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?
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.