Camera Lens Testing - Physical Condition
Part 2 - Physical condition check (dust, scratches, bubbles in glass)
The first thing to do when you get a new or used camera lens is to check the physical condition of the lens, especially the optics.
A new lens should obviously look new. Focus and zoom should be smooth with no "rough spots". With a used lens you can expect some minor cosmetic defects of course.
A new camera lens should not have any scratches on the optics, but a used lens may. In general, a small scratch of two on the coating of a lens isn't an optical problem. It may lower the value of a lens, but it really won't affect the optical performance to any significant extent. The major effect of a scratch is to cause some scattering of the light passing through the lens which will in turn result in a slight loss of contrast. The more scratches there are, the more scattering you are likely to get, but unless there are a lot of scratches you're really not likely to see an appreciable contrast loss. If you get a lens that looks like the front element has been cleaned using sandpaper, then you do have cause for concern. The use of a good lens hood can help maximize lens contrast even with an unscratched lens, and it can significantly lessen any contrast loss due to scratches. Scratches on the rear element may have a greater effect than those on the front element, especially with wideangle lenses.
What about a chipped front element? Well, even a pretty big chip in the front element of a lens is less of a problem than you might think. All you need to do is paint over it with some matte black paint! If light never passes through the chipped area it can't contribute to degradation of the image and the very, very slight change in the diffraction pattern caused by a small obstruction is negligible. The blackened area won't show up in the image. Even with mirror lenses which have a large central obstruction, the in focus image isn't affected (though it does show up in out of focus regions). A chip in the rear element is more of a problem. I wouldn't buy a lens with a chipped rear element.
Dust is everywhere and sooner or later some of it will find its way inside all camera lenses. It's not unusual to see a few dust specs even in a brand new lens, and an old lens is almost sure to have some. The effect of dust is usually quite minimal. It won't affect lens resolution, it won't create spots in the image and it's usually not worth having the lens cleaned. As with scratches, a lot of dust can scatter some light and lead to a contrast loss, but a few specs aren't going to cause any problem. If you get a brand new lens with a lot of dust specs inside, you might want to return it for a cleaner copy, if only to make you feel better.
If when you look through the lens any of the elements look a little cloudy, you may have some fungus growth inside the lens. I've never seen or even heard of fungus in a new lens, so this is something you'd only expect to find when buying a used lens. Fungus is bad because again it scatters light and can lower contrast. It will also get worse with time and fungus can actually eat in to the glass itself. You can usually see small filaments in along with the haze. If you do then that's a characteristic sign of fungus growth. High humidity and high temperatures promote the growth of fungus. It initially starts when fungal spores in the are make their way into the lens (along with dust). When conditions are right, they will germinate and start to grow.
The only real cure for fungus is to take the lens apart and clean the elements. There are claims that exposure to sunlight or UV light can kill fungus, but I wouldn't bet on it. Even if light does stop the growth, it won't cure any damage that's already been done. Unless you are skilled in lens repair or the lens is remarkably cheap and you can afford to get it cleaned professionally, don't buy a lens if you think it has fungus.
There's an old story that if glass has bubbles in it, it shows it's good glass. Leica are reported to have said that even the BEST optical glass may have some of them. Well, if that was ever true I don't know, but I do know that a modern lens shouldn't have any bubbles in the glass. If you get a new lens that has a bubble, send it back. Even if it doesn't affect the image, it will certainly lower the resale value of the lens.
But do small bubbles actually cause a problem? The answer is similar to the answers about dust and scratches. They can act as a scattering center, possibly having a small effect on contrast, but they won't affect sharpness and probably won't visibly degrade the image at all. At the right (low) price, I wouldn't worry about one or perhaps even two small bubbles in a lens element.
The coatings on modern camera lenses should look completely uniform. There should be no "spots" or other marks when a lens is brand new.
Older lenses may show some defects and if the lens has been "cleaned" too much they may even show some signs of wear. The effect of the loss of coatings is, as you may have guessed, a loss of contrast. Older uncoated lenses definitely show lower contrast when shooting into the light than modern multicoated lenses. Coatings don't affect lens resolution, but lower contrast images do often look "less sharp".
Small defects affect the value of the lens a lot more than the optical quality. I've bought lenses from KEH which they describe as being in "Bargain Condition" (which is one step above "Ugly"!). The lenses looked pretty used. There was dust inside them and a few scratches on the optics. However their optical performance seemed just as good as a new lens. It really does take a lot of crud to make a significant optical difference. Thanks to the defects, the lenses were really cheap!