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I've got to say, I love the Internet. If I choose my resources carefully, I can get my burning questions answered in minutes (in this case, 4 minutes).
The problem? I had spots! The symptoms seemed to indicate to me that it was the lens, and besides, I had never taken the lens off since I've had the camera, since I've got no other lens. So, I didn't think I could possibly have a dirty CMOS sensor. As it turns out, I did. Here's my post to the Canon SLR forum on dpreview:
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Hi folks,
I've got a D60 with a 24-85mm f/3.5-4.5 lens. I did some "still life" shots against a white sheet recently, and noticed a persistent spot on the images. I've been trying to figure out if it's the lens or the CMOS image sensor, but I think it's the former; the problem is worse at high apertures (f/27) and fades considerably at about f/9.5.
Here are some shots I did against a COMPLETELY white board (except for the word "focus") at f/27. The first image has a couple of spots, most notably toward the topmost middle area (see arrow). The 2nd image shows the extent of the problem, after running the same image through autolevels in PS. Incidently, these are grayscale.
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My questions:
1) Are these spots due to a dirty lens (and not the CMOS)? I don't have another lens to swap out to test my theory.
2) Can the lens be cleaned? About how much will this set me back?
3) I hear that the "L" lenses are sealed against dust and moisture. Would this take care of the problem? (this is probably the result of a wind storm while shooting in Arches NP).
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Here are some of the answers I got:
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1) "You cannot see dust on a lens ... but you can see it on the sensor, which is what you have. Easy and fast to clean, using the Canon recommended blower bulb. You can have a lot of dust on your lens ... and on your eyeglasses ... and it will not affect image quality any noticeable amount."
2) "It's dust on the sensor - a sad fact of life with any digital SLR. Using 'L' lenses won't make any difference - it creeps in when you change lenses anyway. Some 'L's pump dust in as you zoom anyway. Do a search for 'sensor dust' or 'sensor cleaning' - there's loads of stuff about how to do it. I favour Q-tips with Eclipse fluid myself."
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In total, I received 5 responses, all within 20 minutes. The good news is that it's a problem easily remedied; the bad news is that I no longer have a justification for an "L" lens!
Shortly after reading their answers, I followed Canon's procedure to clean the CMOS. This involves taking off the lens and configuring the camera for CMOS access, and blowing out any debris with an air blower. Success! ...mostly. Nearly all of the spots are gone, but one stubborn one remains. So I plan on accosting our local Wolf Camera store to see if they have these Sensor swabs and/or Eclipse fluid.
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