Is the 600mm RD APO Artar rear element symmetrical?

[john_kasaian1]

I am curious, is the rear element of a C.P.Goerz red dot Apo Artar

symmetrical? Can it be 'reversed' without ill effect? I have one

with scratches in either the glass or the coating---not sure which---

and thought I might try polishing it out(with Bon Ami, no less!)

which might remove the coating. If so, I was wondering if reverseing

this element would be beneficial. Hows that for an off the wall

question? Any thoughts, comments, advise, or SWAGuesses would be

welcome!

[jim_galvin]

No. Don't.

To repolish a lens properly, you need a lap, preferably a piece of glass same diameter as the lens, with accurately the opposite curve, so it fits. Grind with graded carburundum. Put pitch on the lap. Polish with cerium oxide. Only optical grade carburundum and ceria. Bon Ami is not graded, it will have some larger particles in it, and scratch worse than you already have. Probably removing the coating will cause worse flare than the scratches. Recoating is nearly as expensive as replacing the lens. Reversing an element is one way to get a soft focus lens.

[dan_fromm1]

The Artar is a symmetric lens so you can reverse it without much loss.

 

That said, how on earth would you reverse an element? The elements themselves aren't symmetric and it seems like a really dumb thing to do. Also physically hard to accomplish.

 

As for reversing the lens, in theory what you do is unscrew the two cells from the shutter, then put the front cell in back and the rear cell in front. In practice, this won't work if the shutter, like Copals, has different front and rear threads.

 

Why adopt half measures? Don't use Bon-Ami, use a belt sander.

 

If you want to have the damage evaluated and the possibility of recoating assessed, send the lens to John Van Stelten of Focal Point Optics.

[tracy_storer1]

I'll second the recommendation to send to John at Focal Point. Artars are easy

to disassemble/reassemble and only one element needs work. I guess it'll

depend how much you've got into the lens already. In any event:

http://www.focalpointlens.com/

Good luck, Tracy

[john_kasaian1]

Actually, I got the thing for next to nothing, which is what it's probably worth. By symmetrical, I meant is the glass element by itsself symmetrical and if it could be reversed, would the scratched area be 'better' on the inside of the barrel or on the surface facing the film? Or would it matter at all? Another option, I guess, would be to find another rear element and see if it could replace the scratched piece and the third option, which is even further off the wall than a Bon Ami polish(hey, it hasn't scratched yet!) would be to take the thing apart and see if I could combine any of those elements with some old Metrogon elements to make a sky lens like the one in Kingslake's book, just for fun. I don't want to ebay the thing since I wouldn't want anyone to think they were getting a "deal" on what would otherwise be a very desireable lens and none of the shops I trade with are interested in taking it on trade(good for them!) FWIW I know diddly squat about optics---just having fun with the idea. Any suggestions???

[c_p_goerz]

A Red dot is a symmetrical lens, the LD Artar is Asymmetrical. Rather than risk really messing the lens up just take some black paint and with the tip of your finger 'wipe' it into the scratch, this will prevent any distorted light bundles reaching the film. A scratch would have to be really bad to cause any noticeable difference in your film though.

 

Although a RD Artar is symmetrical it may not be as easy as you think to switch a cell over. The mounts the cells are seated in are all hand made to each element with unbelievably high tolerances. Slightly different focal lengths of the front and back combined to make the 24" lens and the cells themselves may be slightly different in size. Trust me! they may look like they fit but when you push that glass in and you hear a little tick sound that means you have just knocked a 1/4 chip off the side of the element, when viewed at a slight angle it will look like a gigantic air bubble/chip/crack which will freak you and any potential buyers out.

 

I have unfortunatly learned this the worst and most expensive possible way so learn from my vast amounts of stupidity before its too late ;-)

 

 

CP Goerz

[john_kasaian1]

CP Goerz, Thanks for the tip! I've used black paint on the front of a chipped 19" Artar, but I didn't know it worked on the rear as well.

[s.k. grimes inc]

The groups themselves are symmetrical, that is, the entire front group can be switched for the entire back group. The individual elements are not symmetrical. That is to say the curves are substantially different on each side of each glass; the curves are different enough to tell by looking at them. (Hint, the steep curve of the positive lens faces away from the iris).

[dick roadnight cotswolds]

How much would a new arial 600mm f6 (or a 900mm f6.3) cost if you could get one?

 

How much would it cost to re-pollish a rear element that fills the 54 lensboard?