Found an old PLC filter in my bag. Unused for years (git it for the kit lens of my first DSLR). There is strange spot on it, apparently under the front glass:

Situational awareness: this is the big spot slightly above and left of center:

enter image description here

Close up:

enter image description here

Is it easily fixable (not a very expensive filter anyway, so ready to throw it away...)?

Also, are there good sleeves to carry filters, that protect them from shocks, don't tke an enormous space, and can be fitted with some silicagel (if this is a way to prevent fungus)?

  • \$\begingroup\$ Have you tried cleaning it? What did you try and did it look any different after than before? Can you feel the spot with your finger? \$\endgroup\$
    – Michael C
    Nov 16, 2021 at 8:30
  • \$\begingroup\$ Do you mean CPL (circular polarizing filter)? PLC filters are electrical power line filters to eliminate noise and allow power line communication. \$\endgroup\$
    – Michael C
    Nov 16, 2021 at 8:35
  • \$\begingroup\$ @MichaelC Yes, meant CPL, and yes, although I couldn't feel it with my fingers, it turned out to be cleanable. I still wonder how this thing came there, maybe it's fungus anyway? Also the question about what a good filter box/sleeve looks like still holds. \$\endgroup\$
    – xenoid
    Nov 16, 2021 at 9:51
  • \$\begingroup\$ I keep most of my filters stacked in a cottage cheese tub with index card separators. \$\endgroup\$
    – dandavis
    Nov 18, 2021 at 4:24

1 Answer 1


It looks like dirt/dried mud to me. It does not have the characteristic "spiderweb" look of typical lens fungus.

enter image description here

I've always carried my CPL in the square plastic case that it came in.


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