Lenses covering 20 x 24

[cosmicexplosion]

I just learnt that a dagor goerz 19" converts to a 33"!!

[Tracy Storer]

Not yet, but have had the same thought. Planning on looking into this when things settle down for me in a couple months.

Just out of curiousity, has anyone ever tried popping a good negative diopter on a lens, thereby increasing its focal length and coverage?

[Hermes07]

Just out of curiousity, has anyone ever tried popping a good negative diopter on a lens, thereby increasing its focal length and coverage?

a GOOD negative diopter would have to be achromatic to avoid introducing a lot of fringing and aberrations. In my experience, it's very hard to find decent, coated, negative achromats in large diameters. Anyone have a source?

[E. von Hoegh]

a GOOD negative diopter would have to be achromatic to avoid introducing a lot of fringing and aberrations. In my experience, it's very hard to find decent, coated, negative achromats in large diameters. Anyone have a source?

I've seen them at SurplusShed, but you never know what they will have. Certainly not a reliable source. A couple 60" telescope achromats mounted in a barrel would cover 20x24, and good RR type lenses are surprising. Certainly more than adequate for contact prints.

[Hermes07]

I've been on the lookout at SS for a long time but have never found a decent-sized negative achromat suitably weak to extend the focal length of a lens in the 600mm range.

Given the cost/time/effort of getting the two achromats, mounting them, adding the aperture, e.t.c. It would probably make more sense to just wait for a beat-up or less-hyped aerial or process lens to come along. I've seen telephotos and APO dialytes & tessars around 35" go for very little - whatever their flaws, I'd expect them to hold up lot better than my diy attempts.

[E. von Hoegh]

I've been on the lookout at SS for a long time but have never found a decent-sized negative achromat suitably weak to extend the focal length of a lens in the 600mm range.

Given the cost/time/effort of getting the two achromats, mounting them, adding the aperture, e.t.c. It would probably make more sense to just wait for a beat-up or less-hyped aerial or process lens to come along. I've seen telephotos and APO dialytes & tessars around 35" go for very little - whatever their flaws, I'd expect them to hold up lot better than my diy attempts.

You're correct about the long process lenses. However, inveterate DIY-er that I am, there will at some point be a brass barrel home made RR on the front of my Deardorff V8. The effort of assembling it will just add to the value, for me.

[desertrat]

I've seen them at SurplusShed, but you never know what they will have. Certainly not a reliable source. A couple 60" telescope achromats mounted in a barrel would cover 20x24, and good RR type lenses are surprising. Certainly more than adequate for contact prints.

Just a thought, but the achromats in RR lenses had a meniscus shape, and that was required to get a flat field. Telescope achromats might not be meniscus enough to give a flat field when assembled as a RR. A couple of years ago while mending an injured knee, I spent some time with the free version of OSLO, a ray tracing program, looking at the formulas of some classic lenses I found online and in Kingslakes book, Lens Design Fundamentals. I think ordinary telescope achromats would give a pretty curved field. Of course, at f64 it might not be too bad.

[sanking]

I just learnt that a dagor goerz 19" converts to a 33"!!

True, but the field of view is much narrower so in the end the combined elements and single element cover about the same.

Also, the single element is not well corrected as there is a lot of chromatic aberrations. You can improve quality by the use of a green or orange filter.

Sandy King

[Old-N-Feeble]

Sandy... if one filters out the longer or shorter rays of light... wouldn't one need to know whether to tweak the focus fore or aft the unfiltered spectrum?

[Hermes07]

Just a thought, but the achromats in RR lenses had a meniscus shape, and that was required to get a flat field. Telescope achromats might not be meniscus enough to give a flat field when assembled as a RR. A couple of years ago while mending an injured knee, I spent some time with the free version of OSLO, a ray tracing program, looking at the formulas of some classic lenses I found online and in Kingslakes book, Lens Design Fundamentals. I think ordinary telescope achromats would give a pretty curved field. Of course, at f64 it might not be too bad.

I was going to mention this about Aplanats / RRs. Not only is the meniscus shape of the groups important, but I believe it was originally a design where both glasses are flints, with the negative components on the outside of each group and the positive components on the inside. All this is pretty much the opposite of your standard telescope achromat - they tend to be plano-convex (or very close to it) crown-flint designs with the positive component on the outside. The standard Fraunhoffer doublet using N-BK7 and F2 doesn't appear to meet the prescription for a very low difference in the dispersive properties of the two glasses.

You could still get the benefits of symmetry (zero distortion at 1:1, e.t.c.) with a DIY version but I doubt you would be able to get near the performance of a purpose-built Rapid Rectilinear at large apertures.