How will this affect my lens?

[Ash]

Without going into great detail, an old barrel lens was damaged and the rear element will no longer screw completely into place. It is at least 2-3mm out from the original position (botched screwthread).

This was months ago, and it has sat unsorted.


I've not had a chance to test the lens, but what can I expect from the rear element (or element group) being slightly further away from where it was originally designed to be?


Will I get a softer focus, or some weird effect of some sort, such as loss of focus at the corners?

[Kevin Crisp]

The answer is: It depends. Spacing is relatively critical in wide angle lenses, less so in others. Millimeters of change to the lens spacing is a relatively large increase. I think you need to try it out, preferrably stopped down, and see if performance is still acceptable to you. If the barrel is brass, and you can clearly see the damaged thread(s), you might try opening it/them up with something sharp and strong, like an exacto blade. Perhaps if you can clear that bad patch it will screw in all the way. If you tell people the lens design, you might get a specific answer about the effect of increased lens spacing on a particular design. Optics is one area where this group continues to amaze in its depth of knowledge.

[Ash]

Annoyingly it was the most peculiar of the few barrel lenses I had bought.

Taylor Hobson patent no 113590
Series IX
Cooke Apochromatic Process lens
13inch f/10

Mounting thread on front and back of lens can be mounted either way around? Stiff aperture. What looks like a waterhouse stop slot? no idea how to open and close the gap though. The whole lens needs remounting in a barrel, or a big CLA. Not sure if it's worth it.

It was dropped during cleaning the elements, and so the big dent was tapped out, and I used a small flat screwdriver, but thusfar I couldn't stretch the internal thread enough to get the thread to the correct size again. Too nasty for photo's. Looks like the weight of the barrel/glass was its downfall.

EDIT:

Cooke process lenses made after the early 1920s were provided with a removable lens-hood that could be covered with a leather cap. The screw thread that attaches the hood could alternatively receive any process prism fitted with the standard screw. Process Prisms are extremely rare now and are the stuff of museums instead of Ebay. An iris diaphragm was provided at that time instead of the stops with circular openings. A narrow slot in the lens barrel near the front of the lens could receive process diaphragms or gel filters (of graphic arts quality) if desired. The slot can be opened and closed by revolving the inscription tube.

Series IX, f/10, f/12, f/16 is an Apochromatic process lens for photo engraving and three-color work. It was equally suited for half tone and line negatives, for engravers' cameras, and for use on view, commercial and copying cameras. The glasses were not antireflection coated. The lens was made between about 1924 and 1952. In 1952, the following speeds and focal lengths were produced: f/10, 12 inch for 9x13 format; f/10, 16 inch for12x15; f/10, 18 inch for 13x18; f/10, 21 inch for 16x20; f/10, 25 inch for 18x25; f/16, 30 inch for 20x30; f/16, 36 inch for 24x36, f/16, 42 inch for 30x40, and f/16, 48 inch for 36x48. Today you can get one for about $100 or less.