Re: [APML] Petzval hot spot?

[Chris1011@aol.com]

Speaking of hot spots in scopes, I have seen a marked deterioration of 
quality in binoculars of late.

Last weekend I was at Astrofest. Lots of vendors selling stuff. Since I was 
in the market for a good pair of binocs, I checked out just about everything 
that was available. Every one of them had the same problem - uneven 
illumination of the exit pupil. Yes, most of them had Bak4 prisms. That in 
itself does not produce even illumination. Some binocs had 60 and even 70mm 
apertures, but with internal field stops that limited even the on-axis 
apertures to between 40 to 50mm. Edge illumination on most of them was equal 
to about 30mm aperture. The reason for this is that the prisms are too small 
in virtually all binocs - even models costing $400 and up. The effect is the 
same as undersize diagonals on a Newtonian. Why do they put big lenses in 
front and small prisms inside? Because the general public is not aware of 
simple optical principles and it saves money, size and weight. 

I guess I will have to spring for costly Zeiss or similar binocs to get full 
illumination of the field (or maybe put up with some light falloff).

Roland Christen

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