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Thread: Inducing spherical aberration in an aplanat/rr?

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    Mark Sawyer's Avatar
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    Inducing spherical aberration in an aplanat/rr?

    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?
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

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    Re: Inducing spherical aberration in an aplanat/rr?

    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.

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    Mark Sawyer's Avatar
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    Re: Inducing spherical aberration in an aplanat/rr?

    Quote Originally Posted by goamules View Post
    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...
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

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