Bounce flash Canon A85 - Vivitar 2800 question

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I need to understand the blue and red auto range on a Vivitar 2800.
The flash is used as a bounce flash with a Canon A85 by mean of the Sunpak
digital camera flash adaptor DFG5.

I want to use ISO 100 for decent digital noise but the flash red auto
range call for f2 when the A85 only open to f2.8.

Can I just use f2.8 and mentally adjust the red auto range to max at 28
feet instead of 40 as pictured at the back of the flash. What about the
min range?

The red blue mode setting switch does not seem to do any thing else beside
controling the amount of reflected light reaching the sensor by mean of a
set of holes of different diameter. This is weird as the red range (the
broadest 4-40') is the one with the large hole. If the flash max power is
the same for both auto ranges then the only limiting variable is the flash
duration 1/2000th-1/30.000th sec.

Any one to explain how these auto range can be modified if the default f2
and f4 aperture are not the one you want to use?
--
Alain