If you hang around vintage camera circles long enough, you eventually run into the dreaded "F" word: fungus. It is the absolute bane of film shooters and vintage lens collectors everywhere. There is nothing quite as heartbreaking as scoring an incredible deal on a classic piece of glass at a flea market, getting it home, and realizing the inside elements look like they are growing a small science experiment.

Most of us know that fungus can sometimes be cleaned. You crack the lens open, hit it with some hydrogen peroxide and ammonia, and wipe it away. But there is a darker, more permanent side to this problem that a lot of people do not talk about until it is too late. I am talking about etched fungus damage.

Let's talk about what happens when that little fuzzy spiderweb decides to leave a permanent mark on your vintage glass, how to spot it, and what it actually means for your daily photography.

What is actually going on inside your lens?

To understand etching, you have to understand what lens fungus actually is. Dust and moisture inevitably find their way inside older, unsealed lenses. Vintage camera gear was meant to be used, and using it means exposing it to the elements. Spores floating in the air settle on the glass elements inside the lens barrel. When you put that lens away in a dark, slightly humid environment—like a leather camera bag sitting in the back of a closet for twenty years—those spores wake up.

Fungus needs something to eat. Unfortunately, it absolutely loves the organic compounds found in vintage lens coatings, and sometimes even the adhesives used to glue certain lens groups together. As the fungus feeds and grows across the glass, it naturally secretes an extremely corrosive acid.

If you catch the fungus early, you can often just wipe it off. The glass underneath will be totally fine. But if that acidic secretion sits on the glass for years, or even decades, it quite literally eats through the anti-reflective coating and directly into the glass itself. That permanent scarring is what we call etching.

Surface fungus vs. etched glass

The trickiest part about buying older lenses is that surface fungus and etched fungus look basically identical when you first inspect the lens. Both show up as those familiar, sprawling, branch-like patterns that look a bit like frosty roots or lightning bolts stretching across the glass.

The difference only really reveals itself when someone finally opens the lens to clean it. You can scrub at etched fungus all day with the strongest optical cleaners available, but the marks are not going to budge. That is because etched fungus isn't something sitting on top of the glass anymore—it is the physical absence of glass and coating. The acid has carved a microscopic trench into the element. Trying to wipe away etched fungus is like trying to sweep away a pothole in your driveway. The damage is baked in.

How to inspect your lens like a pro

You do not need fancy equipment to check your lenses for fungus or etching. All you need is your phone and a dark room. Here is my exact process when I am evaluating a new vintage pickup to see what kind of shape the optics are natively in.

Step 1: Use a strong, single-point light

Turn on the flashlight on your phone. Regular room lighting is way too diffuse to show internal glass flaws. You want a bright, harsh beam of light to punch right through the lens elements and illuminate every speck of dust, haze, and fungal growth.

Step 2: Open the aperture completely

Set the lens to its widest aperture so the internal aperture blades are tucked away. If you are inspecting a lens that has an automatic aperture pin on the back, gently press the pin down so the blades open up. You need a clear, unobstructed tunnel right through the center of the lens.

Step 3: The angle is everything

Do not just point the flashlight straight down the barrel and look directly into the light. That will just blind you. Instead, hold the flashlight behind the lens, shining through it, but look through the front of the lens from a slight angle. Move the lens around in your hands. Tilt it back and forth.

What you are looking for is anything that catches the light. Dust will look like tiny isolated white specks. Cleaning scratches will look like thin, sweeping spider silks. Fungus will look like a distinctly organic, branching network.

If you know the lens has already been cleaned, but you still see faint, ghost-like outlines of those branch patterns when the light hits it just right, you are looking at etched glass. Sometimes etching does not look like distinct branches; if the fungus was dense and clustered, the etching might just look like a cloudy, dull, permanent patch where the coating has been entirely eaten away.

Does etching actually ruin your photos?

This is where I tell you to take a deep breath. Finding out your lens has permanent etching feels terrible, but it does not mean the lens belongs in the trash. In fact, you might not even notice it in your images.

Cameras do not "see" the surface of the lens the way your eyes do when you are staring at it with a flashlight. Depending on where the etched element sits in the optical formula, light might just scatter slightly as it passes through.

If the etching is minor, the impact on your photos will be nearly zero. You might lose a tiny, microscopic amount of contrast in heavy backlit situations, but in everyday shooting? It will look identical to a pristine copy. I actually have an old 50mm lens with terrible scarring on the front element, and it is still incredibly sharp.

However, if the etching is severe, thickly clustered, or located right on the rear element (which sits closest to the film plane or sensor), the results will suffer. Bad etching diffuses the light violently. Your images will look washed out, colors will seem muddy, highlights will bloom out of control, and you get severe flaring every time you point the camera anywhere near a light source. Sometimes people enjoy that low-contrast, dreamy look for portraits, but it gets old pretty fast if you just want a normal, clean photograph.

Stopping it from spreading

If you have an etched lens, the most important thing is to make sure the fungus is actually dead. The etching is just the scar, but if the active fungus is still alive in there, it can technically release spores and spread to the rest of your gear.

Leaving the lens out in direct, bright sunlight (without a camera attached, to avoid burning your shutter curtain!) for a few hours is a great way to let UV rays neutralize the active fungus. Make sure to store your equipment in a well-ventilated, dry place. Skip the dark camera bags for long-term storage and use a transparent plastic bin with lots of silica gel packets.

When it is time to move on

Sometimes a heavily etched lens just isn't worth shooting anymore. If the contrast is totally gone and every photo looks like it was taken through a dirty fish tank, it might be time to retire that specific piece of glass to the display shelf. The incredible thing about vintage gear is that there is always another beautiful copy out there waiting for a new home. If you're tired of fighting cloudy optics and want to grab something that will give you bright, punchy, contrast-rich images right out of the box, you should absolutely check out the selection available in the shop. You can browse through a massive, rotating inventory of thoroughly checked manual focus lenses to find the perfect clean replacement for your setup.

Don't beat yourself up if you end up with an etched lens. It is a rite of passage for every vintage camera enthusiast. Learn how to spot it, test how it shoots, and keep moving forward. Happy shooting!