Second-hand glass is where the value is. A lens that cost a small fortune when it was new can change hands a few years later for a fraction of that, and if it has been looked after, it will produce exactly the same photographs. The catch is that lenses age in ways cameras do not: cement can haze, coatings can scratch, lubricant can creep onto aperture blades. None of that shows up in a listing photo taken at arm's length. Here is what to inspect, what to ignore, and what to ask before you send anyone money.

Dealer or private seller?

This decision shapes everything else. A dealer charges more, but in the UK you get statutory rights under the Consumer Rights Act 2015: goods must be of satisfactory quality, fit for purpose and as described. Buy at a distance and you also have a 14-day right to cancel. Private sales are different. The lens must match its description, but there is no satisfactory-quality guarantee, so your inspection is the only protection you have.

Before travelling, ask the seller three specific questions: the serial number, whether the lens has been serviced or repaired, and whether it has ever been dropped or exposed to water. Clear answers are reassuring. Vague ones are not.

Dust, haze and fungus

Take a small torch and shine it through the lens from both ends with the aperture wide open. You are looking for three things.

• Dust. A few specks are normal on almost any used lens and rarely show in photographs, even stopped down.
• Haze. A milky film on an internal element, usually condensed lubricant. It flattens contrast and tends to spread.
• Fungus. Fine white threads, often fanning in from the edge like a spider's web. It etches coatings and can transfer to other lenses stored beside it.

A hazy or fungal lens is a project rather than a bargain. Only take it on if you know a repairer who will strip and clean it, and factor that work into what you offer. Do not keep it in the same bag as your other lenses while it is still infected.

Scratches, coatings and the rear element

Shallow marks on the front element look worse than they perform; a deep gouge or a chip is a different matter. Tilt the lens under a bright light and look for cleaning marks where someone has polished through the coating, which shows up as a dull patch rather than a line.

The rear element matters far more. It sits close to the sensor, so scratches, haze and dust there affect image quality much sooner. Check it first, and be wary of any lens whose rear glass has been cleaned aggressively.

While you are there, inspect the filter thread. A distorted ring means a filter will not screw in, and it often points to a knock.

Aperture blades and mechanical feel

Set the lens wide open, look through the front, and stop it down while you watch. The blades should snap across evenly and form a clean polygon. Watch for an oily sheen, which makes blades sluggish and eventually sticky, and for bent or misaligned blades, usually the result of impact.

Now work every control. The focus ring should be smooth, with no grit or tight spots. Zoom rings should move without creeping when you tilt the lens downwards. Switches for autofocus and stabilisation should click firmly rather than flop between positions.

Mount, barrel and signs of impact

Brassing on the mount is cosmetic and expected on a working lens. What you are hunting for is structural damage: dents, cracked plastic, bent flanges, loose or missing screws, or play between barrel sections. A wobbling zoom is a repair waiting to happen.

Look closely at the mount screws — if they are chewed up, someone has been inside. Check the tripod collar, the hood bayonet and the condition of the caps and hood, because replacing missing accessories adds to the real cost.

Autofocus, stabilisation and electronics

Test on your own camera body if the seller will allow it, and bring a memory card. Focus on something close, then on something at infinity, and listen for grinding or squealing. Fire a burst in continuous AF to check consistency.

Turn stabilisation on and off while watching through the viewfinder; you should see the image settle. Check the electrical contacts for corrosion or scoring, look for error messages, and confirm that both the camera and the lens accept the same mount and firmware. Third-party lenses sometimes need a firmware update to work properly on newer bodies, so check that it is available.

Warranty, returns and payment

A dealer warranty is worth having, but read what it excludes. Fungus, water damage and impact are commonly left out, and cover may be limited to mechanical or electronic faults. Keep the receipt and the original listing in case you need them.

Pay by a method that gives you some comeback. Credit cards offer protection on qualifying purchases, and debit card chargebacks exist; a bank transfer to a stranger offers nothing. If the lens arrives not as described, contact the seller promptly and in writing. If a dispute escalates, take proper advice — Citizens Advice or your local Trading Standards service is a sensible starting point.

The ten-point checklist

1. Serial number, service history and accident history confirmed in writing.
2. Glass checked with a torch for dust, haze and fungus.
3. Front and rear elements checked for scratches and cleaning marks.
4. Filter thread and mount inspected for dents and impact damage.
5. Aperture blades opened and closed slowly, watching for oil and uneven movement.
6. Focus and zoom rings tested through their full travel.
7. Autofocus, stabilisation and switches tested on your own body.
8. Mount screws, barrel play and accessories checked.
9. Warranty terms, return window and exclusions read before paying.
10. Payment made by a method that protects you, with the listing saved.

Test it properly in the first hour

However careful the inspection, the real proof is a few photographs. Meet in daylight somewhere public if you can, and shoot the lens wide open, at its mid aperture and fully stopped down. Point it at a plain wall to check for decentring, where one side is softer than the other, and shoot into a bright window to see how it handles flare and any internal haze.

Once you are home, check the images on a computer rather than the back of the camera, and give yourself a day or two to use it properly. A lens that seemed fine on a café table often reveals a stiff focus ring on a cold morning. If it does not match the description, act quickly: your rights and the seller's goodwill both have a shelf life.

Photo: jplenio / Pixabay