Eroded Lens Coatings

The elements of most lenses come with a coating to reduce light reflection and boost light transmission. High-end lenses actually feature multiple layers of different coatings which boost light transmission up to 99.9%!

These coatings, while not fragile, are not exactly tough either. Exposure to harsh environments, solvents and even incessant rubbing with cleaning fluid can erode the coating of the front and rear elements.

To check for eroded coating, you should first remove the dust and oil off the front and rear elements. Start by blowing off dust on the front element with a rubber blower. Don't blow on it with your breath - your spittle will make things worse!

After blowing the dust away, apply some methyl alcohol or lens cleaning solution onto a good lens cleaning pen or lint-free lens paper. Apply the wet pen or paper onto the surface of the front element in circular motion, starting from the center and working your way to the sides. Of course, if you are using the carbon-based cleaning pen, you will not need any alcohol or cleaning solution.

Clean up the alcohol or cleaning solution with a piece of dry lens paper. The front element should now be clean of dust and oil. If necessary, do the same for the rear element. Once done, you will be ready to examine the lens elements for eroded coatings.

Now, look at the front and rear elements in bright light. By varying the angle of the light and the angle at which you examine both elements, you can see if the coatings are eroded. You can also look through the lens at a bright light source. The bright sky or a sun-lit wall works well too. By varying the distance of the lens from your eyes, you can focus on the different elements in the lens and look for eroded coatings.

Slightly eroded coatings should not affect picture quality. Significant erosion though will reduce light transmission in affected areas and give your pictures a blotchy look. You may also notice less contrast and more flaring with eroded coatings.

You can check by using the lens to take a picture of the bright sky at the smallest aperture possible. You should, of course, take the biggest picture possible with fine JPEG compression. Next, extract the picture and examine it at 100%. If you do not see any variation in light intensity in the picture, then you have confirmed that the eroded coatings do not affect picture quality.

If the erosions affect picture quality, you should really forget about buying the lens. Repairing it may cost you quite a bit of money, unless you can buy the replacement element cheaply in online auctions. If the erosions do not affect picture quality, you can still use that as a tool to help you bargain down the price of the lens. Just remember that someone else will do the same to you when it's your turn to sell the lens!

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