"It is mentioned in the Olympus OM System brochure. See the following link for
a scan of the F280 data.
http://www.glink.net.hk/~photogrp/F280.jpg
The F280 is intelligent in Super FP mode, the flash duration varies, you can
even get higher GN with OM707."
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.
So the question of _why_ there is a difference in duration remains.
"It would seem logical that since the F280 acts like a constant light source
added to the picture its light would be added into the automatic shutter
calculation. That would certainly increase the number of successful F280
exposures. Otherwise Olympus would have to use the extra contact on the flash
to cause the automatic setting of the shutter to ignore light from the flash
and how would they do that? Easier just to let the camera operate normally
integrating all the light in the scene into the final shutter calculation."
This is almost certainly correct. However, there's a subtle point to this issue
that's easily overlooked.
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.
To put it another way -- if there were a very bright light (or unduly dark
area) at the extreme left side of the scene you're photographing, you'd wind up
with an underexposed (overexposed) picture -- "center-weighted" readings
notwithstanding!
I appreciate Mr. Swier's intelligent remarks. Perhaps we will gradually be able
to fill in the gaps about how the F280 "really" works.
"William said: "multi-spot metering -- both are terrific ideas, but they don't
_quite_ work they way you think they "should"... Have I missed something? Why
doesn't multi-spot work the way I think it should?!"
That _was_ a rather presumptuous statement -- I can't read other people's
minds. What I should have said is that it doesn't work the way Olympus would
like you to think it works.
The OM manuals give the impression that there's something "magical" about
multi-spot metering -- all you have to do is spot-meter every important area in
the picture, then (if you like) take a few extra readings of those areas you
want to have good detail in (highlight and/or shadow), and the camera will
miraculously give you an aesthetically pleasing exposure.
This cannot always be true. If the highlight and shadow areas span a brightness
range that's wider than what the film can record, there's no _single_ exposure
that can capture detail in both areas. (Note that some digital cameras provide
for two exposures to capture highlight and shadow detail, which can then be
melded with the appropriate software.)
Nor is the effect of multiple readings of the same area obvious. Suppose you
take a reading of the brightest highlight, then take one of the darkest shadow
area, then finish with a second reading of that shadow. The darker the shadow
area relative to the highlight, the more the second reading of it will "bump
up" the overall exposure. This means that the exact aesthetic effect of
multl-spot reading is not easy to (pre)visualize.
As I see it, the major advantage of multl-spot reading is that it lets you
easily analyze the image, so you can intelligently choose the appropriate
exposure. For example, you can read the highlights and shadows, then see where
the mid tones fall with respect to them. No other camera offers anything even
remotely like this. It's certainly a lot easier than fiddling with a hand-held
spot meter!
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