converting a apo tessar to a triplet

[Lou Baleur]

Long story short: I just inherited a junk 60cm apo tessar. nice shape except for the dust and the front element which looks like it was cleaned a LOT over the years without much care. So the front element is almost frosted in appearance and I was wondering if I can get a soft focus lens or a reasonably sharp lens out of this by substituting the front element with a eyeglass blank or other cheap single element (or available doublet). I have none of these laying around so an eyeglass blank is probably a good starting point. So a positive meniscus lens with 60cm apo tessar rear elements to make a triplet is what I'm thinking. What focal length would I be looking to use then to make this work?

A puzzler for you lens designers and lens historians that know what the individual elements are in this lens. It is 60cm carl zeiss apo tessar.

[Pere Casals]

You may consider just re-polishing the front surface, any coating there will be lost but...

You may check instructions for DIY telescope hobbyists to make and polish lenses.

http://www.newportglass.com/share.htm
https://www.youtube.com/watch?v=tMp0sI3aYEU&t=682s

[Bob Salomon]

You may consider just re-polishing the front surface, any coating there will be lost but...

You may check instructions for DIY telescope hobbyists to make and polish lenses.

http://www.newportglass.com/share.htm
https://www.youtube.com/watch?v=tMp0sI3aYEU&t=682s

Repolishing changes the lens so this would not work.

[Mark Sampson]

Something to have fun with. You never know until you try... all you have to lose is some time and a few sheets of film.

[Mark Sawyer]

Repolishing changes the lens so this would not work.

A modest repolishing, correctly-done wouldn't change it much, if at all. But just for the good, clean fun of it with soft-focus in mind, replacing the front element is a cheap, enjoyable experiment. I'd start by disassembling and checking the front element for focal length and diameter, then looking for a diopter lens (as in those $10 for a +1, +2, +3 macro "filter" sets) to replace it.

A note on Tessars: soft focus (spherical aberration) can be induced by moving the front element spacing forward, ala the Velostigmat Series II. But with an f/9 Apo-Tessar, you may have to move it a ways. But the different front element may change all that anyways.

[Pere Casals]

Repolishing changes the lens so this would not work.

This depends on the amount of microns that have to be removed... but a fine job can be done with a 1 micron synthetic diamond polishing paste, a variable speed Dremel, and polishing wheels. I'd not try to remove all scratches, just I'd try to make a very light polishing to make the surface clear, basicly removing the coating and not eroding much glass.

[Jeff T]

Lou, I've polished uncoated lenses similar condition to your lens with Mother's Mag & Aluminum wheel polishing compound with a dab of carnauba wax. I used a foam applicator from my wife's cosmetic make up kit and apply with very light pressure. I put the lens up to my eye to see the frost then polish the glass by hand until the frost clears up.

[Lou Baleur]

Thanks everyone.

Polishing sounds maybe better if I can't get it apart.

But great suggestion--Yes--I totally forgot about the lens close up diopters I have somewhere. Hopefully the focal lengths are long enough. So I'll see what's involved in getting this thing apart. Although, now I'm thinking I may just shoot it "as is" since it may just be diffused enough from the front frosting to give a softish effect all by itself. First I have to take off the giant aperture selector lever and get it mounted on a board.

Yes--this may be a fun experiment if I'm able to get it apart--to compare before/after and different combos.

Before I start digging--does anyone know if this is a regular process lens with 4 separate elements or is this an actual tessar design with a 2 element in it?

[fuegocito]

I would mount it on the camera and have a look at what it does first. As long light gets through it just might work as a soft focus lens right off the bat without you having to do anything. I have an Apo Kollinear that has a nasty crack that splits the front element in half, the balsam is all yellowed and cracked throughout the front group, but man it produce a killer soft image on the ground glass.

[Dan Fromm]

Before I start digging--does anyone know if this is a regular process lens with 4 separate elements or is this an actual tessar design with a 2 element in it?

Apo-Tessars are Tessars. Four elements: two singlets in front of the diaphragm, a cemented doublet behind the diaphragm. See https://en.wikipedia.org/wiki/Tessar for a cross-section.

[Lou Baleur]

Thanks! So it is a "for real" tessar. I also found this article: https://www.arnecroell.com/czj.pdf. Thank you for Mr. Arnie Croll for this information.

This article has a whole section on these lenses and is VERY informative. I have the serial number range of the "old" design, so it's a definite doublet in the back.

So now this can be like a verito design if I put a single element in front. I finally got the front and rear cells off--let me tell you, they were very tight. It worked out good with the large aperture levers--I was able to use them like filter wrenches and was able to break loose the cells with them! See the pic--they are perfect filter wrenches.

So now the rear cell doublet has a much shorter focal length--like 12"--and the front cell with 2 elements is a net negative lens. The very front damaged element looks like it would be positive and the second one a more powerful negative which makes a net negative cell. To test this, i'd have to take the front cell apart. It will not unscrew easily and I see there is a single screw with a mark on it and a matching mark on the retaining ring that won't currently budge (see pics). I assume the screw is holding this from moving and this is a very sensitive adjustment.

Does anyone know if I take it apart further if the things inside can get hopelessly misaligned? Or is it safe to pull out the screw and unscrew the retaining ring so that I can reverse the operation and put it back to spec? I don't want to pull it apart if that's going to be a one-way ticket to "never back in spec land." If the operation is reversable, then I'm going to take it apart and start experimenting. First, of course, I'll shoot it at the factory assembled settings just to see how it does "as is."192189192190192191