Re: [Rollei] OT: retrofocus & inverse square

["Richard Knoppow" <dickburk >]

- ----- Original Message ----- 
From: "RUBEN BLÆDEL" <ruben  
To: <rollei  
Sent: Friday, January 09, 2004 2:34 PM
Subject: Re: [Rollei] OT: retrofocus & inverse square


> Richard
>
> You mentioned Goerz Hypergon so I looked it up - what
excactly did the
> "star" do
> The Goerz is mentioned as something very special for Large
Format -
> where they rare or common among prof. photographers
>
> Regards
> Ruben
>
    Its an exposure equalizer. It performed the same
function as a center filter, blocking light from the center
of the lens and increasing the input of light toward the
periphery. Its really not a stop but a mask. For some reason
no one thought of making a filter photographically at the
time so this ingenious mecanism was made. I've explained its
operation in a previous post.
   I've never seen any exploration of center filters on the
MTF of a lens. Since obcuring stops have a definite effect I
would expect a center filter to have at least some. An
obscuring stop for the center of a lens tends to lower the
mid frequencies of the MTF and raise the higher ones,
probably resulting in an effect on edge contrast. I've also
not seen any explanation of the effect of the tilting
entrance pupil on MTF variation with image angle. Presumably
it could increase resolution away from the axis beause the
stop is not vignetted in the same way as usual. However
there may be other effects.
- ---
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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