Monday, March 3, 2014

Rear curtain synch

Most digital cameras have something called rear-curtain synch. This means that the flash fires just before the second curtain of the shutter closes. Perhaps I need to go back to basics for people that didn't fully understand what I just said.

Most cameras have a shutter that opens and closes to let light in for a brief period in order to make the exposure. This can take one of three forms. Some cameras (which won't be discussed here) have a leaf shutter which is a blind that raises and falls over the film. This means that the top of the film does indeed have a fractionally shorter exposure than the bottom. Some cameras have a vertical travel shatter or a horizontal travel shutter. A few have the shutter built into the lens. Just to complicate matters, some electronic cameras don't even have a physical shutter.

Digital SLRs and some of the Interchangeable Lens Compacts have shutters. As these are universally horizontal or vertical travel, where there are two shutter curtains, it's possible to use rear curtain synch. In normal operation, the flash fires as soon as the first shutter has fully opened. With rear curtain synch, the camera knows that the flash is going to be between 1/15,000th of a second and 1/40,000th of a second. Thus, about 1/20,000th of a second before the second curtain closes, the flash is fired. As normal flash synch speeds are up to 1/250th of a second, any variation is unnoticeable.
In this photo, the shutter speed was 1/15th of a second. The movement is the green blur. That happened before the flash fired. This is in contrast to a normal flash exposure which would have frozen the green part of the image and the rest would have been blurred. It was just something that I tried and didn't get great results with and never really felt the urge to try again. I have seen great results with this technique but they're few and far between. In the camera and flash catalogs, there are plenty such photos and they look great. Real world application, however? I have yet to see anything really that benefits from the technique.

It just seems to me that camera manufacturers are piling capabilities into cameras and flashes, not because anybody really wants or needs them but in order to persuade people that they do need them and that they really do want to pay extra in order to have them. All 90% of people need from a flash is that it illuminates the scene well. Canon's has a lot of facilities and then costs $700. An equivalent flash without all the bells and whistles will do everything most people really need and will cost maybe $70. This is what I call trashing a camera range. Canon has trashed their entire camera range by adding too many features and jacking up the price too far.

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