Re: [APML] Petzval hot spot? (Somewhat OT)

[Mark Bohrer <lurchl@ix.netcom.com>]

As I said in an earlier post, the Zeiss 15mm Hologon for Zeiss and Leica 
M-series cameras was sold with a radial gradient filter to ameliorate the 
vignetting/corner-darkening problem. Extereme wide-angle camera lenses have 
often had this problem. Before Photoslop, the trick was to photograph 
scenes with discontinuous edge detail to make it less obvious, like this 
shot with a 12mm rectilinear lens: 
http://www.kokophoto.com/2001races/SS01wmnStrkSTXC.htm. (Main problem was 
avoiding tire tracks down my forehead at the close shooting distance.)

Covington talks about edge-of-field falloff on p.124 of "Astrophotography 
for the Amateur". In the case of Leica's 50mm F1 Noctilux lens, the mount 
intentionally vignettes the extreme corner light rays with the lens 
wide-open at F1. This helps to conceal some edge problems at F1. As the 
Noctilux is stopped down, this vignetting goes away, being almost entirely 
absent by F4. See Erwin Puts, "Leica Lens Compendium", 2001, if you're 
interested. The Noctilux is a good wide-field star lens, if a bit pricey: 
http://www.kokophoto.com/California/Orion_nite_sky.htm, a 7-second exposure 
at F1 on E100 film.


>>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

Mark Bohrer
www.kokophoto.com
Pro mountain bike racing on the web


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