Perhaps it is easier to use the formula

u/f = 1 + M

where u is the distance to the subject and M is the ratio of the subject size to the image size. You measure a linear dimension in the subject and the corresponding dimension in the image on the ground glass. M is the ratio of the former to the latter. You add 1 to M. You then measure the distance to the subject. In principle, this should be measured from the front prinicpal plane, but if you measure it from the front of the lensboard or even the front of the lens, it probably won't make a lot of difference in the calculation, You know u/f = 1 + M because you know 1 + M. You take its recipprocal f/u and multiply it by your measured distance u. That gives you f in the units you used for u. If, as you say, u is 10 meters, you will get the focal length in meters. Multiply by 1,000 to get the focal length in mm.

This method will only be an approximation because of possible errors in measuring the subject size, the image size, and the distance u to the subject. If you do the calculation for different subject distances and take the average of the results, you will get a better aproximation. Finally, it might make sense to round it up or down to the nearest probable lens focal length. For example, if you measurments give you 104 mm, it is more likley the focal length is 105 mm.

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