Rear curtain flash sync – potential problems
With flash photography, suggestions are often given that you should use rear-curtain sync. Most often for incorrect reasons. So here is when NOT to use rear-curtain sync flash … whenever you don’t need it. Sounds confusing? Well, let me explain …
Rear-curtain vs first-curtain sync
Your camera’s shutter consists of two curtains that open, and allows the light to hit the sensor or film. This is true for all focal plane shutter film cameras, (as opposed to those with leaf shutters), and true for all digital cameras that have a mechanical shutter, (as opposed to an electronic shutter.)
To allow light to hit the sensor (or film), the first shutter opens, and then some short time duration later, the second curtain closes. Now, in using slower shutter speeds than max sync speed, most D-SLRS and speedlights allow you two points at which you can sync your flash.
You can either have your flash fire right at the point when the first shutter has opened, (first curtain sync), or juuust before the second curtain (rear curtain) closes.
This gives two different effects when your subject is moving laterally 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. If you’re photographing people (or any subject) in low light, and you use flash .. while that subject is moving, then rear curtain sync will most likely give you the best effect. (More about that in a follow-up article later on.)
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.
To explain why, let’s look at this diagram again:
In calculating the TTL flash exposure, the camera and speedlight emits a preflash sequence. It is from this pulse / pulses of light BEFORE the main burst of flash gives you proper exposure, that the camera calculates the TTL flash exposure.
When you’re using first curtain / normal flash sync, then the time between the preflash and the main burst are so close to each other, that you 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 an appreciable 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 attractive.
You can actually see this delay in the preflash and main burst:
Set your camera to any ISO and aperture.
Since this is just a test, you can set a high ISO (doesn’t matter how high), and a wide-ish aperture. f4 is good.
Set your shutter speed to 1 second.
Bounce your flash. (There’s no real reason. Just do it because it is a good idea anyway.)
Keep your flash to first curtain sync, and trip your shutter while noticing the burst(s) of light.
It will look like a single burst of light.
Now set your camera or speedlight to rear curtain sync.
– with Nikon it is usually a button on the top of your camera that you push, while rotating the dial. The button will have a lightning symbol on it.
– with Canon, it is a setting on your speedlight. It is the button with the lightning symbol and triangles. Push the button until you have to the triangles set.
You will now see two distinct bursts of light. The first is the preflash with which the camera determines TTL flash exposure. The second is the actual flash burst that gives you the exposure you will see in the photograph.
That delay causes the preflash to appear distinct from the main burst of light. This will often cause people to react by blinking.
Rear curtain flash will NOT give you better flash exposure than first curtain sync.
I frequently see advice on the forums that using rear curtain flash sync will give you better flash exposure. This simple is not true.
My advice as to when you should use rear curtain sync – use it when you have a specific reason to. Such as when you want to avoid the ghosting effect you see with movement and you’re using flash. Any other time, just use first curtain sync. You will have far fewer ‘blinkers’, and your life will be simpler.