Re: My 7p Realist!!!

[P3D john bercovitz <bercov@xxxxxxxxxxx>]

>> Dennis wrote about choice of lens focal length on 7-perf stereo 
>> cameras: If you measure the diagonal of the 7-perf frame, it's 
>> exactly 37mm-thus it's the "normal" length. Like a 50 or 52mm 
>> lens on a 35mm SLR.
 
> The geometrically challenged Paul Talbot wrote: Oh no, geometry!  
> I haven't done that in over 20 years either!  But somehow these 
> numbers seem a little off.  7-perf has a 37mm diagonal, but 8-
> perf has a 50 to 52 mm diagonal?  Isn't 8-perf closer to 42mm on 
> the diagonal?  I had heard a long time ago that the so-called 
> "normal" 50mm lens on a 35mm format camera is actually slightly 
> telephoto.  Am I remembering something incorrectly?
 
Jomuttry!  Horrors!  Actually, it's even simpler than that.  SLR 
cameras have a mirror box between the lens and the film.  So a 
lens of ordinary design has to be fairly long in focal length so 
that the mirror won't crash into it when it flips up.  Hence the 
first 35 mm SLR lenses were around 55 mm.
 
There is a way around it and that is with a retrofocus lens 
design, which has its cardinal points shifted to the rear while 
the glass stays well out in front of the mirror.  A retrofocus is 
also called a "reversed telephoto" since a telephoto has its 
cardinal points shifted out in front of the glass to reduce 
overall length of lens.  (A telephoto is not, by definition, a 
long lens.  It is a lens with cardinal points shifted by the 
addition of a negative lens after the positive front lens.)
 
Retrofocus designs introduce aberrations so it was a while before 
the lens designers were able to tame them.  Now you can get pretty 
short rectilinear lenses with good resolution that are obviously 
retrofocus, since they fit on SLRs, and you can also get good 
telephotos.
 
BTW, I think Minolta made/makes a 45 mm lens if you want to be a 
purist about the definition of a normal lens being equal to the 
diagonal of the format.  I feel it's kind of an arbitrary 
definition too.  It is good in that it tells you what the coverage 
of the lens is (53 degrees) and that tells you something about the 
design of the lens.
 
John B
 


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