Today's post is inspired by a message I read on some photography forum. The user had discovered a 1cm (less than half an inch) scratch on the front element of a lens and he was panicked. A scratch on the lens sounds like a really serious thing, right? Well, not exactly. Let's see what are the most common flaws lenses can have, and then we'll see how much of a...catastrophe it really is (hint: it usually isn't).
Unless if they are extensive and deep (technically they would then be cracks), it is extremely unlikely a small scratch will be noticeable in everyday use. I have used plenty of scratched lenses, at all apertures, in all lighting conditions - including straight into a light source - and never noticed anything out of the ordinary. No loss of contrast, no weird ghosts or flares, no loss of sharpness. And I did know what to look for, because I have also seen the effects of cracked glass. So, unless the entire front element is like a spider web, you have nothing to worry about. The only problem is that it lowers its resale value. But that's about it.
|Taken with a Nikkor 18-55 whose front element had couple of long scratches. Would you have ever guessed?|
I only mention it here because so many beginners actually worry over it and believe it's a serious flaw. It isn't. Even a lot of dust means virtually nothing, and it certainly doesn't affect the optical quality more than, say, atmospheric conditions. Unlike a scratch, dust is expected in all used lenses, so it doesn't reduce the resale value of a lens - unless if it is present in unthinkably large amount. In such quite rare cases, the lens might lose a bit of contrast in certain conditions. But I've never seen anything like that, and I've handled countless of old lenses.
The beginning of fungus looks like very tiny snowflakes. It looks a bit like a small point with a bit of halo around it. Later, when fungus starts to spread, it looks nothing like dust, and it's more like a network of threads. Again, the optical quality is generally not affected, unless the fungus has spread a lot. In those cases, contrast will be reduced. The biggest problem with fungus is that it spreads, so the problem tends to get worse. Although you can have your lens decontaminated, it's a solution I'd recommend only with very expensive lenses (in those cases a professional decontamination would be more financially sense-making than a new lens).
Aperture Blades Issues
The aperture blades are the small metallic blades that control the aperture. Two of the most common issues these can have are a) stickiness; b) rust. The latter is much less common, but not unheard of. As long as it doesn't affect the pictures (read below about sticky blades to see how), there isn't cause for alarm - although, in theory, continuous use could cause small pieces of rust to detach and contaminate the optical elements further inside the lens.
|Rusty aperture blades in a Vivitar 135mm f/2.8|
This is when one of the lens elements is separated and becomes misaligned. You might be even able to see this (i.e. to physically spot the misaligned element looking into the lens), but the easiest way to realize such a problem exists is simply by looking through the viewfinder. The image seems to be out of focus no matter how accurately you try to manually focus - autofocus usually hunts back and forth. It really depends on the amount of misalignment and the focal length to know what's going on. In long teles it's much easier to notice. Actually, I once used a Sigma 70-300mm with such an issue, and the problem was so tiny that I could notice it only after 200mm. At 70mm even autofocus worked normally.
These are by far the most serious. Autofocus failures, abnormally stiff or even stuck zoom and focus rings, VR failure, are all issues that indicate a significant problem. You should never try to force a stuck zoom/focus ring. If a mechanical issue exists, have your lens serviced - if it's a valuable lens at least.