Used lenses: how to check for fungus, dust, and scratches before purchase

Every week, lenses arrive at the counter that owners swear are perfect. I inspect them under raking light, and in one case out of five, I find something: a film of fungus on the edge of the second element, an aperture blade that sticks, or a scratch the seller had never seen because they were checking the lens in the wrong backlit position. Nobody is usually lying; they simply don't know where to look. A lens is inspected like an engine, with a fixed procedure and always in the same order.

Here is the procedure we use in our lab, adapted for those buying from a private seller or an online listing with nothing but a phone torch. I’ll explain what to look for, under what light, and, most importantly, which defects actually affect your photos and which are purely cosmetic. Because the point isn’t to find a flawless lens, which doesn’t exist at 15 years old: the point is to figure out how much of a discount the item in front of you deserves, and when it’s better to turn around and walk away.

What you need to inspect a lens (and it’s not an optical bench)

90% of the defects that matter are visible with a 15-euro flashlight and a white wall. Optical benches are used to measure decentering and MTF, not to decide whether to buy a used 50mm for 180 euros. If someone tells you that you cannot evaluate a lens without professional tools, they usually have something to hide.

LED torch, white wall, and five minutes of good light

The complete kit fits in your pocket. Here is what you really need:

• 200-500 lumen LED torch, with a concentrated beam. Your phone's flashlight in torch mode works too: most recent smartphones sit around 50-80 lumen, enough for a 50mm lens, less so for a telephoto with ten elements to pass through.
• A uniform white background: a wall, an A4 sheet, or your phone screen displaying a white page. This is needed for the backlit inspection, when you look inside the lens against the light.
• Low ambient light. Not pitch black: a dimly lit room. At the counter, we turn off the fluorescent lights and use only the torch.
• A clean microfiber cloth, to determine if that spot is external dust or something under the glass.

The key point is the angle. Hold the flashlight at approximately 45 degrees to the optical axis, never frontally. Raking light strikes the internal surfaces at a shallow angle, making everything protruding from the glass plane visible: filaments, dust particles, and coating halos. If you point the flashlight straight into the lens, the light reflects back into your eyes and you see nothing. It’s the difference between looking at a floor at noon and looking at it with the low sun at 7 PM, when every scratch stands out.

Why a test camera body is needed

Three categories of defects are invisible with the lens detached: the actual aperture speed under electronic control, autofocus, and electrical contacts. A lens may have perfect glass elements yet fail to communicate with the camera body because a contact is oxidized.

If you buy from a private seller, bring your camera body. If the seller doesn't have a compatible one and you can't bring yours, treat that lens as untested: deduct 15-20% from the price or walk away. This isn't nitpicking, it's arithmetic — an aperture service starts at 120 euros.

The inspection order: from the outside in

Five steps, always in the same order:

1. Exterior and threading: mount, filter thread, signs of drops on the barrel.
2. Front element: raking light at 45 degrees, looking for scratches and coating wear.
3. Internal group in transmission: lens pointed at a white wall, aperture fully open.
4. Rear element: same technique as the front, but with more attention.
5. Mechanics and aperture: dials, blades, barrel play.

The order is not aesthetic. It goes from the defect you see in three seconds to the one that takes two minutes. If you find stripped screws and misalignment on the mount, you're done: that lens has been opened by someone and it makes no sense to spend another eight minutes on the glass. Better to find out now than after spending half an hour negotiating.

Fungus: how to identify it and when to discard the lens

Out of every ten lenses that arrive at our workshop from private sellers, two or three show traces of fungus. Almost always, the seller is unaware of it, because looking at the lens straight on under a ceiling light reveals nothing. Fungus only becomes visible with raking light, and at that point, it is evident even to an untrained eye. It is the defect that causes the most concern in the used market, and it is the only one that can worsen on its own while sitting in a cabinet.

