Does Rear Sync really help background exposures in darker rooms?

Question

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:

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.

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?

Answer 1

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.

Answer 2

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).

Answer 3

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.