William Sommerwerck wrote:
>
> The guide number listing for Super FP mode shows that the square of the guide
> number divided by the shutter speed is a constant at all shutter speeds. This
> means that, even if the duration of the Super FP flash varies, it has _no_
> effect on the flash's output.
>
Right.
> So the question of _why_ there is a difference in duration remains.
>
To save battery power.
>
> Fill-in is almost always used for objects or people near the center of the
> frame. (That is, unless you're photographing a group hug, there are rarely
> things near the edges of the frame that intercept the fill light.) If auto
> operation gives a shutter speed of (say) 1/500, the second curtain will be
> released before the first curtain has travelled even 1/8 of the way across the
> frame! As this part of the picture is unlikely to contain anything the fill-in
> flash is illuminating, the light from the flash will have _no_ effect on the
> exposure.
>
> If you don't mind my getting a bit off-topic, the same issue could be raised
> about non-flash OTF exposures. At auto shutter speeds above 1/60, the exposure
> is actually determined by the light striking the right side of the film plane
> (as seen from the back of the camera)! At 1/1000, less than 3mm's width of the
> film-plane area determines the exposure.
>
No, with speed higher than 1/60s it is off the curtain not off the
film. Exposure are determined at the start of exposure by light
reflecting off the curtain not off the film.
C.H.Ling
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