How to Test a Used camera lens Before You Buy (20-Minute Check)
A used camera lens can be a great deal — or someone else's problem. This is the exact 20-minute test to run before you hand over cash, with the real tools and the red flags that mean walk away.
The test kit
Cheap, Prime-fast tools that make this test reliable. (affiliate)
- LED flashlight / phone torch →shine through the glass to reveal fungus, haze, dust and scratches
- Lens cleaning kit →clean surface dust/smudges and budget for what won't clean off
- Rocket air blower →remove dust from the rear element and contacts safely
The step-by-step test
1. Shine a light through the glass — front and back
In a dark room, shine a bright torch through the lens at an angle and look through the other side. You're hunting for: fungus (spidery web-like growth — avoid, it spreads and can etch the coating), haze (a milky fog reducing contrast), separation (rainbow/oily patches where cemented elements delaminate), scratches, and excessive internal dust. A little dust is harmless; fungus and haze are not.
2. Test the aperture blades
Set the lens to its smallest aperture (on a body, or via the DOF preview) and watch the blades snap closed instantly and evenly, then open cleanly. Oily, sticky, or sluggish blades (oil on the diaphragm) cause exposure errors and need a service. The blades should form a clean polygon with no oil sheen.
3. Check autofocus and image stabilization
Mount it and test AF in good and low light — it should lock quickly and quietly without hunting or grinding. Listen for the AF motor; a loud grind or failure to focus means a dead/dying focus motor. If the lens has stabilization (IS/VR/OSS), enable it and confirm it steadies the view and you can feel/hear it engage.
4. Shoot a flat target for sharpness and decentering
Photograph a flat, detailed surface (a brick wall or a newspaper) square-on, wide open and stopped down. Check all four corners and the centre at 100%: one corner consistently softer than the opposite corner means the lens is decentered (often from a drop) — a real, hard-to-fix optical fault. Compare wide-open vs f/8 to gauge overall sharpness.
5. Work the rings and zoom
Turn the focus and zoom rings through their full range — they should be smooth with no grinding, gritty spots, or excessive play. On a zoom, check for 'zoom creep' (the barrel sliding under gravity) and that the barrel doesn't wobble when extended. A loose, rattly barrel suggests internal wear or a past drop.
6. Inspect the mount, contacts and filter thread
Check the metal/plastic mount for wear and that it seats tightly with no play. Clean and inspect the electronic contacts (dirty contacts cause communication errors). Confirm the front filter thread isn't cross-threaded or dented (a dented thread means filters won't screw on). Look for a clear rear element with no scratches in the critical light path.
Red flags — walk away if you see these
- Fungus (web-like growth) or heavy haze inside the elements
- Element separation (rainbow/oily delamination patches)
- Oily, sticky, or sluggish aperture blades
- Decentering — one corner consistently soft (often from a drop)
- AF motor grinding or failing to lock, or a wobbly/creeping zoom barrel
See camera lens listings on eBay → (affiliate)
FAQ
- How do I check a used lens for fungus?
- Shine a bright light through the lens at an angle in a dark room and look for spidery, web-like growth between elements. Fungus spreads and can etch the coating, so avoid lenses that have it.
- How do I tell if a used lens is decentered?
- Shoot a flat, detailed surface square-on and check all four corners at 100%. If one corner is consistently softer than the opposite corner across apertures, the lens is decentered — usually from a drop, and hard to fix.
- Is dust inside a used lens a problem?
- A small amount of internal dust is normal and has virtually no effect on images. The real problems are fungus, haze, element separation, and oily aperture blades — those degrade image quality or get worse over time.
These are practical buyer checks, not a professional appraisal. For high-value items, get an expert opinion before paying.