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I have a vintage Rolleiflex camera which I want to restore and repair: Xenotar, 2.8, 80mm. The coating on the lens seems to be worn. Will this create poor images?

  • Hi gvidotto, Welcome to photo.stackexchange. The images will be prone to flare which affects the result. Some LOVE the effect and use it creatively. Search the word in the tags link in our site header. Also, It's a film camera and the film will be difficult to locate and bothersome to process. Again, some LOVE the challenge and use it creatively. Very likely, the camera would be happy in its new role as an interesting and elderly conversation starter sitting on your bookshelf. – Stan Aug 14 '17 at 17:46
  • ^^ Should this be an answer @stan? – Digital Lightcraft Aug 14 '17 at 18:25
  • @Stan Film is not so rare as to be difficult to locate! Lenses can be re-coated. I would never condemn my Rolleiflex to a life sitting on a bookshelf! – osullic Aug 14 '17 at 19:00
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The purpose of lens coating is to increase the light transmission of a lens. About 3% to 5% light is lost on a lens element due to reflection off the glass (polished surface). To mitigate a super thin layer of minerals is applied. This works by interference, the coat is ¼ wavelength of the color to be controlled. With a coat, the light loss is about 1% per surface. In an uncoated system, the light loss accumulative per surface and will be as high as 50%. Don’t get me wrong, light loss is not what you want but the front element losing is coating is not the real issue. The rest of the story is; the internal lens surfaces reflect light and this likely hits and re-reflects from adjacent surfaces. Each re-reflection adds to the extent of the tray light that will intermingle with the image forming rays. This stray light bathes the sensor (of film) during the exposure. The net result is increased flare. Flare light in an optical system is unavoidable but is mediated by coating the internal lens surfaces. Flare can be devastating as it robs our images of contrast.

Not likely that a loss of the coating on the front of your Rolle will make any significant different and the internal elements have not been scrubbed of buffed – so you are OK.

Factorial: The English optician Harold Taylor, in 1892 observed that old lenses transmitted 4 to 6% more light than new ones. He figured out why. Seems old lenses were blemished with soot. This was during the industrial revolution and the air was laden with smoke and soot from the coal fires that powered the steam engines and gave warmth. This coating of atmospheric pollution settled on lenses on the shelf and etched them. He discovered that this thin transparent coat somehow reduced surface reflections allowing more light to transverse the lens.

Taylor experimented and found a way to artificially bloom (age) lenses. This truly was an important discovery because new lenses suffer a 4 to 6% loss in light due to light being reflected from their polished (mirror like) surfaces. Now lenses used in cameras and telescopes are complex systems with many lens elements sandwiched together. Thus multi-lens element systems can suffer a loss of 40 – 50%.

This discovery and remedy is important as modern lenses often use many elements and groups. Losing 4 to 6% at each junction translates to quite a high loss. Most loss is from internal junctions (glass to air and glass to glass) within the barrel. Each internal reflection caused light rays to go astray and many misdirected rays bathed the film/chip with light scatter called flare. Flare is devastating; it degrades the image by reducing contrast. Gross reflections cause glare spots.

Many coating methods are used. One method is to place the lens to be coated in a vacuum chamber. The air is evacuated and the mineral that will be the coat is heated causing it to vaporize. This vapor condenses on the glass lens and coats and etches. It is the thickness of the coat plus the material that does the trick. Each coat is optimized for just one color of light. A modern lens has multiple coats applied. Each coat is different in thickness. A high quality lens can have as many as 7 thru 11 coats.

A tip of the hat to Harold Taylor.

  • Thanks a lot for this answer, Alan. You make "no coating" sound terrible, however one shoots with a coatless lens, and if the scene is lit enough and the person shoots RAW, they should be able to balance the contrast/saturation afterwards in post, right? Or at least one would assume these would still make great BW lenses – MicroMachine Mar 1 at 1:56
  • @ MicroMachine -- It takes a multi-element lens design to mitigate lens aberrations that degrade. Each element has two polished surfaces that reflect light. Some of this reflected light will intermingle with the focused exposing light. The result is flare light. Flare light robs contrast and reduces the dynamic range of the image. Nevertheless some images made with un-coated lenses are spectacular. In modern times, we likely would not chose the handicap presented by un-coated lenses --- would we? – Alan Marcus Mar 1 at 4:32
  • Agreed, obviously. However, sometimes people are limited by factors such as budget, or simply some lenses are rare and the few remaining ones are damaged. Until lens coating repair is made easy or cheap, and anyone can just go get their favorite lenses fixed quickly, many people will be looking for ways to palliate the limitations of worn coating. The more challenges appear, the more faith it takes :) – MicroMachine Mar 1 at 4:39

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