Should you buy a used Canon lens with dust inside?

Usually, yes — a used Canon lens with some internal dust can still be a good buy, especially if the price reflects the condition. Small amounts of dust are very common in used lenses and often have little to no visible effect on images.

What matters most is how much dust there is, where it is, and whether it’s actually dust:

• Dust: often harmless in small amounts, though a lot of it can be a concern.
• Fungus or haze: avoid these if possible, because they can affect image quality and may be harder or more expensive to fix.
• Rear-element issues: dust, scratches, or residue near the rear element are more concerning than a little dust elsewhere.

If you can test the lens in person, mount it on the camera and:

1. Shoot a bright, plain surface at a small aperture like f/16 or f/22.
2. Check the image for spots, softness, or obvious defects.
3. Make sure autofocus and aperture operation are normal.

A practical rule: buy it if the lens is otherwise clean, works properly, and the price is good enough to account for the condition. If the dust is heavy, the seller won’t let you test it, or you suspect fungus/haze, I’d pass.

As a rough negotiating point, minor dust alone may justify only a modest discount, but there isn’t a reliable universal percentage.

Also, don’t assume Canon will clean minor cosmetic dust under warranty if the lens has no functional problem; Canon support may treat that as non-warranty service.