PDA

View Full Version : Burnished lens mounting



swmcl
13-Jul-2010, 16:41
Hi,

Could I be given a good explanation of the situation where a lens is 'burnished' into the frame ? How is this done ? Can it be un-done ? At what risk ? By whom ?

I have a Xenar lens that does have a front lens ring with a couple of notches but was told that it had a burnished lens mount when I tried to get it cleaned.

The lens was then returned to me but I want it cleaned. So I now ask the community for advice and help.

Should I purchase a lens spanner from MicroTools and proceed as far as I can myself ? I must admit that I do tend to go this way because I think others aren't as interested as going the distance as I am.

I'd be interested in your thoughts,

Rgds,

Steve

Arne Croell
13-Jul-2010, 23:18
A burnished lens mount means that a thin lip of the lens cell or barrel is crimped over the lens edge by mechanical force. No retaining ring is involved. If your lens has a burnished mount, the ring that you see is there for decorative or other purposes.

It is pretty much a non-reversible mounting. To get the lens out, you would need te bend the thin metal "lip" over the glass back up, and then fold it back to its original position when you put the lens back in. In most cases, the metal will not withstand this multiple bending and break.

swmcl
14-Jul-2010, 04:52
Thanks Arne,

Is it a common practice to do this? Going by what you've said it means the lens can never be fully cleaned. This is a rather interesting manufacturing decision!

I wonder too if there is a manufacturer or period of lens making when this has occurred.

Now I'm a bit frustrated :(

Rgds,

imagedowser
14-Jul-2010, 06:02
When I got my Dallmeyer No. 3, the rear lens was out and and the bezel that held it in was turned up. Apparently, someone took the lens apart. Having studied jewelry making, it was fairly simple to remount the lens by re-rolling the bezel with the lens in place. There was an article on lens re-cementing on this site that talked abt the fragile nature of thin brass bezels and the poss of fracture. You might check it out. The guy to talk to is: dag camera repair.com, or S.K.Grimes. Good repair isn't always inexpensive but you get what you pay for... Good Luck, Bill

imagedowser
14-Jul-2010, 06:24
.......also, John, at Focal Point.

GPS
14-Jul-2010, 06:33
Hi,

Could I be given a good explanation of the situation where a lens is 'burnished' into the frame ? How is this done ? Can it be un-done ? At what risk ? By whom ?

...

Steve

There are several methods of burnishing the lens (a press or a special turning device is used). The joint is regarded as non reversible. It is used for rather smaller lenses or as a cheaper way of lens assembly. I don't think that a repair centre would take the lens for cleaning as the risk of breaking it is considerable.

Steven Tribe
14-Jul-2010, 10:11
The bending of the thin lip over the mounted lens in the brass is (was!) done by mounting the lens assembly in soft jaws in a lathe and applying a "pusher" - piece of hard wood to slow push the thin brass to make a sealing contact. The system was used on all sorts and sizes of objectives. In the first decades of the photographic revolution it was expected that their objectives would only be in use for a few years so thoughts about cleaning and repair were not foremost!

Lifting the edge can be done professionally in a lathe with a similar set-up - but I doubt whether anyone has experience and soft jaws (box wood) these days. I have opened 4 of this seals without damage. The problem is to get the first small first gap. I have found that a very solid hobby knife is the way to do it. This is, of course, a considerable health hazard! Moving a flat thin edge slowly around the circumference will create a gap without damage to the lens surface. This gap can be widened in steps by inserting non metallic flat feelers. You will not be able to avoid a degree of cosmetic damage to the brass edge. Brass will be able to stand up to this treatment without becoming brittle. Remember that the lens may be stuck with excess balsam which has been leaked. Remember too that the hidden edges of the lens may be complex as not all the components may have the same diameter.

Resealing is a quite easy process. Just imitate a lathe's movement on the brass and apply pressure with a solid hard wood edge.

GPS
14-Jul-2010, 11:17
Steve, I gather that you are speaking about old lenses, which is not the case of OP's Xenar. The problem with resealing the lens is more complex as a more modern lens needs to be centred while burnished. The correct tool for that (if you want to use a lathe) is a 3 parts instrument moved while the lens is held centred. Also, the greatest danger in removing the edge is not to damage the brass but the lens itself.

Steven Tribe
14-Jul-2010, 12:30
Yes GPS, my experience is with pre-1900 objectives. I totally agree with you about the need for some helping mechanism to keep the glass central if it is not an exact fit in the recess. I suppose one could introduce some sort of packing material between the glass edge and brass "hole" to ensure the lens was held in the exact centre of the mount.

There is a wonderful book by Henry Orford called "Lens work for Amateurs" which shows all the processes involved in making an objective.

Glenn Thoreson
14-Jul-2010, 15:48
Many lenses were made with a removable ring on the front and the spun mount on the rear. If your front ring has spanner slots AND threads aroung the inside of the front of the barrel, it can be, at least, disassembled enough to clean the dust out. The rear cell in a spun mount was usually a cemented doublet, and usually it is not necessary to remove it from the mount if the front group will come out. Those rolled over spun mounts were someone's idea of a joke, methinks, and a bad one at that. Good luck. :D

swmcl
14-Jul-2010, 16:53
Many, many thanks to you all.

The lens is not small. It is a large Xenar 7.25 inch f3.5 Xenar. I have not been able to loosen the front lens retaining ring. This ring does have two notches 180 degrees apart for the purpuses of installing / removing. The rear ring is able to be shifted (has been able to be cleaned?)

The repair centre was asked to fix a broken aperture blade as the primary job and clean the lens also. I think (feel) that they may have fixed the blade and given it a quick wipe and sent it on its way. The reason I say this is because the cost was not too excessive - which makes me think they couldn't get very far with the cleaning or didn't get around to it. At the time, I was on a very extended road trip and communication with the repair house was sporadic. The repair house was the SK variety. They said the lens was 'rolled' or 'burnished' - I assume they mean the same thing!

Because the front retainer has notches I think it should be able to be removed. Because the front retainer is as solid as the day I sent it means it hasn't moved (to my way of thinking). For the front lens to be 'rolled' or 'burnished' I would've expected the burnishing to be against the glass underneath the retainer. Since the retainer can't be moved (and there be no evidence of a tool being used in the notches) how could one know if the lens were 'burnished'?

It is a long way to the US from here. I must prove to be more self-sufficient to some extent. I have to be prepared to go some of the way myself relying on good info from the LF community. I have no local expertise and the LF community are a lifeline for people in my situation.

To complicate issues, the lens does seem to be rather rare.

I shall consider your thoughts carefully. Again, I am in debt to you all.

Cheers,

Steve

swmcl
14-Jul-2010, 17:13
Looking at the website for Focal Point, it seems that these guys would be the best for the job. The website at least is very easy to navigate and is very clear and open about their abilities.

Thanks for the link! It is the first time I've heard of this crowd.

Cheers,