P3D Re: Normal lenses and other myths...

[bercov@xxxxxxxxxxx (John Bercovitz)]

>> For these thoughts and observations I conclude that the definition of a
>> normal lens as matching the diagonal of the image is an arbitrary but
>> practical one since we normally view images from distances close to the
>> diagonal, which is of course related to our field of view...

> Hmmm, I am a bit worried... I have not heard anyone comment on these
> speculations so I really don't know if they have any truth in them...

I don't think we view from distances approximating the diagonal.  I can't 
see a 4"x6" (10x15 cm) print from a distance of 7.2" (18 cm), for instance.
People tend to put a print at a comfortable viewing distance such as 16" 
(41 cm) if the print is hand-held or 32" (81 cm) if it's hanging on the 
wall.  If you know the print size and the method of viewing ahead of time, 
you can maintain correct perspective by choice of camera lens.

> I've heard the following method for finding the "normal" FL in an SLR:  Use
> a 35-75 mm zoom, or something similar, and start zooming while looking
> through the viewfinder with one eye while having the other eye open.  When
> the two images match in size, this is the normal FL.  It seems to me that
> this happens when the FL matches some internal distance (from the eyepiece
> to the screen where the image is projected?) built (arbitrarily) into the
> camera.  Is that correct?

If the view through the viewfinder matches the nekkid eyeball's view, then
the focal length of the camera lens matches the focal length of the lens
which is viewing the ground glass in your SLR.  Usually that's around 50 mm.
I don't think you should infer too much additional.

You could say the diagonal as a measure of normal is arbitrary but I think
it represents an optically easy focal length to make by modern design methods 
and in that sense it is normal.

John B


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