Just purchased a 15 in. B+L projection triplet, not a petzval design, and I'm wondering if any of the lens masters here know what the aperture of this lens is? I've seen some of Jim Galli's photos with the 18 in. version, does anyone else have samples? Thanks
Divide the focal length in inches by the diameter of the lens in inches?
Kevin's suggestion will be 'close' on a long f.l. lens. The curvature of positive lens elements alters the apparent (visual measurement).
What really determines it is the 'effective' f.l. Richard Knoppow (I usually find him on rec.photo.equipment.large-format) has a procedure at his fingertips to measure the e.f.l.
On a long f.l. lens, with a net low diopter equivalence (following the principle that 1000 mm = 1 diopter), the magnification is low so it doesn't 'magnify' the physical to visual appearance as much.
Symmetrical long lenses are probably the safest to measure with a ruler. Lenses with negative elements or odd nodal positions might do something to alter what you see as you attempt to measure it also.
A short f.l. lens, say 50 mm = 20 diopter makes a strong magnifier.
I know that a 6" Metrogon looks very different when you take a cell out of the Fairchild shutter body & see the aperture unmagnified.
Murray
You can make your own estimate of the actual aperture by placing a small flashligh around the focal plane i.e. bout 15" behind the centre of the lens. Illuminate the lens backward and place a piece of translucent paper in front. You'll see an illuminated circle, this is your actual aperture, it will probably not coincidate with the diameter of the front lens.
Measure the illuminated diameter, divide the focal length by this diameter, you get the numerical aperture you are looking for.
Standard 150mm projection triplet lenses I have at home for the 6x6 format have apertures around f/3 - f/3.5. I also have a Leitz 150mm-2.8 triplet .
Expect something which is in the range of 2.8 to 5.6 but probably not f/8.
I used to have one just like it. Mine was a 12" around f/5. Beautiful lens, very well constructed and the images were great. You have to bear in mind that it doesn't contain any achromats so focusing adjustments are necessary. However, the results can really be wonderful. I have an image or two done with one I could email to you if you'd like.
- Paul
Thank you all for the excellent replies!
Emmanuel, how far should the paper be from the front of the lens? Touching the barrel or at the effective focal length of the lens, ie 15 inches? I assume in contact with the lens.
Paul, I would love to see any samples you have. You have a pm.
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