Appearance: spiderweb filaments, not dots

The fungus appears as thin, white-grey branches starting from the edge of the lens and growing towards the center. The typical shape is that of a star or a coral branch: a main filament that divides into smaller filaments, with a clearly organic pattern. It is never regular, never geometric.

Comparing it with other defects helps avoid misdiagnosis:

• Dust: isolated, round specks scattered randomly across the field. No connection between one speck and another.
• Fungus: interconnected filaments that branch out, originating from a point on the edge.
• Cement separation: iridescent halos or spots with sharp edges between two glued elements, often with purplish reflections. No filamentous structure.

Where it lurks: lens edge, internal group, cemented elements

Moisture accumulates where air circulation is lowest, so fungus almost always starts at the edge of the element, hidden under the retaining ring. In long telephoto lenses and zooms with eight or nine optical groups, it develops in the central area of the optical scheme, where no one looks because it is difficult to illuminate.

To do this: remove the rear cap, set the aperture to fully open, and hold the lens in backlit mode against a white wall while rotating it. Then, repeat the same check with a flashlight held at a low angle against the front element, viewing from the side opposite the light source. Together, these two tests cover almost the entire optical scheme.

Active fungus or dead residue: how to tell (and why it changes everything)

An active fungus feeds and continues to grow; in doing so, it produces acids that etch the anti-reflective coating. A trace that has been cleaned in the past remains visible but will not worsen further. The difference is visible at the edges: the translucent filament, which appears to sit on top of the glass and reflects light like a haze, is surface material. If, however, the edges are sharp, matte, and almost pencil-drawn, the coating has already been etched, and cleaning will not reverse it.

There is also a risk that almost no one considers: spores migrate. A lens with active fungus, stored in a bag with two other lenses, will contaminate them within a few months if the environment is humid. If you buy a used item with suspicious traces, keep it out of the bag until you have verified its condition.

How much does cleaning cost and when it's not worth it

Disassembly with peroxide and ammonia-based cleaning costs approximately between 80 and 200 euros depending on the complexity of the optical scheme, and on a zoom lens, re-collimation must be taken into account. On an 80-euro 50mm f/1.8, it makes no economic sense. On a 70-200 f/2.8, it does, and often by a significant margin. In our lab, every used lens undergoes raking light inspection before being listed in our catalog: those with active fungus or etched coatings are not put up for sale, period.

Internal dust: how much is actually needed to ruin a photo

It happens all the time: someone brings me a lens, holding it up to the light like a bottle of wine, to show me three white dots between the elements. The question is always the same: how much does it ruin my photos? In most cases, the answer is zero, and it’s not just a commercial courtesy. It’s optics.

Why dust is not sharp in any photo you will take

A speck of dust resting on an internal element is located several centimeters from the sensor, on a plane that the lens does not focus on and never will. It does not produce a defined shadow: it produces a minimal diffusion of the light passing through it. The measurable effect is a loss of transmission of fractions of a stop and a contrast dispersion so low as to be indistinguishable from measurement noise.

The case of dust on the sensor is different, as it sits exactly on the image plane. There, a single grain is enough to see a distinct gray spot, which becomes evident when stopping down to f/11 or f/16 against a uniform sky. When a listing mentions "spots in the photos," the culprit is nine times out of ten the sensor, not the lens.

The practical threshold: when contrast truly drops

The threshold exists, but it is much higher than what is often claimed in Facebook groups. For a noticeable drop, you need widespread and uniform haze, not countable spots: typically lenses with over 20 years of use in dusty environments. Here is the relevant overview:

• 5 to 20 isolated spots: no detectable impact, even in backlight.
• Dense dust but only on the first group: negligible impact, the front group is the farthest from the focal plane.
• Diffused milky haze across multiple groups: here, contrast drops and blacks lift in backlight.

The test takes thirty seconds. Point the lens toward a window or a streetlight, look through the rear, and observe whether the light passes through cleanly or if a milky haze forms. If you have the camera body, take a shot with the sun just outside the frame and compare the blacks: a gray veil covering the entire frame is the real sign.

