Chuck/Dr. Flash,
Thanks for a lucid explanation about determining the flash's power. It was a
great help. I did a few calculations and discovered that with a guide number
of 80 at 50mm, F8 gives me a max distance of about 10 feet (probably less).
My subject was about 12 feet away, so that explains why earlier the camera
gave me flash confirmation at F5.6 (max range is about 14 feet) but not at
F8. Underexposure.
You were right on about the flash's auto settings' apertures being chosen
for a reason. I'm simplifying things a bit because I think there are some
variables I'm not taking into account, but my conclusion is that generally,
TTL mode on the 266D allows one to pick apertures between and including
those two aperture values marked on the two auto settings. I can only infer
this, because the Sunpak can't test in TTL mode.
To test that inference, I used 400 speed film, set the flash to TTL, and
took four shots at F16, F11, F5.6 and F4. Shots two and three represent the
two auto settings' apertures, or what I believe to be the smallest and
largest f-stops that can still give good exposure in TTL. Both those
apertures produced flash confirmation in TTL. Shot two, just barely - which
supports the notion that my flash, like others, is a bit weaker than spec.
Neither shot one or four produced a flickering LED confirmation in the
viewfinder, so shot one will likely be underexposed, and shot four
overexposed. I repeated this test at a slightly further distance with
accordingly different fstops, and got the same results. Now I know alot more
about my flash, including how to intentionally over- or under- power it with
some degree of control.
One other thing I noticed about the auto settings is that when bouncing, the
auto setting with the larger of the two apertures must be used to get an OK
signal from the flash. The smaller apertured auto setting and the large one
both give the OK lamp for head on flash, but only the larger aperture will
give the go-ahead when bounced. This seems to be the case no matter what
film speed the flash is set to. So, my rule of thumb for bouncing TTL will
probably be to use apertures within 1 stop of that larger auto aperture.
PB
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