To bolster my previous posting I offer a quote from "How to Select and Use
Olympus SLR Cameras" by Shipman:
Chapter 10- "How To Use Flash" section titled: "X-Sync Speed with TTL
Auto Flash":
" If the TTL auto mode is Programmed OTF, the camera sets both aperture
and shutter speed for the ambient light according to a program built
into the camera. If a flash is installed and turned on, it will fire or
not fire depending on the brightness of the ambient light and the resulting
camera-selected shutter speed.
" If the TTL auto mode is Aperture-priority OTF, you have some control
over shutter speed because you set aperture size manually. Suppose you
want to use flash and set aperture so the camera indicates that it will
use the x-sync speed exactly, such as 1/60 second. If the light changes just
as you press the shutter button, shutter speed may become faster than
x-sync and the flash won't fire. To be sure that the flash will fire, set
aperture so the camera chooses a shutter speed that is slower than x-sync.
Suppose the indicated speed is 1/30 second. [italics] This will not cause
a longer exposure time than using the x- sync speed. [end italics] The
reason is that, with OTF control, the camera turns off the flash and closes
the shutter as soon as it measures sufficient exposure of the film- no
matter what shutter speed was indicated before the exposure. "
So, you'd set x-sync or slower to cover any changing light during
exposure that will bump the necessary shutter speed higher than x-sync, not
because your fingers may "bump" the shutter ring.
Regards,
George S.
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