----- Original Message -----
From: "Tom Scales" <tscales@xxxxxxxxxxxxxx>
Sent: Saturday, July 20, 2002 5:57 PM
Subject: Re: [OM] Fill-in Flash with Olympus Flashguns
> I'm not sure your conclusions are correct.
I am not convinced that they are correct either, which is why I listed them
in my previous post! The problem is that I cannot see how the F280 can
possibly do accurate fill-in flash, as it does not have enough information.
Let's consider an example.
Suppose you have an OM4Ti set on "Auto", with an F280 connected, powered-up,
and set on "Super FP". You adjust the aperture so that the ambient light
gives a shutter speed of 1/125 or faster. You press the shutter button, the
mirror flips up, and the OM4Ti meters from the first shutter curtain. What
happens next? Well it seems to me that there are two possibilities:
(1) The OM4Ti ignores the fact that there is a powered-up F280 connected,
and sets the shutter speed appropriate to the ambient light -- let's say
1/1000. It then triggers the flash, and releases the first shutter curtain.
At 1/1000, the shutter is a narrow slit that traverses the frame (i.e. not
all of the frame is exposed simultaneously). Once the second shutter
curtain is released, the actual shutter speed used cannot be altered (if it
were, the frame would not be exposed evenly).
(2) The OM4Ti knows that there is a powered-up F280 connected, and so sets
a slightly faster shutter speed than the ambient light alone requires --
let's say 1/1200. It then fires the flash and releases the shutter.
The problem I have is that neither (1) nor (2) can work accurately, because
the OM4Ti does not know the distance to the subject. This is where an
autofocus lens that reports the focused distance has the advantage -- the
camera knows the distance to the subject, knows the amount of ambient light,
and can therefore calculate what flash power is needed to provide the
appropriate fill-in at the subject (say 1 to 2 stops less than ambient,
according to preference).
Note above that I said "flash power" and not "flash duration". With normal
TTL auto, the camera can vary the flash duration in order to give the
appropriate exposure. But it can do that only when the frame is uncovered
completely, i.e. shutter speeds of 1/60 or slower. In "Super FP" mode, the
camera cannot vary the duration of the flash; instead the camera must vary
the power (full, three-quarters, half, quarter, ....). Can the F280 choose
what flash power to use in "Super FP" mode (as distinct from flash
duration)? If it does, it is strange that it does not offer multiple manual
power settings (as the T32 does). Another possibility is that the F280
varies flash power DURING the exposure as the slit traverses the frame,
controlled by data from the OM4Ti's light sensors. However, I think that
this is unlikely because the OM4Ti would need light sensors that could read
from only the slit as it traversed the frame, whereas I think that they read
from the entire frame.
But the fundamental problem is that the OM4Ti does not know the distance to
the subject. If the OM4Ti implements algorithm (2) (giving slightly faster
shutter speeds with an F280), the difference in shutter speed can be only
approximate, derived from the study of thousands of fill-flash scenarios
that result in a rule such as "give a shutter speed that is 20 0.000000aster
than
ambient if a powered-up F280 in "SuperFP" mode is connected" (the 20 0s
just by way of example -- I am not claiming that 20 0s a sensible value!).
So my conclusion is that the F280 designers studied thousands of fill-in
flash scenarios, and decided on a flash power (guide number 28) that gives
the right amount of fill-in for the average case, and a result that is
better than no fill-in for cases that are not too different from the average
case.
But I am happy to be corrected!
-- from Cy in the UK
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