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Lens Recoating

gunteach

Well-Known Member
Has anyone here ever had a lens front element recoated to address a scratch or something similar? I've got a small but very noticeable scratch on the front surface of a CZ 80-200 zoom, near the outer edge of the lens. It never shows up in pictures, but is truly bugging me to look at it.

Tocad wants to send it back to Japan to have the front element replaced, at an astronomical price. A local camera repairman who is well thought of suggested having that element repaired and recoated. I know this would not result in the Zeiss T-coat being on that surface, but will it matter that much if professionally done?

Has anyone here ever had this sort of work done, and if so, how did it work out?

Thanks
 

jsmisc

Well-Known Member
Oooh er! I think that the T* coating is a lot to do with the perceived benefits of CZ lenses. Maybe you lose it at your peril?
 

grumpoid

Well-Known Member
Hi,

I think the chances are that re-coating a Zeiss T* lens would cost quite a lot and just suceed in probably ruining the lens and certainly de-valueing it. If it is performing ok I do not see any point at all in paying someone to remove the T* coating and butcher the lens!!

Cheers Steve.M.
 

wang

Well-Known Member
I e-mailed Zeiss Camera lens division concerning a damaged coating of the rear lens element of my C-Y 35 1.4. They said they could do nothing because no more spare parts are available.
I don't think they re-coat, they just replace the part if they can.
 
I thought that I would jump into this discusiion as I have some relevant experience. My background is in physics and optics (24 years). I have designed optics, managed the design and manufacture of optics by contractors (including 2 space instrument designs by Leitz).

To recoat the lens, the existing coating will first have to be removed. This will be done by polishing with an abrasive compound. The radius of curvature of the front end of the front element may change and the thickness of the front element will be slightly reduced. These changes may affect the image quality. Without the lens prescription, it is difficult to predict the affect on the overall lens performance.

The lens can then be coated, but the coating must be designed for the specific spectral range and angles of incidence. The spectral range is easy to figure out and the angles of incidence can be reasonably estimated with some judicious ray tracing with an optical design program. The coating will not be the proprietary T* coating, but it can be an excellent coating. The coating may have to be designed by the coater, if they don't have one in their design "catalog".

Given that you have to dissassemble the lens, polish off the coating, recoat the lens and reassemble/align the lens, I am estimating that the work will cost you about $3,000 to $5,0000. There are places that can do this work, but they may not be interested, as it is a small job for them and they don't always like dealing with "amateurs". If you find a place that will do all of the work for less than a few $k, then you should be very cautious.

If the scratch is really bothering you, buy another lens and sell this one on ebay. The 80-200's are going for ~$400 on ebay, with a maximum of ~$750. They seem to show up regularly. You won't get the maximum price for yours, because of the scratch, but if the price differential is $400 between what you sell yours for and what you pay for another one, you will have only spent 10% of what it would cost to repair the lens and had much less aggravation and probably have a better result.

There are good reasons why replacing the front lens was quoted as an astronomical price. It has to do with hand building one item versus mass production. Replacing one lens is a hand-built effort.

My recommendation is to buy another lens, rather than try to get the lens recoated.

Good luck, if you decide to recoat. I wouldn't. I would rather put the $4,000 into a few more lenses.

Regards,
 

wang

Well-Known Member
Can I ask Doug a question ?

I have used many Zeiss and Leica lenses in 35 mm photography. For those lenses which showes divergence of the saggital and tangential lines on the MTF graph, I always notice a strange skin tone compared to other lenses which do not. Am I correct to say that the strange skin tone is due to astigmatism in these cases ?
 

nickser

Well-Known Member
Hi Tom,
I asked a fellow zeiss user anout this as I had heard that he had a lens recoated in the past. Here is his reply to me for your benefit:

"Malcolm Taylor is still doing lenses.
His contact details (+44) 1568 770542
Upper Lye Farm
Aymestrey
Near Leominster,
Hereford
HR6 9SZ.
UK.
His charges were reasonable; GBP125 for a full rebuild and recoat on a 35-70 VS but he is
very sloWWWWWWWWWW. It took over 6 months.

Wilson"


Regards,

Paul
 

gunteach

Well-Known Member
Paul,

Thanks so much for taking the time to contact your friend on my behalf. I'll contact Malcom when I make up my mind whether this is worth doing or whether I'm just going to shoot pictures and forget the scratch.

Cheers!

Tom
 
The question about astigmatism causing a strange skin tone is an interesting question and I have to be honest and say that I had never thought about it before.

My first thought is that astigmatism shouldn't cause a color change over an area with a fairly uniform color. There may be an effect on color due to astigmatism, but I can't think of the connection offhand.

I am going to take a stab at a possible explanation, but I am writing while thinking and that can sometimes be dangerous.

One possible explanation is that the coatings and glasses in different lenses are different. Each glass has a unique spectral transmission, but the main difference is the blue wavelength cutoff. Each surface AR coating is designed for the specific glass to be coated and the specific range of angles of incidence for that surface in that particular lens. The overall spectral response of the lens is a product of all glass and coating transmissions. Every lens design will have a different spectral response. The T* coatings that are used on the Contax lenses are not likely a single identical coating for every glass and angle of incidence. They are likely a family of similar coating designs. One other point about coatings is that they have different spectral transmissions for each polarization component. The differences are larger for larger angles of incidence.

That you have noticed a connection is a very interesting observation. In your opinion, which lenses are most affected and is there a difference with different films? Is it just skin tones that you see differences or other colors as well?

The bottom line of my rambling post is that I don't think that astigmatism is the direct cause of the color difference that you have noticed. However, it is possible that the lens configurations that have the most astigmatism have glasses and coatings that affect the color more.

Unfortunately, I don't think that I can give a definitive answer. The optical designers at Zeiss probably could. Although I have the tools to investigate the question analytically (ZEMAX optical design software), I would need both the lens prescriptions and coating designs to do so. I can use patent data to get approximate lens prescriptions, but the T* coatings are not likely in the patent literature.

Regards,

Doug
 
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