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IanG
8-Aug-2016, 12:26
A question for Nodda Duma, Dan Fromm etc.

An observation which needs testing in practice. I've been making lens boards, looking at using more of my older lenses two in particular an early (pre 1912) 135mm f4.5 Tessar in a sunken mount, and a 5¾" TTH Cooke Luxor, an original Cooke Triplet. What's surprising me is the Cooke Triplet has significantly better contrast.

Both lenses are very clean and in excellent optical condition the Cooke triplet though is quite different to a later Triplets as the barrel length is longer and the spacing between cells larger. Am I right in assuming this is helpng the contrast, inverse square law affecting internal reflections.

Ian

jp
8-Aug-2016, 14:17
The triplet has one less piece of glass, so two less reflection possibilities. I'll leave the spacing question to the others.

Nodda Duma
8-Aug-2016, 15:03
Just to get the basics out of the way: contrast in the lens derives from how well stray light is controlled. That includes veiling glare from the quality and number of air-glass interfaces along the primary optical path, control of secondary paths from reflections and scatter in the lens from in-field sources, and control of stray light paths from out-of-field sources (the reason we use sunshades). All three contribute to reduced contrast in various amounts depending on the optical design as well as the opto-mechanics.

Inverse square law doesn't necessarily apply to stray light since secondary ray paths strike each point on the image plane across a broad range of angles.

Once you understand where contrast comes from, it gets a little clearer what makes for better contrast in a given lens. The number of air-glass interfaces can reduce contrast but not necessarily so. More interfaces increase the opportunity for the near-focus ghost reflections that cause veiling glare, but I can (and have) design a lens with more air-glass interfaces which would have better veiling glare characteristics even if the coatings are the same.

Clean optics, ghost focus on lens surfaces, surface roughness, edge blackening, barrel inner surface treatments, baffling, etc all contribute. Some may be more important than others for any given design.

That said I'm sure your barrel length and the number of lenses are making a difference.

Here's a professional tip: If they're not blackened already, edge blacken your lenses (the ground edges) with a Sharpie marker and you'll noticeably improve contrast. Sharpie is soluble in isopropyl and acetone so you can reverse this if you care about originality.

Dan Fromm
8-Aug-2016, 16:25
Ian, Jason mentioned baffling. Some of the old crocks we use have straight barrels painted black inside. This is second-best. I like TTH's approach to the problem as embodied in several of their lenses for aerial cameras. The rear tubes are threaded internally as for filters but all the way to the rear element and blackened. This is better for suppressing glancing reflections than plain blacking.

Not relevant to y'r question, but useful for those of us who use adapters for hanging lenses in front of shutters. Machinists who aren't aware of the importance of glancing reflections in the tubes will make adapters with smooth interiors. These are just horrible even when anodized black. TTH's approach works well here. Thread, then anodize. So does flocked paper as sold by Edmund (edmundoptics.com, I think).

Jim Jones
8-Aug-2016, 18:25
Hobby and craft stores should have flocked paper. Some velvet from fabric stores also works.

Mark Sawyer
8-Aug-2016, 19:08
...Machinists who aren't aware of the importance of glancing reflections in the tubes will make adapters with smooth interiors. These are just horrible even when anodized black. TTH's approach works well here. Thread, then anodize. So does flocked paper as sold by Edmund (edmundoptics.com, I think).

What Dan said. I have a projection Petzval that was completely unusable on a camera until I covered its shiny black interior surface with flocking. Flocking cloth for inside telescopes is the best solution I've found.

Nodda Duma
8-Aug-2016, 19:24
Dan: In regards to the "thread all the way to the lens". That is very common in modern designs for stray light reduction and I always request the opto-mechanical engineer incorporate the feature for my objective lens designs. Every internal surface is treated in this way, including retainer rings. I call it a helical cut or helical baffling. Guarantees at least a double-bounce on black anodize surfaces for a maximum 1% stray light transmission path before reaching the image plane.

IanG
9-Aug-2016, 05:09
Thanks Jason, Dan etc. These Cooke triplet lenses (I have 3 5¾" & a 6 7/8") are significantly different to othe LF Triplets I own, the aperture is between the front element and rear pair, on the longer one the interior of the rear tube is threaded as Dan mentions but overall lenghts are quite significantly longer.

Now I have one fitted on a lens board so can do some comparisons using a Speed Graphic.

Ian

neil poulsen
10-Aug-2016, 09:46
I had a Schneider Dagor 14" that for a while that was very contrasty. No wonder, given it's minimal four air-to-glass surfaces and the fact that it was multi-coated.