This topic is mainly aimed at focal plane shutters. Lens shutters, due to their design, open fully at any speed. So they sync at any shutter speed. Focal plane shutters are only opened fully at the sync speed and slower. At any faster speeds the opening becomes a slit. Faster the speed the narrower the slit. A flash’s duration can be as short as 1/20,000 of a second. So only the portion of the frame where the slit is when the flash fires would be exposed properly.

To simply test whether or not the flash sync contacts work a small flash should be kept in the tool kit. This flash should have both shoe contacts and a pc cord connection.

The tricky part is figuring out if the flash is firing when the shutter curtains are fully open. A quick check is to open the camera back and fire off a few flash shots (in a darkened room) at a white wall while looking through the back of the camera. You should see the whole frame lit up.

I find this technique a bit too subjective. Maybe my eyes are slow but I can never be sure if the shutter is FULLY open or not. So I made a simple tester. Amazingly it is just like the Combination Shutter Speed/Curtain Speed Tester only the phototransistors are the full frame width apart. When attaching the tester to the focal plane care must be taken to assure that the phototransistors are exactly lined up at the edge of the frame. Two different dimensions between the phototransistors must be used for vertical and horizontal shutters... 24mm for vertical and 36mm for horizontal. I actually mounted mine about a half mm less than each of these dimensions.

Use it exactly the same way as described in that article EXCEPT use this one in a dark room without the Adjustable Light Source. If you see two peaks, in CoolEdit, exactly above each other on both the top and bottom graphs the flash is firing when the shutter is fully open... as it should.

If you only see one peak, either the tester isn’t aligned properly or one of the curtains is covering a phototransitor... meaning sync is off.