At 22:48 9/8/02, Pete wrote:
I'm confused again. If Super FP mode allows full synchronization at all
speeds, then the flash duration should be dependent upon the shutter speed
and not be a constant of "about 1/25 sec", no? I seem to remember a post
(by John L.?) explaining that Super FP mode operates as follows: flash
starts, shutter opens, shutter closes, flash stops.
Yes, that's the sequence.
If true, and I believe it is, then the flash duration will be the
shutter speed plus the time before and after the shutter operates. And,
the longer the shutter opening, the less flash power that will be
available, according to the charts.
No. With focal plane shutters, total time it takes for a curtain to travel
across the film gate remains constant, regardless of shutter speed. You
have two curtains, an opening one (leading) and a closing one
(trailing). What varies with shutter speed is the delay between releasing
the opening one and releasing the closing one. In other words, it's the
*gap* between them that changes. Shutter speeds faster than the X-sync
speed have a gap that is narrower than the film gate. At those speeds, the
closing curtain is released before the opening one has completely traveled
and the result is a slit moving across the film gate. The faster the
shutter speed, the narrower the slit, but the slit itself *always* moves at
the same speed. Therefore, for an "FP" flash to work, it must always have
the same, long burn duration, regardless of shutter speed. And . . . the
narrower the slit traveling across the film gate, the less light from the
flash that's allowed to reach the film.
Because of the radically reduced flash power as shutter speed
decreases, it seems that Super FP mode is *only* useful as a fill
flash, and nothing more. No?
Super FP imposes noticeable limitations on working distance compared to
using the same film speed at X-sync with TTL Auto. The faster the shutter
speed, the greater the limitations.
-- John
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