Canon Lens Dirt.

June 24, 2009 2:09 AM

CanonFilter: It appears I have dirt inside the my Canon 28-135mm lens... Rather, it looks to me like it's on the inner optics, and not the prime lens...

I was cleaning my UV filter which has been on the lens ever since I bought it, a year ago, and noticed that some specks seemed to still remain. I removed the filter and looked carefully at the prime lens, but I've never touched it and, frankly, the cloth I was using to clean the filter wasn't aces, so I wasn't about to touch my actual lens with it.

There appeared to be no dirt or dust on the lens itself, but two or three specks were visible on the moving piece of glass beneath the prime lens... which was, to the best of my ability to determine, not an optical illusion, though I could be wrong.

Is it at all possible there's dirty on the inner optics? I also pulled the lens off and looked at the back, but couldn't see anything there at all. The specks seem to bend and fit the inner lens when I move it... so how do I clean it? Do I care?

Some of my pictures at 28mm show dirt specks, but I think that was just the obvious stuff that was on the filter. I haven't shot again since cleaning...

Do I need to worry or care at all? It's my only lens for my XTi, but still, it bothers me... I imagine about the only thing I could do is send it in for repair, so I guess my question is this: is it at all likely or even possible for there to be dirt on the inner optics, or am I just being tricked?

posted by disillusioned to sports, hobbies, & recreation (6 comments total)

The specs at 28mm are, I suspect, dust on the sensor itself, where they will be in focus, rather than on one of the lenses, where they will not.
posted by zippy at 2:45 AM on June 24

In almost all cases any specs or blobs visible in your photos will be a result of dust on the camera's sensor.

Dust on a protective filter or the front lens element will result only in diminished contrast in the pictures; not specs or blobs. To do even this the amount of dust would need to be substantial.

The only situation where dust on lens elements might create specs or blobs in your photos would possibly arise if you were shooting extreme macro close up pictures with the lens stopped way down. This could result in the individual dust particles almost being in focus, and hence being visible in the photos..

It is quite possible that you have a little dust inside the lens, especially since some zooms do tend to generate a slight partial vacuum when zoomed or focused. However, only in a fairly extreme case will this affect your pictures, and once again it's more of a threat to contrast than it is to sharpness.

I'd say don't worry about the lens, but clean that camera sensor.
posted by imjustsaying at 3:30 AM on June 24

Yes generally dust in the lens is not a big deal. It's probably sensor dust showing up in your pictures. Cleaning your sensor is a delicate but not impossible task.
posted by sully75 at 4:17 AM on June 24

Third-ing sensor dust. Dust inside the lens won't be noticeable, though on the front of the lens it can catch the sun and cause some flare.

Most dust can be removed without actually touching the sensor. Get a good blower like this one: http://www.bhphotovideo.com/c/product/259157-REG/Giottos_AA1900_Rocket_Air_Blower_.html
posted by paanta at 5:33 AM on June 24

Is it at all possible there's dirty on the inner optics?

Yes. These lenses are not sealed, so dust can get inside. Some of them can even generate their own dust from wear of the moving parts.

so how do I clean it?

You don't! I say this as a person who is usually not afraid to take just about anything apart. I suggest that if you try to disassemble the lens yourself to clean it, at best you will end up with a lens kit (collection of parts that could be assembled into a lens) and no instructions for assembling it correctly. At worst you will break something trying to take it apart. It's a specialist job.

Do I care?

In general, no. Dust inside the lens has approximately no effect on picture quality. Sometimes if direct sunlight hits it, you can get a bright patch in the picture, but usually that just means you should have been using a lens hood to keep sunlight out of your lens in the first place.

Some of my pictures at 28mm show dirt specks

Nth-ing those who have said this is probably dust on the sensor rather than the lens. Here's something you can try to see if this is what it is: go outside on a clear day, set focus at infinity, and shoot the clear blue sky at the smallest aperture you have (e.g. f/22 or something like that). Do this at 28mm and at at 135mm.

You will probably get a picture with lots of awful black specs, and they'll be in the same place regardless of focal length. If so then they're definitely dust spots on the sensor. You can clean the sensor yourself if this bothers you, although you need the right tools and you have to be a bit careful about it.

Dust spots show up worst in areas without detail (like a clear blue sky) and at small apertures. If they only rarely appear in your pictures, you might as well just Photoshop them out instead (since you can never get all the dust off, you'll end up doing that anyway).
posted by FishBike at 7:20 AM on June 24

These will not show on your pictures. You know how a reflector telescope has a small, flat, diagonal mirror at the viewing end, to reflect the image sideways into the eyepiece? Here is a relatively large object sitting smack in the middle of the viewing field. It diminishes the light ever so slightly, but it is so far out of focus that you can't see it. Dust is nothing, optically speaking. You'll do more damage opening up the lens.
posted by weapons-grade pandemonium at 8:17 AM on June 24