Lens coating damage

greenspun.com : LUSENET : Large format photography : One Thread

My knowledge of optical physics is sadly limited, but perhaps someone here can help me.

I have an old Apo-Ronar with some fairly substantial damage to the coating on the front element. The damage is central, and covers about 50% of the area. The glass otherwise seems perfect. Since it is a symmetrical design I could simply swap the front and rear groups around, to make the pristine rear element into the front element.

My question is, would there be any real point? I can't quite decide whether I think a damaged front coating matters more than a damaged rear coating, or whether there would be no discernible difference. If there is even a small benefit I may as well take advantage of it.

-- Huw Evans (hgjevans@hotmail.com), May 10, 2001


If you feel the lens is worth it (I would, for an Apo-Ronar, but it's up to you)there is a fellow in Lafayette, Colorado who recoats lenses. His information escapes me at the moment, but he often has an ad in the boxed ads near the back of Shutterbug.

-- Anthony J. Kohler (arbitrator@uneedspeed.net), May 10, 2001.

Hi Huw

Re. change between front and rear elements should not work at all because the rear element is smaller in diameter! So you are loosing f stops. Are you loosing some sharpness on the negs? I would do a test shooting and if I would see a visible loss on the neg. then I would buy a other one! Good testing!

-- Armin Seeholzer (armin.seeholzer@smile.ch), May 10, 2001.

I am not an optical expert, but I believe that I have heard that the rear surface is more important than the front.

-- David Rose (DERose@msn.com), May 10, 2001.

Huw: David is correct in that the rear element's condition is more important than the front. I would make a few shots with it and see if it works fine as it is. If the coating bugs you, try having it recoated, but use the front element where it is.


-- Doug Paramore (dougmary@alaweb.com), May 10, 2001.

Thanks to all who have responded.

Armin, this is the only process lens I have used, so I can't generalise too much, but certainly this 360mm Apo-Ronar (early 60s vintage) seems to be precisely symmetrical in every respect, so I think the option of reversing it does apply.

As Anthony suggests I may look into getting it recoated. I'm on the other side of the Atlantic, but I know there is a company over here which can do the job. I haven't yet had the chance to expose any film through this lens, but aside from the coating it looks immaculate. My simple examination of the aerial image suggests that the lens could well turn out to be one of the sharpest I have, so I'm sure it would be worth it in the long run (especially since I only paid the equivalent of about $140 for it in a compur!)

From the sound of what David and Doug are saying, it seems as though it may be as well to leave it the way it is, although clearly it will do no harm to try it out both ways.

Thanks again for the advice. Huw Evans.

-- Huw Evans (hgjevans@hotmail.com), May 10, 2001.

Don't swap the front and rear groups over, just forget about the coating damage and buy a lens hood. The loss of coating will reduce the effective speed of the lens by maybe 1/5th of a stop at most, and a lens hood will more than compensate for the slight extra tendency to flare. Think of all the great pictures taken before lens coating was in common use.
The main difficulty with having the lens re-coated is in removing the remaining coating without causing further damage. In effect this means having the lens re-polished. Not cheap!
You could always ask if a local spectacle grinder could re-coat the lens for you, as is. Multi layer AR coating of spectacles is so commonplace these days that many of the larger opticians workshops have their own coating chambers.

-- Pete Andrews (p.l.andrews@bham.ac.uk), May 11, 2001.

Huw, Coatings main function is avoiding internal flare within glass elements. The first surface, obviously, don't contribute much to flare. So, as stated above, keep the lens and relax! And, of course, use the shade.

Cesar B.

-- Cesar Barreto (cesarb@infolink.com.br), May 11, 2001.

It looks as though the jury is well and truly in now! Thanks again guys. I do hereby faithfully promise that I will never again contemplate reversing this lens, and will use a well adjusted lens shade at all times - Scout's honour! :-)

-- Huw Evans (hgjevans@hotmail.com), May 11, 2001.

Moderation questions? read the FAQ