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Franken-lens question


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Obnoxious question, I'm sure.

What would happen if I were to take the front from a 28mm f2.8 Nikon lens (everything in front of the aperture) and put the back (everything behind the aperture) of an 18mm f3.5 Nikon lens behind it? Even though the rays from both converge in the middle, if I'm not mistaken, I doubt it would make a functional lens and strongly doubt the focus scale would line up at all, but would it even create an image? If so, what would that most closely resemble?

I've read the Tegea 9.8mm and 5.7mm share the same front half, but have a different rear half and some Angenieux lenses can be converted between FF and crop by swapping the rear group. So I'm just curious what this would do.

Also, if Panavision anamorphic lenses are based on Nikkor taking lenses, how do they have a t1.1 model when the fastest 50mm Nikkor is f1.2? Or how is there a 75mm model at all?

Edited by M Joel W
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Only some rays would pass through the lens at the aperture but you wouldn't have a real convergence. You're creating a lens that will not draw a sharp image at all since the 28mm nodal point is very different than the 18mm nodal point. It won't focus, even if you could swap the front groups. What you might try instead is get a Kodak Retina Reflex and swap lenses around on that. All of the optical groups between the leaf shutter and the film plane are the same and all the groups in front of the shutter are designed to work with them. The rear optical unit has its own focal length and nodal point. Designing the front optical units to work with them is how they did it. You could do it with the rear of the 18mm Nikkor, but you have to have a lens design in mind, have to know what kind of glasses you're using and where, have a raytrace program, and then you'll be able to see what your aberrations will look like. Try not to destroy those nice Nikkors. Get some bargain basement lenses or garbage lenses from a thrift shop, take them apart and see what they do. 

Phil Forrest

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Thanks, Phil. Are there any textbooks on optical design that are comprehensible to someone with a fairy limited understanding of physics? I want to be able to read a lens diagram and understand roughly what it's doing.

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16 hours ago, M Joel W said:

What would happen if I were to take the front from a 28mm f2.8 Nikon lens (everything in front of the aperture) and put the back (everything behind the aperture) of an 18mm f3.5 Nikon lens behind it? Even though the rays from both converge in the middle, if I'm not mistaken, I doubt it would make a functional lens and strongly doubt the focus scale would line up at all, but would it even create an image? If so, what would that most closely resemble?

Rays don't converge to an intermediate image anywhere in the lens, and in any system the aperture stop is a place where rays are not focused. So the front and rear parts of, say, a symmetrical Planar/Double-Gauss lens aren't independent in a sense an objective and an eyepiece of a telescope are. 

Two halves of different lenses will still focus light at some distance. Even in wide retrofocus lenses components before and after the stop are both low-power positive (converging). You can get a very soft image with severe aberrations if you're lucky; more likely the spherical will be so bad you'll hardly see an image.

Actually, some lenses do have independently corrected components that can be used separately to an extent. Some tele zooms consist of a quasisymmetrical "main lens" and a variable afocal converter (a variable-magnification galilean telescope in substance). A lot of wide-angles are basically a quasisymmetrical lens with an afocal converter. But the components aren't separated by the stop - it's always inside the "main lens". 

Edited by Michael Rodin
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