A salesclerk told me that manufacturer's lenses are better than third party lenses because the lens coating on the manufacturer's lenses do not fade as much as 3rd party lenses in the long run. Do lens coating fade at all?

-- Ronald R. Gregorio (gregorio@ksc.th.com), June 29, 2000

Answers

Hmm, well, just shows you the level of "expertise" that gets you a job as sales clerk, or what some people will stoop to, to increase their commission. Lens coatings aren't coloured in the first place, and so cannot fade. They are layers of metal fluoride or oxide coating which have a refractive index between that of glass and air. The colour is caused by light "interference" across the very thin layer(s) of the coating, like oil on water, or the colours seen on detergent bubbles. Anyway, the technology, machinery, and materials used by 3rd party lens manufacturers is likely to be identical to that used by the Camera manufacturers these days. The time when a Camera manufacturers name on a lens meant something is coming to a close, I'm afraid.

-- P.L.Andrews (p.l.andrews@bham.ac.uk), June 29, 2000.

As the other answer points out, lens coatings do not "fade". They can be scratched or worn, but no fading.

WRT original manufacturer versus third party, I somewhat disagree with the other answer. While buying an OEM lens deosn't guarantee quality like it did in the past, the top lenses from the camera manufacturers are still just about the best lenses available. Unfortunately to compete on price, they all make lower priced lenses to compete with the third party lenses, and they do compete. They are comparable in price and in quality, and not as good as the top lines from the same manufacturer.

-- Terry Carraway (TCarraway@compuserve.com), June 29, 2000.

I'm not in any way supporting the claim of this salesperson, but I think we ought to pause a moment and think about the fact that the metal oxides used in these coatings (often deposited as vapors onto the glass) are probably molecules whose atoms are COVALENTLY bonded. (It seems to me less likely that they are some type of simple metal "salt", whose constituents are held together by non-covalent, electrostatic charges, in which I imagine they would be very vulnerable to polar solvents such as water, ammonia solutions, or even less polar solvents such as alcohols). At any rate, I find it hard to imagine that these metal oxides are entirely immune to potentially damaging photochemical reactions that prolonged exposure to sunlight can induce. Suppose we took a piece of fine, multicoated optical glass and measured its absorption and reflection characteristics. Next, suppose it was then placed against and below a skylight (protected from wind and precipitation) for a month or so, in which it was subjected to the Sun's direct light, especially ionizing UV. I wonder if this sort of prolonged exposure would harm the multicoating, and if its absorption and reflection characteristics would show some degree of degradation upon re-testing? While the cumulative amount of exposure to direct sunlight that many photographic lenses are subjected over prolonged usage is likely to be quite small in comparison to this imaginary experiment, it is still an interesting question to ponder. Hypothetically (and for the sake of argument, ignoring the potential fire hazard), how many of you would feel comfortable leaving a fine lens or multicoated filter sitting on a window sill, encased in a clear box, with the optical glass exposed to direct sun for days or weeks?

-- kurt heintzelman (heintzelman.1@osu.edu), June 29, 2000.

That reminds me of what a mail order sales clerk told me, that the gray market Nikon lens(that they listed for cheap and didn't have in stock of course)had plastic elements while the US version was all glass. I've also been told that Pro Optic filters were made of ED glass. I wonder if there is a book that teaches these guys this stuff, or if they just make it up all by themselves. In defense of camera store salespeople ,however(and I was one for several years), many I've worked with were very competant photographers supplimenting theire income with a day job, and didn't resort to tall tales to get a better spiff on an item.

-- Andrew Schank (aschank@flash.net), June 30, 2000.

Kurt, without spending a huge amount of time to research the numbers, I'd initially think that solar UV, at its current levels (ozone holes notwithstanding) are not at the levels which would be required to overcome whatever intiation energy required to initiate degradation of the metal oxide bonds. Maybe with several hundred years of exposure, or under a high-intesity lamp, you'd see some amount of dissociation, but in the real world, I doubt it'd be an issue. I'll have to dig around my CRC or Lange's later, though.

Actually, I'd be

• disappointed in spending so much for coated lenses if they did degrade so freely;
• inclined to think flourite would be at least as susceptible to this sort of damage (which, again, I'd expect to be minimal at best)...

-- Scott (bliorg@yahoo.com), June 30, 2000.

Kurt: I doubt if any lens manufacturers are still coating their lenses by simple vaccuum evaporation/deposition. Even simple spectacle lenses are coated by electron-beam nowadays. Using the electron-beam current to carefully control the rate of deposition and the electrical charge on the coated surface, it's possible to get the molecules to form a near monocrystalline structure. Naturally, this has far greater strength, adhesion and integrity than the old vapour deposited films. Magnesium Fluoride is still the mainstay of anti-reflection coating, and this material has almost zero solubility in water and all common organic solvents. Another material commonly used is Silicon Dioxide (Quartz), an extremely hard and tough material. I believe both these materials are highly transparent to UV light, and so will have little interaction with it.

Where OEM lenses do tend to differ from 3rd party lenses is in the matching of the coating properties to the glass. Nikon, for one, controls the overall colour balance of their lenses by using the AR coating to shift the spectral transmission of the different glass types used in the lens elements. Some 3rd party lenses seem more concerned with appealing to people who'd rather look at lenses rather than through them, by creating multicoloured rainbow effects with the coating.

-- Pete Andrews (p.l.andrews@bham.ac.uk), July 03, 2000.

Gentlemen: Thank you for your input.

-- kurt heintzelman (heintzelman.1@osu.edu), July 03, 2000.