[rollei_list] Re: 500/5.6 Tele Tessar for SL66 Comments
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sat, 6 Jan 2007 08:39:24 -0800
----- Original Message -----
From: "Ellestad" <ellestad@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, January 05, 2007 11:44 PM
Subject: [rollei_list] Re: 500/5.6 Tele Tessar for SL66
Comments
Richard -
Is true APO performance focus distance specific?
Tim
It probably is to some extent but this is getting to the
limit of my knowledge. Since coma and some other aberrations
are distance dependant its likely that something like
spherochromatism is also. Note that lenses with fixed
position elements can be corrected for all aberrations at
one distance only. Some lenses are more sensitive to change
in distance than others. For instance, the very slow four
element air spaced lenses used for process work (like the
Apochromatic Artar) are relatively insensitive to change in
distance. So the Artar will work well at infinity focus if
stopped down a little despite being optimized for equal
image and object distances (unit magnification). Probably
the important factor is the angle of the light rays inside
the lens. The greater they are the greater the deviation
with distance. So, fast lenses and wide angle lenses are the
most sensitive. For "normal" photographic lenses, which are
corrected for infinity focus or near it, the rule of thumb
is that corrections will stay pretty good down to about 10
focal lengths from the lens. I don't know if this applies to
telephoto and retrofocus lenses. \
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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