usher99@xxxxxxx wrote:
> Thanks again for the clarification. I checked the archives
and use of the T20 in non TTL auto seems to be a source of
confusion. You know your flash units---I do have the Vivitar 283.
I am still puzzled how the T20 can modulate its output without
help from the OM-2. I have been unaware of any thyristor circuit
in it.
The fact that you are unaware of it doesn't mean that the T-20 doesn't
have a thyristor circuit. Believe me, it does. That's how it gets its
non-TTL auto modes.
Thus how do you set the IS0 setting on the flash? I
believe the calculator dial just gives one the 2 available
f-stops to set the camera.
Sorry, I've probably confused you on this point. You set the ISO on the
calculator dial but it's intimately tied to the aperture setting as
well. They're just different aspects of the overall exposure control.
I used my usual strategy given to me
long ago in a sign in organic chem lab "If all else fails, read
the directions." This did not work in this instance.
See pages 12 and 13 of the T-20 user's guide. This tells you how to do
normal auto mode. However, the confusion point may be that what it's
telling you about is how to use the auto mode to have the flash be the
*main* light.
In fill flash the sun is going to be the main light. The fill light
needs to be 1-2 stops less bright. Therefore you need to lie to the
flash about the aperture or ISO settings on the camera. You want the
flash to put out 1-2 stops less light. For example, if you were
shooting ISO 100 film you could set the calculator dial for ISO 400 and
then choose one of the two apertures available as indicated on the
calculator dial.
But now the real world of fill flash intervenes. You can't really just
choose an aperture from the dial as you would if the flash was the main
light since your main light exposure is already determined by the
amibient sun light. In order to sync with the shutter the shutter speed
is already determined at 1/60th (could be slower but not likely). Since
you can't change the shutter speed your only way to control the main
light exposure is via the aperture... and it is what it is.
So, when doing fill flash, you have to work backwards. Determine the
main light exposure and then try to figure out how to control the flash
to produce 1-2 stops less than the ambient. Neutral density filters may
have to be used.
When you've got a camera that syncs at 1/500th life is much easier.
Then you can simply behave as though there are really two separate
exposures going on. The ambient light exposure can be controlled by the
shutter speed and the flash exposure by varying the aperture. They're
not independent, of course, but, if you want to vary the aperture to
accommodate the flash, as long as you have flexibility in the flash sync
speed you're free to change the shutter speed to adapt to the new
aperture setting without influencing the flash exposure. No matter what
the shutter speed is the flash exposure will only ever be controlled by
the aperture. The flash is always far faster than the fastest shutter
speed and will never be influenced by it.
Chuck Norcutt
> Mike
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