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I've bought a CPL lens 77mm and a 77-55 step down adapter, but the problem is the step-down adapter is not fitting on the lens. The CPL fits fine on the step down but the step-down is not fitting on the lens, could some one point out what I'm missing?

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    \$\begingroup\$ It should work. Check the lens for a small dent around the filter thread (unless the step down thread itself is damaged). \$\endgroup\$
    – xenoid
    Sep 22, 2023 at 19:30
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    \$\begingroup\$ How is it not fitting? Are you sure it is a 75mm-55mm step down ring? Please describe how it is not fitting in more detail. \$\endgroup\$ Sep 22, 2023 at 22:09

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If you are adapting 55mm filter threads on the lens to a 77mm filter you need a step UP ring; 55mm male to 77mm female.

If you bought a step DOWN ring it will have 77mm male threads and would fit onto the front of the filter; but the 55mm threads will be female and unable to mount onto the lens... this should be fairly obvious visually. You could adapt it with an additional 55mm reversing ring (55mm male-male)...

If you actually do have a 55-77mm step up ring and it won't mount; I would guess the threads or roundness are slightly off. Cheap aluminum rings are notorious for this, and for getting jammed on the lens/filter due to poor tolerances/machining... quality brass rings are far better.

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    \$\begingroup\$ Also, even if they could somehow get it to work: With the CPL turned around backwards it would only act like a linear polarizer. Light entering the lens must have passed through the 1/4 wave plate after passing through the polarizing material for it to work as a circular polarizer. \$\endgroup\$
    – Michael C
    Sep 23, 2023 at 16:54
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    \$\begingroup\$ @MichaelC True, but that wouldn't matter with the (mirrorless) RF cameras. \$\endgroup\$ Sep 23, 2023 at 17:54
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    \$\begingroup\$ That all depends on the AF technology used. Anything with dual pixel type AF is still based on Phase detection. \$\endgroup\$
    – Michael C
    Sep 24, 2023 at 21:15
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    \$\begingroup\$ @MichaelC; most modern mirrorless use PDAF to some degree; that's not why re-polarization is needed by a DSLR. It's needed because the DSLR uses a beam splitter (pellicle mirror) to divide the light between the AF module and viewfinder; based on polarization. Without re-polarization it is probable that one or the other will receive a greatly reduce amount of the available light. \$\endgroup\$ Sep 25, 2023 at 0:08
  • \$\begingroup\$ These comments miss the bigger point: a CPL filter (that's turnable) turned around backwards (using M-M or F-F threaded adapter) won't work, because the optical filter itself won't be turning; it's only the original lens thread ring that would rotate, while the filter glass itself doesn't move. \$\endgroup\$
    – scottbb
    Oct 27, 2023 at 4:14

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