Zoom and pumping: why they take in more air

Extending barrel zooms function like pneumatic pumps. Every extension from 18 to 135 mm displaces air volume, and that air enters along with whatever is present in the environment. An 18-135 or a 24-105 with five years of use and internal dust is a structural norm, not a maintenance defect on the owner's part.

Negotiation must be handled with honesty. Asking for a 30% discount on an extending zoom lens because it "has dust inside" is asking for a discount on physics. However, if the seller presents it as a serious anomaly, they are misdescribing their item: in that case, it is worth verifying the rest of the equipment more carefully.

Scratches: front, back, and anti-reflective coating

It happens almost every time: someone points a lens toward the light, sees a line on the front glass, and rejects it. Then the same lens, mounted, produces images indistinguishable from a flawless example. The front scratch is the defect that scares people the most and weighs the least. The real problem, when there is one, is almost always on the other side of the lens.

Front lens: the most visible and least damaging defect

A 2-3 mm scratch on the front element of a 50mm lens is not visible in photos. The surface is so far out of focus relative to the sensor that the damaged portion of glass merely subtracts a minimal fraction of light, less than 1% of the usable area at full aperture. What truly changes is the behavior in backlit conditions: with a strong light source inside or just outside the frame, the scratch scatters light and produces a localized veil of flare, resulting in contrast loss in the area near the light.

You test it with two shots: one with the sun at 45 degrees out of frame, and one with the light in the frame. If the second one holds up, the scratch is cosmetic. The interesting point is economic: the used market discounts a scratched front element by 25-35%, much more than it technically deserves. It is an honest negotiation lever, and for those who work almost always with the light behind them, it is one of the smartest purchases you can make.

Rear lens: here position matters

The rear element operates just a few centimeters from the focal plane, so a defect there is optically much more significant. A 2 mm scratch on the rear element can appear as a visible artifact on point light sources, typically closing beyond f/8, when the beam narrows and concentrates on a reduced area of glass. On telephoto lenses, where the rear group is small, the effect is even more pronounced.

Inspection should be performed with the lens detached from the camera body, using a raking light from one side against a dark background. A scratch on the rear element is sufficient reason to walk away, unless the price is significantly below market value and the intended use is explicitly at full aperture.

Cleaning marks and worn treatment

Cleaning marks are fine circular lines left behind by years of cleaning with a dirty cloth or rough paper. They almost never affect optical performance. They serve as an indicator: they show that the lens was cleaned poorly and frequently, and whoever cleaned it that way probably didn't store it in a dry place either.

Worn coating, on the other hand, is immediately visible. Tilt the lens under a light and observe the reflection: a healthy coating returns a uniform color, either purple or green depending on the manufacturer. If transparent or whitish spots appear, the coating has worn off in those areas, causing a permanent increase in flare. It cannot be restored.

Real impact of optical defects on images

Aperture, focus and mechanics: the checks almost no one makes

At the counter, I've seen the same scene for years: the customer picks up the lens, points it toward the window, inspects the elements against the light and says "clean." Then they mount it, fire off a burst, and the first three shots are overexposed by two stops. The problem wasn't in the glass; it was in the aperture. The mechanical parts of a lens fail more often than the optics, cost more to repair, and aren't visible when looking inside the barrel.

The aperture test in 30 seconds

With the lens mounted on a DSLR, set aperture priority to f/16 and press the depth-of-field preview button. The diaphragm blades must close firmly, in a single motion, and reopen instantly upon release. Repeat this ten times in a row while watching the front element: if the closing action slows down or the blades remain partially open by the fifth or sixth cycle, oil has migrated from the control group to the blades.

Without the camera body, on lenses with a manual ring, close it by hand and inspect against the light. The key indicators:

• Dark streaks or shiny reflections on the blades: oil. The worst defect because it worsens over time.
• Blades that do not return to a uniform position: the aperture becomes irregular, not circular.
• Half-second delay in closing: already enough to ruin the first shots of every burst.

