This means curtains for Flash ‘Arry!

I was giving my “Movement in Photography” talk at a photo club recently and, whilst talking about use of flash and long shutter speeds, I was asked if I used first or second curtain flash synchronisation.  So what does this mean, how does it work, and what are the effects you get?

Flash synchronisation is to do with the timing of the flash while the shutter is open. First (or front) curtain synchronisation means that just as the shutter opens for the exposure the flash fires.  The shutter stays open for the ambient light exposure and then closes.   Second (or rear) curtain synchronisation means that the shutter opens for the ambient light exposure and then just before it closes the flash fires.

The type of flash synchronisation is a choice you can make when you want to use a long shutter speed to balance an ambient light exposure with a flash exposure AND your subject is moving, or you are moving the camera.  There’s no effect if your subject isn’t moving (and if you keep the camera still), but there may be an unwanted effect if you are taking portraits.  I’ll come on to that later.

The default setting for most cameras is first curtain synchronisation, and most of the time you can just leave it there.

So, what changes if you change your flash setting from front curtain to rear curtain?  In both of the images below my hand was moving diagonally upwards from the bottom left of the frame towards the top right.

1st-curtain-flash-synch-for-blog

This image was taken with the default, front curtain, setting.  The flash fired at the start of the ambient exposure, so there is a sharp flash-lit image of my hand overlaid with a blurry ambient-lit image going off to the right.  The light trail from the head torch goes up to the right too.

2nd-curtain-flash-synch-for-blog

This image was taken with the second curtain setting.  The flash fired at the end of the ambient exposure, so there is an ambient-lit blurry image of my hand overlaid with a sharp flash-lit image further to the right.  The light trail from the head torch appears to go down to the left.

The main effect is that the apparent direction of movement for the first curtain flash is in the opposite direction to the actual direction of movement.  We intuitively want the light trail to follow rather than lead.  If you are photographing a moving car at night, and you want the rear light trails to appear after the sharp flash-lit car, then use second curtain to make them appear behind the car.

So if it gives a better impression of movement why wouldn’t you set your camera to use second curtain flash all the time?  Well, if you are taking portraits, even using the normal flash shutter speed of 1/125th of a second or thereabouts, it can cause problems.  Most modern cameras use TTL flash metering.  They fire a metering pre-flash just before the shutter opens and then measure the light coming back from the subject.  It all happens in the blink of an eye, and that’s the problem.  The pre-flash can make people blink, and if you have the flash set to second curtain you can pick up that blink, so your subject’s eyes are closed.  Sticking with first curtain can avoid this, as the pre-flash and exposure flash are so close together.

The “curtain” name came from a time when camera shutters had fabric in them, which looked a bit like curtains, and it’s sort of stuck, even though shutters don’t use fabric these days.

Don’t worry about the name, just have fun changing the settings and seeing what they do.

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