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I have a Nikon D7500 and it came with a cheap Xit "pro series 0.43x high definition wide angle 55mm lens". The lens cap that fits the Xit is 65mm diameter (Ø65 is printed in the cap). I'm trying to find a polarizer for it but I can't find any online that are 65mm.
This is my first DSLR so I'm still learning how it all comes together.
Can anyone help me understand what size polarizer I need?
A cheap 0.43× wide angle converter will not warrant buying a polariser for. The polariser would be more expensive than the converter (and the converter gives a comparatively low quality image).
Additionally, the main use case of a polariser is to remove light of a particular polarisation. However, a wide angle scene more often than not features a flurry of different polarisation grades and directions. You'll get reflections and dispersions removed at different levels throughout the scene; for example, showing a darkened blue of the sky that considerably differs in darkness across the scene.
65mm threads are not a standard thread size for lens filters. This means you'll have a very difficult time finding a thread-on filter (any filter) for your wide-converter.
There are slide-in filters (e.g. Lee Filters, Cokin, etc.) that slide into an adapter, but again, even the filter holder needs an adapter ring that matches the thread size of the lens and you don't have a standard thread size ... hence no pre-made adapter rings are available.
Lee Filters does offer custom-made adapter rings in any thread size ... but custom adapters are going to be expensive. You may prefer to eliminate the wide-angle converter and go with a lens that offers the focal length you want (and it would have standard thread sizes to accept filters).
It is also possible to get custom-machined step-up adapters for threaded filters offered in standard sizes. E.g. if you want to mount say, a 77mm thread-on filter onto your 65mm thread size, you buy a 65 to 77mm "step-up" ring.
Both step-up rings and slide-in filters have the benefit that you can buy one size filters (large enough to fit the largest lens you own) and then get step-up rings (or adapter rings for slide-in filters) for any smaller lens sizes you own (avoiding the cost of buying a circular polarizing filter in every lens size you might eventually own.)
I offer the "custom" adapters as a solution to your direct question (because you asked how to get a filter to fit wide-converter), but if it were my money, I wouldn't go with the custom solution. I'd get rid of the wide-converter (for a lot of reasons... quality, metering accuracy, focus accuracy/reliability, etc.). They are usually ... not great. Part of the point of owning a camera with detachable lenses is that you can get a lens to fit your needs rather than using one lens and use focal length adapters.
