This must also be the instant that flash must fire. This simultaneous sequence is what we called "synchronization."

There are a number of modes in synchronization; each one is designed to match the requirement of the user. Some of them maybe considered obsolete but will be presented here for better understanding on this topic.

Older mechanical cameras have different terminals for each supported mode, while others use selector switch to be set by the user. However, this approach leads to countless spoiled shots due to wrong choice or simply forgetful users. Thankfully, this problem has been addressed by today’s digital cameras.

"X" is the maximum shutter speed that cameras with focal plane shutters can synchronize with electronic flash.

At any speed lower than X, there is no problem with synchronization. At shutter speeds above X (say 500th/second), the whole burst of light from the flash can not reach the film or image sensor. Only a part of the frame (image) will register. Shots are diffinitely ruined and photographers call it "half frame." The missing part did not recieve flash illumenation because the light had been blocked by the shutter curtain. (See box. Two-Curtain Shutters) This situation is a result of wrong shutter speed setting. To synchronize, simply use appropriate shutter speed that should not be higher than X.

Focal Plane shutters are composed of two curtains: a front and a rear curtain. The front curtain slides open to begin the exposure, and then the rear curtain slides closed in the same direction to end the exposure.

At slower than X sync shutter speeds, the first curtain opens from right to left, and stays open during exposure according to the shutter speed setting. At the same time, it closes the flash circuit contacts which fire the flash. Then the second curtain follows the first curtain, thus closing the entire frame. This sequence is repeated when the shutter is cocked again and the curtain moves back to the starting position, ready for the next exposure.

At faster than the X sync speed, the second curtain begins closing even before the first curtain has reached the left edge of the frame. It has the effect of a slit opening between two curtains that travel across the image sensor plane during exposure. In this order, the image area is not exposed simultaneosly. The X synchronization at high shutter speed is therefore not possible.

M Sync

This mode is used to synchonize flashbulbs only. Considering that flashbulbs are no longer used today (except on specialized applications), this mode is already history. However, there is an interesting attribute of this mode – it fires the flashbulb earlier than the actual shutter opening. This is due to flashbulb’s characteristics that take some time to reach the maximum light output.

Focal plane high-speed sync, or FP-sync is a variation of M sync. It was originally designed to fire slow-burning flashbulbs that maintained nominally constant light output over the time it took the shutters traveling slit to complete journey across the entire image frame. But again, that was history. However, high-end electronic flash today is capable of emulating same effect, finding its use in high-speed photography. Such scope of discussion shall be covered in our future topics on Advance Flash Photography Applications.