To test a used lens, inspect the glass and body, mount it on a camera, and shoot a few frames at different focal lengths and apertures. Doing this in person, with your own camera body, takes about fifteen minutes and shows most common faults. Bring a lens cloth, a torch and a memory card.
Before You Mount It
- Look at the outside. Dents on the filter thread or hood mount suggest a drop.
- Shine a torch through the lens from both ends. Look for dust, scratches, fungus (thin web-like lines), haze and separation at the edges of glass elements.
- Turn the focus ring and zoom ring through their full range. They should move smoothly, without grit, sticking or loose play.
- Check the mount for scratches, bent pins or worn metal, and that the rear cap and contacts are clean.
- Open and close the aperture by looking into the lens at different settings, if the lens has a ring. Blades should be dry, not oily, and should move cleanly.
On the Camera
- Attach the lens and confirm the camera recognises it with no error messages.
- Try autofocus on a near subject and a far one. Listen for grinding or unusual noise and watch for hunting.
- Switch stabilisation on and off if the lens has it, and listen for a smooth hum rather than rattling.
- Take sample photos at the widest and narrowest ends of the zoom, using a wide aperture and a mid aperture.
- Photograph a flat, detailed surface such as a page of text on a wall, and check both centre and corners at full size.
- Point the lens at a bright window to look for flare and odd colour casts.
What to Look For in the Pictures
Compare the left side of a frame with the right side. If one side is much softer, an element may be out of alignment, which is a sign of a past knock. Check that autofocus lands where you expect, and that manual focus reaches both infinity and the closest distance.
Condition Guide
| Finding | What it can mean | Action |
|---|---|---|
| Small dust specks | Common with age | Usually fine |
| Web-like lines | Possible fungus | Walk away or ask for a lower offer |
| One soft side | Possible misalignment | Avoid |
| Stiff or gritty rings | Wear or dirt | Ask about servicing cost |
FAQ
Is a scratch on the front element a deal breaker?
Not always. Light marks on the front often have little effect on pictures, but deep scratches can cause flare and loss of contrast. Judge by sample photos taken into bright light.
What if I can only buy online?
Ask for clear photos of the glass, the mount and the rings, and ask the seller what return terms apply. Check the lens as soon as it arrives, using this same list.
The Short Version
Check the glass with a torch, move every ring, try autofocus and stabilisation, then shoot sample frames and compare the corners. Walk away if you see fungus, misalignment or damage the seller cannot account for. A calm, methodical check protects you from most bad buys.
