Fungus (?) inside RF 70-200 F4 L

Started 3 months ago | Discussions thread
Karl_Guttag Senior Member • Posts: 1,152
Re: Fungus (?) inside RF 70-200 F4 L

Sinawang wrote:

Karl_Guttag wrote:

Please, don't get stress out. It is not that bad (yet).

First, ALL lenses have dust in them, and they will get more dust with time, even the best L lenses in the world. You can't have lenses that move in and out and seal against all dust. Also, note that waterproof is not the same as vapor proof. No lens is mold spore proof as mold spores are very tiny.

The good news is that if the dust and other imperfections are not near the sensor, they will not appear in the image. Note that even if the dust spot is on the sensor filter (just before the sensor), you can't see them until you stop down, generally to f16 or smaller. I don't think it happens today, but back when Canon used to say that a few bubbles in the glass were "normal" even for L lenses and would not degrade the image quality.

To a first approximation, all parts of the lens contribute to all parts of the image. So even if that mold spot were a black dot, you probably would not notice it even if you shot a picture of a white object. All the spot does is dim the whole image by the percentage of the light that is blocked (which is probably much less than 1%).

For a very simple but dramatic and simple example, see this video at the point linked to here:

Is the point you trying to show me in that video is at the 8:35 time mark?

Yes, Sorry, when inserting, there was an extra period at the end of the link, so instead of starting at the right time, it started from the beginning.

The correct link:

The thing to note is how much of the lens he could block off, and it still worked to form the same image and not one with a dark spot or a line through it. This is an extremely simple example with one lens element and you can't block off so much with a camera lens.

If it didn't work this way, you would have to constantly clean your lens. In fact, many experts will tell you not to clean the lens so much as you will wear away the coating with constant cleaning. If you get a big smudge on it, it will affect the image. Also, you might want to shine a flashlight through from the camera side all your lenses and see how much dust in them that you never knew was there. I just did that on my new RF100mmf2.8 macro and it has dust in it and it is not even a zoom lens.

See for example:  and or do a search on "does dust in a camera lens matter."

If you think about it, as the f-number gets smaller for the same focal length, the lens gets bigger (compare the 70-200f4 to the diameter of the 70-200f2.8). The lens is "collecting light." The rays coming through the outer part of the lens must get bent the most are thus are the hardest to focus. Thus when you stop down, you will notice that not only does the depth of focus increase, but the whole image gets sharper even in the center.

It is also why when you stop down, you don't get a dark circle on the outside of the image. In fact, the image uniformity improves.

To a first approximation (not absolutely true), every part of the lens contributes to every part of the image unless blocked by the aperture. You are cutting out by stopping down the light rays from the outer part of the front elements.

Also, it appears that this is occurring on the outer part of the lens, which primarily contributes to the image only at the wider apertures so that it will be even more out of focus/invisible if it has any effect at all.

It is kind of like you scratched the outside of the lens. It does not make you feel good, but it is not hurting the image significantly. Take some pictures of a white sheet of paper or some other uniform white object at various apertures and see if you can see it.

That said, since you live in a humid climate, you don't want it to get worse. You could get a nasty surprise someday when you put a lens out that you have not used for a long time. Also, mold can eat at the lens coatings and not be reversible via cleaning. You definitely want to take precautions regarding Humidity and UV light to kill/control the mold.

Even if you were to kill all the mold in the lens today, you would get more tomorrow. All lenses have microscopic mold spores in them. If they get water vapor, then they will grow. You have to keep the humidity down and kill them with UV (sunlight counts) light.

Thanks. Lesson learnt!

 Karl_Guttag's gear list:Karl_Guttag's gear list
Canon EOS R5 Canon RF 15-35mm F2.8L IS USM Canon RF 24-70mm F2.8L IS USM Canon RF 70-200mm F2.8L IS USM Canon RF 24-240mm F4-6.3 +13 more
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