New article: Soft-Focus Lenses and Techniques

[Ernest Purdum]

Robert, the f number of a lens with only one cell can be found just by dividing the focal length by the diameter of the opening. (This doesn't work with combined lenses because it is not the actual aperture but the apparent aperture that counts..You have to measure what the diameter of the aperture appears to be from the front.)

I don't have a Verito to measure. Wollensak gives the lens diameter of the 14 1/2" size as 3 1/2". If the diaphragm opens that far, this would give an f number of 6.8 as the top aperture.

[robert_4927]

Ernest, that makes sense. But let me understand this.... with just the rear cell the normal f settings would be as follows? ....on the old studio shutter f4 becomes f6.8......f5.6 becomes f10.2......f8 - f13.6......f11- f20.4.....f16 - f27.2......f22 - f40.8.....Am I figuring this right?

[David A. Goldfarb]

Ernest (and others who know their optics better than I do offhand)--does that rule apply if the stop is way out in front of the element? On my 14.5" Verito, the stop is 3" in front of the rear element. I've never tried using the single element, because I don't have a camera with enough bellows (but I have a new 36" bellows on order for my 11x14" camera, so maybe in a couple of weeks...).

[Ernest Purdum]

Robert,

I knew I should have been paying more attention in math class. The series of f stops on your lens barrel is based on the square root of two, which is roughly 1.414. Now whatever starting point you measure, 1.414 X that figure will be the next stop down. Two times the starting figure will be two stops down, and so on. f6.8 may not be the right starting point, since the opening may be less than the full diameter of the lens, but if we open the diaphragm to a reading one stop down from wide open, (f5.6 in your case) the actual f number should be 1.414 times the wide open figure. The next marking (f8) will be twice the starting point, and so on.

David,

The distance from the lens to the stop won't make any difference in the f number. The stop placement is chosen to avoid coma and field curvature.