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I recently watched Adorama's video on rear curtain sync flash photography. At 4.12 to 4.42, the presenter claims that when using front-curtain sync, the background/long exposure blur will be put on top of the frozen object, while when using rear-curtain sync the frozen object will be put on top of the blur.

I was under the impression that the sensors captured light in an additive way, i.e. the final image is a representation of the sum of all light that was exposed to the sensor, but then it wouldn't make any difference if the frozen object was exposed before or after the background blur.

Is there some kind of non-linear behavior of the sensor's recording that would justify what is being told in the clip above, or is this just plain wrong?

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  • I guess it's nonsense... unless the object is moving in a vertical way :-) – FredP Nov 6 '14 at 13:19
  • I would guess he's trying to make the point in this question: photo.stackexchange.com/questions/6667/… – Dan Nov 6 '14 at 15:49
  • @Dan Which point do you mean? All I see is a general discussion about the differences between first and second curtain sync regarding background trails etc, but that is not what he's talking about during the segment I link to above. In other parts of the video, yes, he does address this, but the whole video is about this difference. – Daniel R Nov 6 '14 at 16:08
  • @DanielR fair enough, I didn't watch the whole video :) It sounded like what he might have been trying to say/thinking about was "if your subject is moving, rear curtain sync will make the blur look better" but he could just be talking rubbish! – Dan Nov 6 '14 at 16:11
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    @Dan No, what he's saying is that the frozen image will be put on top of the blur if rear curtain sync is used. This misconecption is apparently quite common as shown by mattdm's answer below. – Daniel R Nov 6 '14 at 16:13
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This is wrong; you are right. The sensor can't tell which part of the exposure goes "on top" of the other. You don't have to take my word for it, either; here's a quote from the Flash basics guide on Scantips:

Some people imagine that the delayed result of rear curtain sync causes a sharp stopped image superimposed on top of the blurred image (so is sharper), which may appear true of the leading edge, but the opposite is true of the trailing edge. So while there definitely are two separate exposures, and the flash does freeze the action when it triggers (and the continuous light continues blurring it), the effect is not "on top" of anything. Each pixel can only contain the one total accumulated pixel exposure value, regardless of when.

It is, sadly, pretty easy to find other photographers on the Internet getting this wrong. I probably should avoid linking to them, but I can't resist this one, where the author gives the explanation of first-curtain shutter "blurring over your freeze", and then posts an example of how second-curtain is better clearly showing blur which appears "on top" of the frozen subject in addition to apparently in the background. Ah well — sometimes what we believe overrides what we see!

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  • Thanks for confirming my own beliefs and for interesting references. There seems to be no objections in the comments to the post you linked to, which is also a bit sad... – Daniel R Nov 6 '14 at 13:50
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If the object in motion is moving from the bottom to the top of the frame then:

  • First curtain sync would expose the "frozen" part of the image at the beginning of the time the shutter was open and the blur from the ambient lighting would appear above the frozen object.
  • Second curtain sync would expose the frozen part of the image at the end of the time the shutter was open and the blur from the ambient lighting would appear below the frozen object.

Other than that, there is no blur "on top of" or "below" the part of the image created by the flash that freezes the motion of the subject.

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