Re: [APML] Petzval hot spot?

["Joseph O'Neil" <joneil@multiboard.com>]

>The FSQ is an excellent telescope with a *very* flat field, this "hotspot" 
>issue just keeps coming up because people are unfamiliar with the Petzval 
>design.  For 35mm or CCD work you never even see the problem becaue you 
>never use the edge of the field.  The famous old Kodak Aero-Ektar lenses 
>have a similar issue. ( Roland -- do you know if they used a Petzval 
>configuration? )  They solved the problem by placing a radial gradient 
>filter on the front of the objective.


         In large format photography there are still many  lenses around 
(usually older models)  that use centre spot filters for exactly the same 
reason.  I don't know about the Aero Ektar (even though I have one), but I 
know that the other lenses were mostly  not Petzval, but exhibited the same 
problem.
         Still a well made lens using the Petzval design can prove to be a 
superb lens for LF and MF work for daytime use.  However, in this case, we 
get to "cheat"  by  using a sheet of film smaller than the image circle, 
and secondly, it always helps when you can stop it down to F22 or more.  :)

joe





http://www.oneilphoto.on.ca
http://www.multiboard.com/~joneil
"Una salus victus nullam sperare salutem"


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