Second Curtain Sync
- Second curtain sync is not available on the EOS 750 or 850 with any speedlite.
- 380EX second curtain sync is available only with the Elan II(e).
Canon SLR cameras have two shutter curtains, first and second.
At the shutter release signal, the first curtain opens; later, the second curtain
closes the shutter. The time between the first and second curtains depends on
the shutter speed or time value (Tv).
At very short time values, the second curtain will start to close the
shutter before the first curtain is completely open; there is effectively a "slit"
opening which travels across the film plane. A flash fired during this short time
value would result in only a portion of the film frame being exposed.
At longer time values, there will be a period of time after the first
curtain has completely opened and before the second curtain has begun to close.
A flash fired any time during this interval will expose the entire film frame.
Most modern SLRs use X-sync for triggering flashes. X-sync is the timing
of the signal to fire the flash just after the first shutter curtain is fully open.
At X-sync time value, the interval between fully open first and second
curtains is sufficient to fire a flash (about 1.5 milliseconds).
X-sync speed of Canon EOS cameras:
- 1/250 second: EOS 620, 1, 1N and 1NRS
- 1/200 second: EOS A2, A2E, IX
- 1/125 second: all other EOS cameras (except Rebels)
- 1/90 second: EOS Rebels (all variants)
Canon’s second curtain sync will fire the flash about 1.5 milliseconds before
the second curtain begins to close. At the X-sync time value, X-sync and
second curtain sync are effectively identical; at longer time values there is a
difference. If you photographed a moving automobile, at night with its lights
on and a time value of 1 second:
- with X-sync:
- The flash would fire at the beginning of the exposure.
- The flash would illuminate the auto and its frozen image would be
recorded on film.
- The film would record an image of the auto’s lights as they moved across
the film frame.
- The image of the car's lights would be recorded as streaks of light in front
of the car.
- The image on film would imply backward movement of the car!
- with second curtain sync:
- The film would record an image of the auto’s lights as they moved across
the film frame.
- The image of the car's lights would be recorded as streaks of light.
- The flash would fire at the end of the exposure.
- The flash would illuminate the auto and its frozen image would be
recorded on film, in front of the light streaks.
- The image on film would imply forward movement of the car!
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