Theorethical question about lens coatings

[erian]

If I had an opportunity to coat an old uncoated lens then do I must coat all the lens elements both side? Should I also separate clued elements and coat them separately?

No, I do not have such opportunity but I would like to understand how this would improve the old lens.

[Mark Sawyer]

It would cut down on internal reflections, increasing contrast, especially in the shadows. The internal air-glass surfaces are the worst offenders. The front air-glass surface less so, as they reflect the light outward, the rear element reflects only what has bounced back from the film/plate, so somewhere in-between the internal and front surfaces as far as its effect. Cemented glass-glass reflects much less, and wouldn't be worth the trouble.

Also, lens designs with fewer internal air-glass surfaces have less reflecting going on, so they'd improve the least. That's why Dagors (only two internal air-glass surfaces) had more contrast than other design with more surfaces. The Plasmat (aka air-spaced Dagor) was essentially the same design but with 6 internal air-glass surfaces, it had pretty soft contrast until coatings came in.

Each air-glass surface scatters about 2% of the light, (there's some disagreement about that number), so the Dagor scatters maybe 4%, while the Plasmat scatters maybe 12%. And as older lenses age, they may develop a "bloom", a natural AR coating, so they wouldn't benefit as much from being coated. Early AR coatings (magnesium fluoride) were about 80% effective, but modern coatings are often over 99% effective. Hope that helps.

[Jac@stafford.net]

Should I also separate clued elements and coat them separately?

With respect, it is highly unlikely you could properly re-assemble, align and center the elements. Bad idea.

[Nodda Duma]

You’d need to strip down the elements and separate them. They need to be perfectly clean and bare to get a good coating.

To Mark’s point, you’ll see little improvement in old lenses that minimized the number of air-glass interfaces anyways.

Rebonding, align and centering isn’t difficult per se, but does require technical understanding of the equipment/jig for a proper setup...at least at a machinist’s level of understanding of tolerances, gauges, etc.

Regards,
Jason