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Rear curtain sync is a flash mode that synchronises the flash pop with the time that the rear curtain is about to come down.

In other words, rear curtain sync comes at the very end of your exposure, and for this reason it is also known as 'dragging the shutter' (ie dragging it at the end). An exposure is typically controlled in the camera by the use of curtains coming down; one to open for the photo and a second to close. Some of the best brands for extreme macro are now starting to use an electronic front shutter curtain but most brands still have a mechanical shutter that controls the exposure.

a variation of an old open shutter trick from the days of film

Rear curtain sync is a variation of an old open shutter trick from the days of film. Open the shutter while the camera sees black (ie a dark room), turn on the light for long enough to make the exposure, then turn off the light, then close the shutter.

Rear Sync Uses

Rear curtain sync is often used in wildlife and action photography, to freeze a subject with a motion trail in the image. In extreme macro, rear curtain sync has several advantages:

Dampening Vibrations

Trailing sync, rear flash sync, second curtain sync and slow sync all describe permutations of the same thing

For the extreme macro studio shot, a nice working sequence per frame starts with using mirror lock-up. Add a 1 second shutter speed with rear-curtain sync onto that and by using a flash for the exposure, the flash will fire only at the end of the one second exposure, and so all mirror lifting and shutter opening vibrations will have dissipated. Since the flash is the only illumination, none of that vibration should effect the shot either.

Preventing Flash Ghosting

Flash ghosting is an ugly shadow rim round your subject, the result of ambient light and flash interfering. Rear curtain sync minimised this phenomenon.

Other Names

Rear curtain sync doesn't always play nicely with radio or IR controlled triggers

Rear curtain sync is named a variety of ways by all the manufacturers. Trailing sync, rear flash sync, second curtain sync and slow sync all describe permutations of the same thing, but do your homework first as there is a deplorable lack of uniformity amongst companies, even though they all do exactly the same thing.

Watch Out For...

Rear curtain sync doesn't always play nicely with radio or IR controlled triggers. This is especially true on Canon and Pentax which require activation of *TTL. On Nikon, you can set rear curtain sync on the body and reports indicate it works just fine. On the Pentax K-3ii, it also works just fine on Metz 58afII, in PTTL and manual mode.

With the more recent 7D and some other Canon bodies you can set the internal flash to a low manual power setting and second curtain sync (unfortunately not on digital Rebels). With optical slaves it is then possible to trigger your flash units at the end of, say, a 1 second exposure.

Canon flash units that have second curtain sync capability can be used at a manual power setting on all Canon bodies (via a TTL cord or directly attached) to trigger a second curtain sync with any other flash attached to one of those small optical slaves.

On Pentax, on the K-5, the only way to use rear curtain sync with manual lenses was to fake it by adding a deglassed -a series+ lens in front of your bellows and then to use a Metz 58-AFII on manual and rear sync to trigger other units.

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