Canon EF 500mm f/4 L IS - inside barrel wall material

[Kayaker72]

but it doesn't look anything like fungus from what I saw online and also the elements are all clean and I can't even spot a spect of dust.

As I was reading, I was thinking you had lens fungus/mold. But you checked for that. Also...elements clean. Have you taken a shot or two, even around the house? How do they look?

I have the Mk II version. There are felt like materials in between each element against the sidewalls. I suspect what you are seeing is the material itself or if you see something sitting on it, perhaps the felt doing its job and trapping dust.

[Sinh Nhut Nguyen]

As I was reading, I was thinking you had lens fungus/mold. But you checked for that. Also...elements clean. Have you taken a shot or two, even around the house? How do they look?

I have the Mk II version. There are felt like materials in between each element against the sidewalls. I suspect what you are seeing is the material itself or if you see something sitting on it, perhaps the felt doing its job and trapping dust.

I had chance to compare a friend's 500 f/4L IS and his showed the same characteristics. The right word I should have used is "sparkly" when a bright light is shined on them. Lens performed flawlessly. I missed an opportunity to buy a 70-200 f/2.8L IS II for $800 two weeks ago because that lens sat unused for years and the AF stopped working so I became paranoid with all my unused gears.