behave the same way, but like I said, I think Kelby is linking slow sync to rear curtain sync for simplicity sake.

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