At 17:12 1/2/01, John Hudson wrote:
The shutter speed goes according to the ambient light but the aperture
setting is based on the guide number for the film speed used divided by
the flash to subject distance [or by what the flash's info dial says
based on whether you're using full power, half power, quarter power or
whatever].
jh
One additional (perhaps obvious) item: For everything but an F280 on a
3ti/4T, the shutter speed needs to be 1/60th or _slower_ for any of this to
work properly.
If the body is in Manual mode shutter speeds faster than 1/60th will result
in a portion of the frame correctly exposed and the remainder under exposed
(perhaps severely so) because it cannot X-Sync properly at shutter speeds
faster than 1/60th.
If the body is in Auto mode, the flash won't fire if a shutter speed faster
than 1/60th would be used for a correct exposure without flash . . . even
if the T-series flash is turned on. If it shows 1/60th (without flash) in
the viewfinder, the flash may or may not fire. It's a coin-toss depending
on what exactly the TTL metering determines is needed for enough
light. The viewfinder display doesn't have quite enough resolution to tell
you which way it will fall, and it's not what the viewfinder metering says,
it's what the TTL OTF metering measures while the shutter is open! (This
can occasionally result in the shutter firing a stop different from what
the viewfinder showed.)
I've found that driving an ambient exposure shutter speed (without flash)
down to 1/30th, or just above it (but not quite 1/60th) by stopping down
the lens will force the flash to fire. To some extent you _can_ influence
how much fill versus ambient you get by driving the shutter speed
indication around with the aperture setting.
This is a bit crude, and you lose some depth of field control to force a
shutter speed. If a proper exposure could be had at 1/30th without flash,
and you turn it on to use a flash at 1/60th in TTL Auto, then half your
light _will_ be ambient and the other half from the flash. To get the
ratio tighter with less fill, you takes your chances with no fill occurring
the tighter you push it!
-- John
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