Re: Plus lenses effect on stereobase calcs.

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

I got to wondering if d = 15 mm was a good guess or not.
The reasons I was concerned are: 1) the 50 mm SLR lens is 
retrofocus so its front may actually be farther back than 
it appears to be; certainly its back is farther back than 
it appears to be.  2) The lens has a deep built-in lens 
shade so the plus lens can't get very close to the front 
of the lens.  I set up my lens bench to find where the 
front of the lens was.  It is indeed buried.  So a better 
representative figure for d is 45 mm.  
 
This is very interesting if we take a look at the lens
combination formula again:
 
f = (f1*f2)/(f1+f2-d)
 
where:
f = combination focal length
f1 = one lens' focal length
f2 = other lens' focal length
d = distance between them
 
If f1 is the focal length of the plus lens and f2 is the focal
length of the 50 mm lens, then if d = f2, f = f2!  So adding a
plus lens to an SLR lens may have virtually no effect on the 
focal length.  Instead, the addition of the lens moves the 
effective position of the back of the lens farther from the film, 
just as if you had focussed it farther out.
 
So what I said before about the focal length being not materially 
affected by the addition of a plus lens is more true than I had 
thought.  In addition, it turns out that the position of the front 
of the lens is less affected than I thought, also.  Numerically, 
we're talking changes on the order of 1 mm in either area. 
 
John B


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End of TECH3D Digest 52
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