Re: Matching lenses - distortion

[bercov@xxxxxxxxxxx (John Bercovitz)]

> How is a circle better than my 8 dot square idea?

I don't know that it is.  Can you refresh my faulty
memory on the 8 dot square idea?  Thanks.

> do you use a cropped circle for the largest one?

Yes, there is an arc in each of the four corners.
Let me see if I can find it.  Then I can describe
it better....   Found it.  OK the circles are 10,
20, 30, and 40 in diameter as they appear at a
35 mm gate.  There is a resolution block (as in
AF res chart) between each of the circles on a
sort of X pattern for a total of 13 blocks.  
Each block has 30, 36, 42, and 50 lppmm.  At the
top of the chart I have written reminders (my
memory again):

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

To find front focus of lens, run object light from 
infinity backwards through the center of the lens and 
note where it comes to a focus.  This point is called 
the front focus.

And at the bottom it says:

You may make a rough check of distortion by measuring the middle 
circle's diameter, multiplying by four, and comparing to the outer 
circle's diameter.

The chart is roughly 700 x 1000 mm in size as plotted.

John B

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.