A dirty diaphragm on a 200 euro lens means it's a write-off. The repair starts at 120-150 euros and on many budget zooms requires a complete disassembly of the central group.

Focus and zoom rings: play, shutter count, hard spots

Rotate the focus ring slowly from infinity to the minimum distance. It must be fluid and constant: a hard spot halfway through indicates dried lubricant or a helicoid bent by a drop. The noise I call "sand-like" — a gritty rustling — is debris that has entered the mechanism, and it cannot be removed without disassembly.

For zooms, point the lens downwards without touching it. If the barrel extends on its own, you have zoom creep: on a used 70-200 it is almost physiological, but on a 24-70 with 3,000 shutter count behind it, it is a sign that someone dropped it.

Autofocus and electronic contacts

Focus on a high-contrast subject at one meter, then at five meters, then at infinity, moving back and forth three times. The AF must lock on without hunting. Then open the EXIF data of a shutter count: if the lens model or aperture is missing or incorrect, the contacts are oxidized. A constant front or back focus error of 2-3 cm can be compensated for with the camera body's microadjustment; if it changes with every distance, the optical group is decentered and microadjustment is useless.

Stabilizer: what to really expect from the "four declared stops"

The stated stops are measured according to the CIPA procedure, on a vibration table that replicates hand shake under standard conditions. In practice, a photographer shooting handheld after a day's work recovers two or three stops. The test: 50mm, 1/15 second, ten certified shutter counts with stabilizer active and ten without. If the count of sharp frames is identical, the unit is dead. Listen as well: a continuous hum or a metallic clack at power-off indicates a loose suspension in the OSS or IS group.

Field test: 10 minutes and five shutter counts that say it all

The visual inspection tells you what's inside the lens. Only the RAW file tells you if that stuff actually matters. A lens can look disastrous under a flashlight yet shoot perfectly, and vice versa: we've seen 24-70s that were aesthetically flawless but had an unrecoverable corner. That's why at the counter we always ask to mount the lens on a camera body and take five shutter counts. It takes just under ten minutes and settles every debate.

The five certified shutter counts

1. Flat wall at f/8, base ISO, tripod or stable support. Fill the frame with a uniform, bright surface. This is needed to check light uniformity and sharpness across the field.
2. Direct backlight, with the sun or a lamp just out of frame. Fungus and damaged anti-reflective coatings show up here: milky haze, collapsed contrast, spiderweb-like flare.
3. Point lights at night at full aperture. Streetlights at 50-100 meters, focus at infinity. Check the corners: marked green or purple fringing and "gull wings" on points are coma, not a defect — but if one corner is much worse than the others, the issue is mechanical.
4. Burst of 15-20 shutter count in aperture priority at f/11. If the exposure varies by one stop between frames, the aperture blades are slow. This is the test that saves you from the most expensive repair.
5. Same subject at 1 meter and at infinity, three shutter count per distance. Verify that the AF locks onto the same point every time and does not "pump" back and forth.

How to read results on a small screen

On the camera's display, you can only clearly see three things: backlit haze, inconsistent exposure in burst mode, and AF that fails to lock. Decentration, coma, and micro-softness require a proper monitor at 100%. Bring an empty SD card and ask to keep the files: a serious seller has nothing to hide, and if they object, you already have your answer.

For testing, a compatible camera body is required. In-store, we frequently use a Sony A6000 at 360 euros for the E mount, while for RF, the Canon EOS R100 at 375 euros or the Canon EOS RP starting from 520 euros are excellent choices; the latter is full frame and therefore unforgiving on the corners. If you are still choosing a body, our article on how to choose your first used mirrorless helps you estimate the overall cost.

The decentering test in 20 seconds

Place the camera perfectly parallel to a wall or a newspaper laid on the floor, f/5.6, focus in the center. Then zoom in on the four corners and compare them. A slight difference is normal in any lens. If one corner is noticeably softer than the other three, the optical group has taken a hit: repair requires a collimation bench and often exceeds the value of the used item. Leave that lens behind.

