On 2015-07-17 14:20, Fred McKenzie wrote:
In article ,
Alan Browne wrote:

I have such indicators on my Sony a900 (dot on when the focus point is
in focus - no indicator of which way however) - but I'm not sure if they
will work with a cat lens.

Alan-

I think it depends on f/stop. I have the Minolta 500mm f/8 AF mirror
lens, which auto-focuses on the Alpha 100 and 900. I expect the focus
indicator would work with a 500mm f/8 manual focus lens.

To be clear I'm referring to the off centre focus points, not the
central point.

blush I'd forgotten that the Minolta cat was AF.



I'm not sure what determines the critical f/stop. I recall that the
split image rangefinder on my old film cameras, could not be used with a
Celestron C-90 mirror telescope (1000mm f/11 as a lens). One half of
the split image would always be dark. I used the ground glass to focus,
but it was extremely critical.

I'm pretty sure contrast (ambient light) is the deciding factor. So an
f/11 lens will have a 1 stop disadvantage. That's the difference
between a sunny-16 day and moderate overcast.