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Re: [Rollei] focus shift

- ----- Original Message ----- 
From: "J Patric Dahlén" <jenspatricdahlen  >
To: <rollei  
Sent: Thursday, January 01, 2004 11:43 PM
Subject: Re: [Rollei] focus shift

> I have noticed focus shift when using Proxar/Rolleinar
close-up lenses. I
> checked with the plate back and ground glass cassette. The
focus is accurate
> with the largest opening, but when closed down to 5,6 I
notice some focus
> shift and have to refocus to get maximum sharpness. This
is only a problem
> when you're using apertures larger than 16, because at
16-22 the depth of
> field is better.
> /Patric

  If the effect is from focus shift its not depth of field
but rather that there is less spherical aberration as you
stop down. I don't think I made this clear in my long post.
SA is proportional to the square of the aperture, so, when
you stop down it is reduced quickly. This is true for both
plain spherical and zonal spherical. If you use a small
enough stop even an uncorrected lens will give a fairly
sharp image. You can try this by making a stop for a hand
magnifier out of black paper with a hole in the middle. A
large magnifier will make a barely recognizable iamge, one
stopped down, say by a quarter inch hole, will make a pretty
decent image.
  The auxilliary lens upsets the corrections of the lens its
used on. Stopping down reduces spherical aberration and coma
and the increased depth of field helps mask any increase in
astigmatism or field curvature which may occur.

- ---
Richard Knoppow
Los Angeles, CA, USA