While the F280 manual is generally useless, it does provide enough
information to definitively answer the questions on flash dumps and flash
duration.
The manual is online at
http://olympus.dementia.org/eSIF/omsif/flashphotogroup/manuals/f280.pdf, and
this information comes from page 51.
Let's first look at the number of flashes you can get from a single set of
AA alkaline batteries. The expected number of flashes are:
Manual: 80 firings
Normal: 80 -- 600 firings
SuperFP: 80 -- 260 firings
Since manual mode results in a constant number of firings, we can conclude
that all manual mode firings are the same. Since 80 firings is the lower
bound on the number of flashes for any mode, and since we'd expect the lower
bound on expected number of firings per set of batteries to be achieved by
dumping the entire capacitor on every firing, it looks like manual mode
dumps the entire capacitor on every firing. This agrees with what most
people seem to think is the case.
In Normal mode, the number of expected firings can vary from 80 to 600.
Since we already know that 80 is the number of firings you get when you dump
the entire capacitor on every firing, it looks like Normal mode is capable
of dumping the entire capacitor. Since the upper bound on Normal mode
firings is 600, it looks like there exists a "minimum dump amount" for
Normal mode, and if you always dump the minimum amount, you can squeeze 600
firings out of a single set of batteries. This agrees with what most people
seem to think is the case.
In SuperFP mode, we see that the number of expected firings is a range from
80 to 260. We already know that 80 is the number of flashes you get from a
set of batteries when you dump the entire capacitor on every firing. So,
SuperFP mode is capable of dumping the entire capacitor. But, it is possible
to get up to 260 firings in SuperFP mode from a single set of batteries, so
there must exist some SuperFP firings that do not dump the entire capacitor.
In fact, if you dump this minimum amount in SuperFP mode each time you get
about 260 firings.
(It's interesting to note that the minimum dump in superFP mode (the amount
that gives you 260 firings) is significantly more than the minimum dump in
Normal mode (which gives you 600 firings), and that both are significantly
less than a full dump (which gives only 80 firings.)
Since both Normal mode and SuperFP mode are capable of producing different
amount of flash output (as determined by the amount of the capacitor that
gets dumped), we would like to know what determines the percentage of the
capacitor that gets dumped, and how differing dump percentages affect the
behavior of the flash in Normal mode and SuperFP mode. From the information
contained on page 51 of the F280 manual, we can get some idea of the
answers, but not entirely.
First, I'll fill in the above chart with the reasons provided for each:
Manual: 80 firings
Normal: 80 -- 600 firings (depending on AUTO working distance)
SuperFP: 80 -- 260 firings (depending on shutter speed)
Now look at the chart on expected flash duration (with the OM-4t (ti)):
Normal "OTF" Flash emission: 1/40,000 -- 1/1000 sec.
Super FP Flash emission: 1/50 -- 1/25
So the flash DURATION in Normal mode actually is variable, from as short as
1/40,000 to as long as 1/1000. My guess is that longer durations takes more
power, so I'd expect that the longest possible duration of 1/1000 sec. is
what you get when you do the largest possible dump of the capacitor. Thus,
if you do a full dump of the capacitor on every firing in Normal mode, you'd
expect each of your firings to last 1/1000 sec. If you get 600 firings from
a set of batteries in Normal mode, you'd expect that each of the firings was
1/40,000 second long. Additionally, you would expect longer flash durations
when the subject is farther away (when the working distance is greater), and
shorter flash durations when the subject is closer.) One thing that is
unclear is whether or not the 1/40,000 sec flash and 1/1000 second flash are
equally bright. It could be that all Normal mode flashes are equally bright,
and the amount of light emitted is controlled by the duration of the
emission. Or, it could be that both the duration of emission and the
intensity of the flash are altered, and Olympus only provides information on
the duration of emission. (One reason to believe that the intensity of all
flashes are equal is the fact that color temperature is fixed at 5800 K. I
would guess that a flash of greater intensity would have a higher color
temperature, and a flash of lower intensity would have a lower color
temperature. Since color temperature is fixed, it looks like flash intensity
if fixed also).
Flash duration in SuperFP mode is also variable, from as slow as 1/50 sec.
to as long as 1/25 second, depending on the shutter speed. We know that with
shutter speeds that are faster than 1/60 sec. two curtains slide across the
film, one which is "opening" a window, and the second that is racing behind
it to "close" the window. At a shutter speed of 1/500 sec, each piece of
film is under the window for 1/500 sec, but the amount of time that passes
between the time when the first piece of film starts getting exposed to the
time when the last piece of film stops getting exposed is much longer than
1/500 sec. People seem to call this second time the "curtain speed" and it
was recently claimed that the curtain speed has an upper limit of 1/60
second. This is consistent with the fact that the SuperFP has a minimal
duration of 1/50 sec.
Putting 1/60 and 1/50 over a common denominator, we have 25/1500 and
35/1500. Thus, if the curtain speed is 1/60, the SuperFP flash will be
flashing at least 10/1500 (that is 1/150 sec) longer than the exposure. This
ensures that at every point during the exposure, the flash is firing. Thus,
every piece of film sees some of the flash.
However, the SuperFP flash duration could be as long as 1/25 sec. 1/25
equals 60/1500. So given a curtain speed of 1/60 (or 25/1500 sec) we could
have a SuperFP flash that is up to 60/1500 - 25/1500 = 30/1500 sec (or
3/150). Why would there be a SuperFP flash that lasts 3/150 sec longer than
it needs to? The fact that such a long flash duration is possible with the
F280 in SuperFP mode makes me think that the curtain speed might be slower
than 1/60 second for some shutter speeds. Why this could be the case, I have
no idea. It might be true that even if the curtain speed is never slower
than 1/60 sec, this super-long flash duration might still be needed, but I
can't think of why that could be.
<taking a deep breath>
I bought an F280 recently for fill-in flash on sunny days with hard shadows,
and for a little extra light indoors with slow film. One thing I'll be sure
of is to do a lot of testing before I start shooting with it for real!
Robert Swier
Rochester, NY
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