Re: 6x6 and lenses shorter than 80 mms

[P3D john bercovitz <bercov@xxxxxxxxxxx>]

Dale Walsh writes:
> what is the relationship of our eyes to focal length?  To me, I would
> understand our eyes to have a variable focal length.  Is this true?
 
Greg answered wonderfully so now you know that if you view from too far
away, the reconstructed image will appear to have the absolute height and 
width of the original object (albeit located farther away) but will have
more depth than the original in proportion to how much too far away you are.
 
Greg mentioned figuring the exact required focal length and said to the
first order your required focal length is 65 mm.  That is correct also.
However, if you take a closeup, to the first order what you want is the 
operating focal length of the lens.  For instance, I sometimes put my 
35 mm focal length lens (set at infinity) on a 15 mm extension tube to 
do a closeup.  So its operating focal length is 35 + 15 = 50 and I need 
to view the resultant images with a 50 mm lens.
 
As to the focal length of your eye.  If the lens in your eye did move to 
accommodate, the movement would be so short that no recalculating would be 
necessary.  As it is, the lens changes shape and even less refiguring is 
required.  So the answer is: ignore it. The important thing remains the 
distance of the eye from the photo.
 
John B


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