O'Reilly logo

Canon EOS 40D Guide to Digital Photography by David D. Busch

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

Exception to the Sync Rule

Triggering the electronic flash only when the shutter is completely open makes a lot of sense if you think about what’s going on. To obtain shutter speeds faster than 1/250th second, the 40D exposes only part of the sensor at one time, by starting the second curtain on its journey before the first curtain has completely opened. That effectively provides a briefer exposure as a slit of passes over the surface of the sensor. If the flash were to fire during the time when the first and second curtains partially obscured the sensor, only the slit that was actually open would be exposed.

However, the 40D and certain Canon flashes provide a partial solution, called high-speed sync or FP sync (focal plane sync). Those flash ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required