Flash Photography: What is rear curtain sync, and when not to use it

In a recent workshop a student argued to only use rear curtain sync: He had read in various forums to only use rear-curtain sync, and not to bother with other settings. Let me be blunt: Never listen to people that tell you to do only one thing, because - supposedly - that's how it's done.

Instead: Try! Play around. If things don't work out the way you want them to: Try again. Figure out what works and what doesn't, and try to understand WHY.

So lets understand "rear curtain sync". Let's start by understanding what "rear curtain" means: Camera shutters on SLRs are mechanical devices (some cameras have "electronic shutters", which is not what we are talking about here). Side note: The machanical nature of shutters is the reason all cameras are rated with a life expectancy of their shutter mechanisms: The amount of "actuations" after you are expected having to replace the shutter.

Now think about it: Getting an even exposure of 1/1000 of a second (or even faster) with a single machanical device, reliably, is almost impossible. So some smart people came up with the idea of employing two "curtains", moving in opposite directions. Looking at your camera from the front, behind the mirror, one curtain is placed "in front of the other". The first curtain ths the "front curtain", the other one the "rear curtain".

Curtain movements during an exposure.

At slower speeds, there will be a period of time when the full sensor is exposed to light (as shown above in step 2 above). At higher shutter speeds, the rear curtain starts moving while the front curtain opens, exposing only a small strip of film/sensor to light at any given time.  It's a mechanical masterpiece!

Now: Your flash only fires a fraction of a second - as short as 30 microseconds or 0.00003sec. When using a flash, modern cameras give you a choice of when, during the above process, to fire your flash: At the beginning of the exposure, when the front curtain openes ("Normal, front or first curtain sync"), or at the end, when the rear curtain closes ("Rear or Second Curtain Sync").

 

Why Would you Care?

For only two reasons, and only when using longer shutter speeds:

  • A moving subject
  • Monitor pre-flashes (TTL flash)

Moving Subjects

When using longer shutter speeds, front and rear curtain sync give you two different effects when your subject is moving across your frame. The way the ambient blur records in relation to the flash-frozen moving subjects looks different for those two ways of syncing your flash, and in most cases, rear curtain sync is going to give you more pleasing results (motion blur is recorded behind yur subject).

Road Runner shot with front curtain sync (1/60 at f5, I believe - he is fast! :-)Same Road Runner, rear curtain sync

Portraits using TTL Flash

However, when your subject is static, there really is no difference in how your photograph will appear .. except when you’re using TTL flash, and you’re photographing people looking at the camera.

Why?

In calculating the TTL flash exposure, the flash emits a preflash sequence to calculate the correct flash power/exposure.

When you’re using first curtain/normal flash sync, then the time between the preflash and the main burst is so small that the human eye can’t distinguish the difference. It looks like one burst of light.

However, when you’re using rear curtain/second curtain flash sync, then there might be a noticable interval between the preflash and the main burst.  The slower your shutter speed, the greater this delay between the preflash firing and the main burst.

When you’re photographing people, then at slower shutter speeds, they DO react to the preflash, and most people will blink in response.  The results is that you will have a greater number of photographs where people are blinking or half-blinking.  Not good.

Summary

"Normal" and "Rear" curtain sync are different tools in your tool belt that give you different results in differnt situations. As a guideline (not a rule):

  • Only worry about this when using slower shutter speeds
  • Use rear curtain sync when using a flash on moving subjects
  • Use normal/front curtain sync otherwise