Canon EF 100-400 L lens fungus attack

note – this is a Mk 1 version of the Canon EF 100-400 L

A working photographer uses their lenses all the time and probably never runs into this. I was into bird photography for a while, about eight years ago, and had the Canon EF100-400 IS L like every other wannabe bird photographer. In between now and then the field has separated the sheep from the goats – real bird photographers use longer primes, because the birds are always at the long end of any zoom. Or they use astro scopes on manual focus 😉

Lapwings landing
Lapwings landing

Anyway, I take time out from birds and photography, because life gets in the way, and I stow the lenses in a relatively cold room. A couple of years back I figured I’d take some long lens pics, and get greeted by this

Lens fungus. Nasty
Lens fungus. Nasty

which makes me curse. Mainly on the front element, though a starting spot on the inner element, which is part of the IS mech. The inner part is magnified by the biconvex front element. The spotty crap is on the inside of the front element, the fine filigree round the edge on the front of the front element.

Markus Keinath is probably right –  the classic way this grows is

In my view, glass fungus forms especially with rarely used items stored in leather cases. This often happens with gear passed on after someone’s died. The camera owner maintains his photographic equipment, and keeps everything neatly in the camera bag. Eventually for some reason he doesn’t take pictures any more, the camera is still packed away a few years , and then sold.

The frequency of the glass fungal occurrence I would describe as high – in unused lenses , microscopes , probably binoculars. At flea markets, many of the old glass optics are contaminated with fungus. Optical devices that were in use until recently , are rarely contaminated with fungus, unless they are often used in humid conditions.

Markus Keinath – any errors of translation are my own

So this lens was used in the field and Britain is not the driest country in the world, many of these birds were on coastal areas, and i stored it in the lens case in a drawer close to the floor. There’s not much more that I could have done wrong from Zeiss’s words on the subject. Oops

Fungus does well with:

Relative humidity of at least 70% (more than 3 days)
No or little airflow
Nutrients (textile lint, traces of grease, varnish, dust and dirt)
Temperatures between 10 and 35°C


Do not use containers made of leather, textiles or wood for storage.

my bad, though I observe Canon’s case for this is made of textile and leather, it’s plastic boxes and silica gel in future!

Noting most of the problem is on the front element, I figure it’s worth taking it to bits to at least try and kill the existing fungus so it doesn’t get worse. Kill or cure though – I could also convert the lens into £1200 worth of scrap metal. I could see if Canon will fix this, indeed if I could purchase a front element I would be a long way to a win. That’ll be £229, Sir. OTOH all that is money that I could otherwise put into a 400 L prime at £950 which is probably what I should have got in the first place, or a replacement s/h lens at £750. It’s a tough call. £229 to restore to original working order is not bad. Getting decent bird pics is not worth £950 to me…

Now I have experience of taking Canon lenses to bits, because I fixed my Canon EF-S 17-85 IS which had the infernal ribbon cable fault. The replacement cable is to be had for about £3 in ebay. It’s the day of swearing and cursing and never having a light strong enough and laying out a bazillion bits and trying to reassemble them that is hateful. I followed Martin Pot’s description successfully, though not without foul language and having to backtrack several times.

I wasn’t afraid of taking this apart, but the risk assessment is different. The 17-85 was knackered – I had nothing to lose. This lens still takes acceptable pics, but the fungus is only going to grow. I have something to lose…

Google is always your friend in trying to fix a lens – if only to find out how to take it apart, and where to stop. The handy guys at Lens Rentals showed me what I needed to know. The front element comes apart reasonably easily,

first these three
first these three

then these – press hard because I think there is some sort of threadlock

press hard and note these are JIS screws not crosshead. I got away without a JIS screwdriver 😉

You can now slide off the white bit to expose the front element mounting – I pinched this pic from Lens Rentals, hope they don’t mind too much.

take picture, mark how the lens goes relative to the body, then unscrew the screws in the middle of the white rollers and the small black screws
take picture, mark how the lens goes relative to the body, then unscrew the screws in the middle of the white rollers and the small black screws

Take pictures of the settings before unscrewing because it is clearly factory settable to take out variations in focus point. Removing the front lens meant I could get some isopropyl alcohol and scrub that barsteward. I used a tissue and IPA. Yeah, I know you should use lint-free this and that, but we aren’t into subtlety here. Fungus makes glue to stick to the food source, so you gotta scrub that baby, scrub hard. Yes, you do run the risk of scratching the lens, life is full of risk, if you don’t want the risk pay Canon to do the job 😉 Trouble is that’s over half the resale price of the lens and about half the cost of a s/h replacement. When I say scrub, you have to scrub hard enough it sounds like this

else that fungus is going to stay put. I don’t know why IPA has such a characteristic sound on glass. You aren’t going to get rid of all the fungus, but in my case the stuff inside was greatly reduced, which will mean less flare. The stuff round the outside was more persistent, and in the end i figured there was only so much I could hope to do. I wasn’t going to go as far as Carl Zeiss’s cigarette ash recommendation, because I dont’ smoke and haven’t got that sort of courage, solvents yes, abrasives, no.

