Flash Photography Sync Speed Explained

For the people who use a Dslr Camera with a flash and don't understand Flash Sync speed, you may like this article which explains it 101 style!! The easy way  😎


This is the link to the full thread  on photo.net


And here is the best of the thread from photo.net

Mark Parker , Jun 14, 2007; 10:50 p.m.

I wrote this for another forum a few weeks ago.


To understand flash sync you have to understand how the curtain on a SLR works.

I'll take a stab at explaining how the curtain works on a DSLR and maybe it will help.

A modern SLR camera has a dual curtain that usually slides from top to bottom. By dual curtain, I mean, one curtain opens sliding from top to bottom and a second second curtain closes behind it also from top to bottom. It is important that the curtains open and close in the same direction so that the frame will be exposed evenly.

Lets say you are taking a picture at 1/30 of a second. The first curtain would open all the way, the flash fires, light hits the sensor, and then 1/30 of a second later the second curtain closes. Keep in mind that the duration of the flash is only about 1/20000th of a second. As long as it fires within the time frame that the first curtain is completely open, and before the second curtain starts to close, life is good.

That's actually a little bit of a simplification. You have to remember that it takes time for the curtain to travel from the top to the bottom of the frame. So the camera has to take that into account when deciding when to start closing the second curtain.

On a Pentax, it takes 1/180th of a second for the curtain to move from one top the frame to the bottom. The only way that the camera can achieve a shutter speed faster than 1/180th is for the second shutter to start closing before the first shutter is even all the way open.

I guess the best way to think of it is, at those higher shutter speeds, the curtain becomes a "slit" that travels across the frame. The higher the shutter speed, the narrower the slit.

Now - How does this relate to sync speed. A cameras "normal maximum sync speed" is the maximum shutter speed in which the camera has the first curtain fully open before the second curtains starts to close. Or in other words, the maximum speed that the camera can operate without going into "slit mode". If you try to take a normal flash photo while the camera is operating in "slit mode". You will actually capture a photograph of the shutter itself. Probably not what you wanted.

The only way that a camera can operate in the "high speed flash" mode. Is to be able to fire the flash multiple times as the slit travels across the frame. That takes quite bit of electronic magic and requires a much more expensive flash. Those flashes have to be in constant communication with the camera during the actual exposure. It's amazing to me that they can even do that!!

Another way to achieve HIGHSPEED sync is to do it totally electronically without involving the mechanical shutter at all.

So there you have it. How a camera works 101. Hope it helped.


Great read on how Flash Sync Speed works!!