Re: How Dr. T does it!

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

Dr. T writes:
> Ed Romney told us how to check focus on a camera by painting 
> a white line and looking at the slide with a 10x magnifier.
> Here is how Dr. T does it:
> He has a very powerful 30x achromatic focusing magnifier,
> made to check camera focus.  
[...]
> Place ground glass (also obtained from
> Shilo, height of 35 mm film) over film gate, ground side facing > lenses. 

If you're tempted to view the aerial image instead of the ground 
glass image, I'm pretty sure the f/number of the magnifier has to 
be as small as the f/number of the lens as it is being tested.  
Think of it this way, if the camera lens is set at 2.8 and the 
magnifier is only an f/5.6, won't some of the rays that would form 
an image on film miss the magnifier?  I also wonder what this 
means in the case of the eye and its entrance pupil used with an 
ordinary single-lens magnfier to view an aerial image.  

The reason you'd want to view an aerial image instead of the 
ground glass is that the resolution isn't compromised by the 
glass' surface.  It's easy enough to tell where an aerial image is 
relative to the film plane; just string a fine wire across at the 
film plane and rock back and forth with your magnifier to use 
parallax to tell you if the aerial image is at the same place as 
the fine wire.

John B


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