What that lens is really worth: defects and a reasonable discount

A 24-70mm f/2.8 with a 2 mm scratch on the front element is worth 10% less than a perfect one, not 40%. The same lens with a dirty diaphragm is worth zero, because the repair cost exceeds the residual value. This is where you decide if a listing is a bargain or a scam, and the difference doesn't lie in the defect itself but in the cost of living with it.

Flaws that justify a discount, flaws that justify a no

The classification we use at the counter is clear. Some defects are grounds for negotiation, others are reasons to end the conversation.

• 5-15% discount — light internal dust, front scratches under 2-3 mm, cosmetic marks on the barrel, missing lens hood, slightly rough zoom ring.
• 20-35% discount — inactive and localized fungus trace, barely visible adhesive separation on the edge of a lens element, noisy but precise autofocus, marked anti-reflective coating.
• No purchase — active and branched fungus, confirmed decentering from testing, oily or slow aperture, focus lock at end of travel, oxidized electronic contacts.

The last category allows only one exception: a symbolic price and a repair already quoted by a workshop, not estimated by eye. Disassembling and degreasing an aperture starts at €120–150 on a simple lens and easily exceeds €250 on a stabilized zoom.

The repair bill before negotiating

The rule I have applied for years is simple: the purchase price plus the estimated repair cost must remain at least 25-30% below the price of a healthy unit with warranty. If a healthy, warrantied 50mm lens costs 300 euros, one with a dirty aperture at 150 euros plus 150 for the repair is not a deal: it is the same price, with the uncertainty of what the technician will find once it is opened. Below that threshold, you are buying a lens. Above it, you are buying a project.

The second number to put on the table is time. A serious optical repair takes 3-4 weeks. If you need that lens for a job in ten days, the defective unit costs as much as an extra rental.

Mount and future: a perfect lens on the wrong system

A flawless lens on a mount you are about to abandon is a bad purchase, period. Before negotiating, ask yourself where your camera body will be in two years. If the answer is mirrorless, consider whether it makes sense to shift the budget to the body and reuse the optics via an adapter.

Today's catalog features three key references for this calculation. The Sony A7 at 400 euros is the most affordable route to full-frame mirrorless and is compatible with virtually any manual lens; I covered this in detail in the article on the used first-generation Sony A7. The Sony A7 Mark III at 752 euros adds autofocus and battery life of a different class. The Canon EOS R at 800 euros remains the logical choice for those already owning EF lenses: with the original adapter, they all work, including autofocus.

Defect, reasonable discount, and what to do

How to store a lens to prevent fungus from returning

A lens refurbished in a lab can develop mold within eight months if it returns to the same environment it came from. Spores are everywhere, even in household air: what determines whether they germinate or remain inert is the environment where you store the lens for the thousands of hours a year it is not in use. Proper storage is not a detail for perfectionists; it is the most cost-effective maintenance available.

Humidity below 50%: the number that matters

Mold on glass germinates above 60-70% relative humidity, in the dark, at temperatures between 15 and 30 degrees. Below 50%, activity stops: there is no need to drop to 30%; in fact, below 25%, lubricants and rubber seals dry out. The healthy range is 40-50%.

• Airtight box + regenerable silica gel: 25-40 euros per container, orange gel that turns green when saturated. It needs to be regenerated in an oven at 120 degrees every 2-3 months. A valid solution for up to 4-5 lenses.
• Electronic dry cabinet: 130-250 euros for 30-50 liters, maintains the set humidity without maintenance. If you have gear worth more than 2,000 euros, the cost pays off quickly.
• 10-euro digital hygrometer: without measurement, you are just guessing. Buy this before anything else.

Avoid: placing the cabinet against an exterior wall, in basements, garages, or on shelves above radiators. In those positions, humidity fluctuates between 40% and 80% throughout the day.

