[rollei_list] Re: Tell the tales of Triotars
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 5 May 2013 11:37:10 -0700
Hello !
Something puzzle me. I own an Agfa Isolette II which is
fitted with a 85 mm F:4.5 Apotar lens. It is a 3 lenses
lens design and use front focusing (i.e; the front element
unscrew for the focusing). This said, it is a very good
lens and has plenty of contrast and detail (to my
surprise).
As this lens dates as back as the early '60s I wonder what
is different from the Triotar lenses ?
Thank you to enlightening me ;-)
The focal length of both the Triplet and Tessar are
strongly influenced by element spacing, particularly the
front element, so its relatively easy to make a lens with
front element focusing. However, the aberrations are also
affected. Note that one of the popular designs for variable
soft focus lenses uses a Triplet with a moving element,
usually the center one. Evidently changing both air spaces
in different directions by moving the center element does
not affect focal length so much as spherical aberration.
Careful design of a variable focal length lens can
result in very good performance but it will not be quite
optimum at any distance. The Tessars used in some of he
Zeiss-Ikon cameras were front element focusing and are
reputed to be very sharp. This may be partly due to the
excellent rangefinder used in some of them.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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