How Shutter and Flash Sync
The burst of light from a flash and the amount of time a shutter is open are both so fast that the two operations must take place with pre-
cise timing. They must be in synchronization, or sync. Contributing to the problem is the fact that on most focal plane cameras, a shutter
speed faster than 1/250 of a second means the shutter is never completely open at one time so that the entire frame can benefit from the
flash. As the speed of the shutter increases, synchronization becomes a greater problem that is countered by increasingly clever scene pro-
grams and more intelligent flash attachments.90 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
As part of the dance of flash
and exposure we just saw in
the previous chapters, one of
the other things
that occurs
when the pho-
tographer presses the shutter button is that the shutter
and the flash begin a nimble pas de deuxcalled
synchronization orsync. On most cameras that use focal plane shutters, or
leaf shutters, only shutter speeds as slow as 1/60 or 1/125 of a second allow the
entire image sensor to be exposed at the same time. These exposures use X-sync. The
flash waits until the shutter’s curtains have completely exposed the image sensor. Then,
in just 1/1,000 of a second, the flash releases its full fury before the shutter can start to
close. The top shutter speed for most cameras using X-sync is 1/500 of a second.Faster shutter speeds require the second
shutter leaf to start covering up the sensor
before the first leaf has finished exposing it
(see Chapter 6). The exposure is made
through a slot formed as the shutter’s two
curtains pass in front of the sensor. An X-
sync flash used with this type of shutter
operation, instead of producing a photo
that looks like this...2
1
...would turn out a picture more like this.