Re: Matching lenses - distortion

[John Bercovitz <bercov@xxxxxxxxxxx>]

[....]
>This chart is 30X scale.  It will appear on film as 23 x 34 mm.
>Newtonian magnification equation: m = f/x or: x = f/m
>where x is distance from target to front focus of lens and f is 
>the actual focal length of the lens.  For m = 1/30,  x = 30 f
[....]
>PS: You could also do a focal length measurement by 
>putting the chart at the nominal distance from the
>front focus of the lens and then measuring what
>actually appears on film.

I hadn't thought of that PS before in the context of my chart so 
I've added this to the bottom of the CAD doc of my chart:

If the focal length of the lens is not the nominal value found on the 
lens barrel, the image size will be incorrect.  Dividing the actual 
image size by the object size will give the actual magnification 
and multiplying that value by x will give the actual focal length.

John B