So what did it look like after? This is about the harshest lighting I could get. Seeing fungus is all about the lighting and the angle of the lighting.

Most of thei nner spotty fungus gone, the filigree stuff round the edge is attenuated
Most of the inner spotty fungus gone, the filigree stuff round the edge is attenuated

I had to push it to pick up the filigree stuff, or hopefully the remains of it 🙂 The inside stuff can’t be seen – after cleaning I could only see it on the inside when I breathed on the inside surface of the front element (still removed) to clean off the residues of the IPA – when the condensation had gone I couldn’t see anything.

Spadger - straight crop from full frame. Shows 1/125s isn't enough to freeze him, but still a decent shot for a salvaged lens
Spadger – straight crop from full frame. Shows 1/125s isn’t enough to freeze him, but still a decent shot for a salvaged lens

So I pointed the reassembled lens at a sparrow on the feeder on a miserable grey overcast day with slight mist observable over more than 200 yards. One of the things that seems to have changed in the last eight years or so is that SLR cameras give decent pictures at higher ISOs which is something I need to learn to take advantage of and favour noisy and sharp over quiet and blurred. That’s a challenge for another day. The spadger isn’t perfect but he’s good IMO, 1/125 wasn’t enough to freeze the motion of his tongue.


Now that I know I can switch the front element for £229 I will take this out and shoot with it as is. If I feel I always struggle for contrast then I may indulge, though I then have the second-order problem of lining up the front element. There is some back and forth adjustment provided. There’s a world of difference between putting the original back exactly where it came from and realigning a new front element spending time in front of resolution test charts.

Brent geese, SWT Trimley Marshes
Brent geese, SWT Trimley Marshes (100% crop)

In theory the front fungus isn’t so bad if I stop down, But in the UK a EF100-400 is always used at the long end of the range and flat out 5.6, because the only reason I am using this is because I can’t afford the EF 500mm f4 L IS ;).

I took this for a spin round SWT Trimley Marshes for the flight shots. I discovered with my more modern camera body I can run 400 ISO and therefore occasionally stop down.



5 thoughts on “Canon EF 100-400 L lens fungus attack”

  1. Hi I have a Canon EF 100-400mm L with some minor mould on the front lens. Have looked at the Handy Guys link but cant see how the front lens is removed. There pictures in the description still show the lens in place. Could you give me some further advice as I would like to remove the mould before it worsens and hope to clean myself. Many thanks and this article was really helpful.

  2. I’ve added a couple of pics – you take off the silver trim and then the JIS screws underneath to be able to slide off the front white sleeve. Press hard on the JIS screws if you aren’t using a JIS screwdriver 😉

    Inspect the front element, which is set in a plastic housing with six screws. Three are in the white plastic rollers in angled slots. Take lots of pictures (I’ve thrown mine out since, which is probably a bit daft if I have to do this again) and somehow note which of the three ways it can go back. This is a factory adjustment of how far the front element is relative to the other bits. Presumably it matters to set it back exactly the same.

    ISTR the corrugated plastic trim can be gently pulled forward once you have taken the six (from memory) screws, three holding the rollers and the three set screws. The front element can now be eased out – it’s in a plastic mount but I didn’t dismantle any more than that.

    Strong light and a small magnet on the screwdrivers make this a little bit less hard.

  3. Cheers Richard,
    Will look at doing this soon and you have made it much clearer for me. Since you have done your lens did you have any reoccurring fungus? They say spores can hang about and fungus can come back.

    1. I think the spores are in the environment anyway. The key seems to be not keeping the lens in a damp, cool environment that means the spores get activated. And let’s face it, the point of getting a lens like this for me was to charge around nature reserves pointing at birds, these places are not free of fungal spores 😉

      I pulled this out and saw a spot on the middle of the lens but that was the old greasy finger and removed with a tissue. I angled it to the light and couldn’t see any regrowth, even on the inside part. So far so good, although the wettest part of the year is the second half round here. I’m cautiously optimistic – the dumb move on my part was stowing the lens in a bottom drawer in a virtually unheated room. I’ve moved all my lenses upstairs into a room normally heated in winter room now. It’s only been six months, however!

      1. Thanks Richard,
        I will make sure its stored correctly to prevent any further issues. Your help is much appreciated.

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