


Measuring the lens to paper distance is a trick, if you use a regular tape measure you have to convert fractions to decimal. I use a metric tape measure, no conversions. Works like a charm. When using 35mm, I have never wanted to make a print smaller than the negative, not even with 21/4 either. So if you into roll film, the simpler lens to paper distance is easier and faster, just use a metric measure.

The problem is not metric or fractional etc... To accurately measure the distance you must have a squrare or something on the baseboard to make sure you're not measuring at an angle. You also need to know the nodal point of the lens.

If the column distance gives you the distance between the negative and the paper, then you have the have the quantity d_o + d_i, when d_o is the object distance (negative to lens) and d_i is the image distance (lens to paper). But perhaps the column distance measures some arbitary point on the head. (This also neglects the separation of the principal planes of the lens, but that shoud only be a couple of mm.)
The focusing equation is 1/d_o + 1/d_i = 1/f, where f is the focal length. Using the equations for magnifcation (e.g., d_i = f(m+1)), and some algebra, one can obtain the equation:
d_o + d_i =f/m (m+1)^2 = f (m + 2 + 1/m)
So from the column height, hopefully a measurement of d_o + d_i, you could solve the above equation for m for both print sizes, then use the (m+1)^2 equation to calculate a new exposure time. Obviously the above equation is not so easy to solve.
Possible approaches: 1) measure the lens position and not bother with the above equation, 2) program a scientific calculator or computer to solve the equation, 3) measure the film and prints to obtain m, 4) use a light meter, or 5) test strips.
To measure the lens distance accurately, you could just mark the optical center line on the baseboard and extend a tape measure from the lens to that point. I would't worry about the nodal points of the lens  measuring to the aperture or middle of the lens should be sufficiently accurate.

My testing has shown if you measure from the same reference points, it doesn't make that much of a difference. If you use the markings on the column, and you change the easel, and its thicker, or thiner, you have a problem. If you just consistently measure from the paper position to the same spot on the lens I find the exposure is close enough for me.

Originally Posted by Max Power
There's actually a PDF floating around in cyberspace which is dead simple to use. I will try to find it and post the link back here.
MTF
Kent
Is it this one? > http://www.bonavolta.ch/hobby/files/ratiomod.pdf
“Do your work, then step back. The only path to serenity.”  Lao Tzu

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Chan Tran
I thought we went through this already. The inversesquare law is based on the lenstopaper distance, not the magification.

Originally Posted by MichaelBriggs
It does. Your equation works with the magnification factor, consequently, you can switch the focal length. It won't matter.

Originally Posted by RalphLambrecht
The inversesquare law is based on the
lenstopaper distance...
Did you forget the lens to negative distance? Any
computation which does not factor in the change in
lens to negative distance will not be correct. Have
any of those interested in this subject found or
derived A formula integrating the two? Dan

You don't need the lenstonegative distance to calculate the change in exposure. Using the lenstopaper distance alone will yield accurate results.

Originally Posted by RalphLambrecht
You don't need the lenstonegative distance to
calculate the change in exposure.
You can not ignore the change in lens to negative
distance when it is that distance which determines
the speed of the lens. You do recall the formula by
which the speed of a lens is determined? Dan