The closed bag is the worst place

Neoprene, polyurethane padding, and technical fabrics absorb water and release it slowly. A lens stored in a bag after a session at the beach or in the rain remains in a dark, humid environment for weeks: the ideal conditions for germination. After every outing, open the bag, remove the gear, and let everything air out for a couple of hours before storing it.

Never leave exhausted silica gel packets in your bag: once saturated, they release moisture instead of absorbing it. If you are considering a second-hand bag, keep this in mind as well — we discuss it in the guide to buying used camera bags and backpacks, where a musty smell is one of the first warning signs.

Caps, filters, and hoods: what actually protects

The lens hood absorbs front impacts better than any filter and costs you no contrast. A 12-euro UV filter in front of a 600-euro lens introduces reflections and flare that are visible in backlight: if you want protection, spend at least 40-50 euros on a serious multi-coated filter. The rear cap should always be mounted: the rear element is closest to the focal plane and most exposed to sensor residue and fingerprints.

What we do in our lab with every used lens

A lens that comes to us via trade-in doesn't hit the shelf the same day. It goes through the workbench, where we always repeat the same sequence of checks — the same ones I've described so far, but with the calm of someone who doesn't have to decide in five minutes at the seller's home. On a batch of twenty used lenses, on average two or three are sent back without even being put up for sale.

The step-by-step inspection checklist

The process is fixed and does not vary based on the item's value. A 50mm lens worth 90 euros undergoes the same steps as a 70-200 f/2.8 worth 900.

1. Internal optical inspection at oblique light. LED flashlight held at an angle, lens set to maximum aperture, inspected from the front and rear. We look for branched filaments, not dots.
2. Aperture diaphragm blades test. Twenty consecutive closures from f/2.8 to f/16 using the lever or stop-down on the body. If a blade lags behind even once, the unit is compromised.
3. AF and contacts check. Mounted on a camera body, focus moved back and forth ten times, contacts cleaned with a dry cloth. We listen to the motor: an irregular hum indicates a failing ultrasonic motor.
4. Test shot on a flat target. Textured wall, lens perpendicular, tripod, self-timer. This is used to detect decentering: if one side of the frame is soft and the other is sharp, the element is off-axis.
5. External cleaning and grading. Rubber, mount, rings. Then the condition stated in the listing, which is a description, not a star rating.

What gets a piece rejected at intake: active fungus, visible decentering, greasy aperture blades, play on the mount. The rest is sold with a full disclosure.

Why the 2-year warranty changes the calculation

Between private sellers, the price is lower, and that’s fine: if you know how to inspect a lens, a good deal still exists. But the risk remains entirely on your shoulders. The fungus that appears three weeks later, in your home, under the right light, is your problem — and you have no one to write to.

On certified used gear, the price difference is paid once and covers 24 months. This also applies to bodies we sell with lenses: a Canon EOS RP at 520 euros remains the most affordable full-frame option for those switching to mirrorless, a Fujifilm X-H1 at 650 euros is the only X-Trans of its generation with in-body stabilization, and a Sony a6500 at 600 euros still delivers serious tracking AF. All units are serviced using the same procedure.

A practical tip: when testing on a flat target at home, use a proper tripod. If you don't have one, we've written what to check on a used tripod before spending.

Frequently asked questions

Want advice before buying that lens?

If you have doubts about a purchase or need advice for your specific case, write to us. We offer free advice because we are photographers first, sellers second, and because a customer who buys the wrong gear stops photographing.

Three ways to get in touch:

• Contact form — response within 24 business hours
• WhatsApp at 342 362 3454 (messages only)
• Phone: 071 291 6347 (Mon-Fri 9-13/15-18, Sat 9-13)

And if you want to see what we have in stock right away, explore the used cameras available today: every camera is certified, comes with a 2-year warranty, and ships within 24-48 hours.

Article written by Matteo for the ReflexMania Team. Last updated: August 2026. Prices and availability refer to the time of publication.