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View Full Version : Inducing spherical aberration in an aplanat/rr?



Mark Sawyer
23-Mar-2009, 17:35
A friend and former student has a small old aplanat/rapid rectilinear (two cemented doublets) that's been in the family for several generations, and would like to use it as a soft focus lens on a 4x5. It's rather sharp, and she doesn't want to use a diffusing filter. We're looking for a way to induce actual spherical aberration into the design.

Ernest PurDum's excellent article on soft focus lenses on the lf homepage has the following passage:

"There are ways of upsetting carefully calculated spherical aberration reduction besides changing element spacing. A thick glass plate is one. Adding various combinations of weak positive and negative elements, Proxar and Distar and the like, might be another."

Any thoughts on placing a quarter- or half-inch piece of glass inside the lense barrel to create aberrations? Other thoughts on how we might get the desired look out of it without causing permanent damage?

goamules
23-Mar-2009, 18:15
I'm sure you've done this with your experiments Mark, but isn't one element of a RR softer? Also, I have a Single Achromat that I believe is close to half a RR. When I got it it was pretty soft and wild edges, which I thought strange for an f16 landscape lens. I disassembled and found it was in wrong, when I reversed it I got a flat field. You could try that too.

Mark Sawyer
23-Mar-2009, 18:30
I'm sure you've done this with your experiments Mark, but isn't one element of a RR softer? Also, I have a Single Achromat that I believe is close to half a RR. When I got it it was pretty soft and wild edges, which I thought strange for an f16 landscape lens. I disassembled and found it was in wrong, when I reversed it I got a flat field. You could try that too.

Yup, that was a thought, but a short lived one, I'm afraid. With about an 8.5" to 9" focal length, a single element would be too long for a 4x5, and rather dark as this is about an f/8 lens as it is. (She'd end up with about an 18" f/16 lens...)

It might be fun on an 8x10, but I don't think her Crown Graphic would handle it...

The front element has a slightly longer focal length than the rear, but both seem to throw a rather sharp image. We might try turning the elements convex-side towards the inside of the barrel, and see what that does. It might just give us a curved field like you got, though...