How do I check a used lens for fungus?

Hold the lens up to a torch (or a bright lamp) and look through it from the front. Fungus shows up as thread-like webs spreading across the glass, often with a milky haze. Haze and separation are also warning signs. Fungus is the one to walk away from, because it spreads and is costly to clean. A few specks of dust are normal and harmless.

Then check the rest of the lens: autofocus and zoom should move smoothly and quietly, and a photo of a flat, detailed wall or a newspaper will show if one side is softer than the other. The full checklist is on Buying used.

Written by Claude from this site's pages.