I just found out that I got hosed for about $12 several years ago, lol. I don't use a polarizer filters as much as I should. With that in mind, I bought a very cheap one for use on a particular lens several years back. It is labeled 'OEC' brand, CPL, 67mm. I tried it quickly at the time and took a few distant shots over water. I stuffed it in my bag and never used it.
Fast forward to a rainy day today. I grabbed the filter and took a few shots out the back door. Ugh. Terrible autofocus and underexposed. I wondered if it was not really a circular filter.
I never really learned what the difference was between a circular and linear polarizer filter so I did some googling. I found that a circular polarizer has an additional layer (on the camera side). In overly simplified terms, it re-scrambles the light so our meters and autofocus don't get confused. So the first two layers do the polarizing job, the final layer mixes things up a bit for the camera sensors.
I found this test to see if a filter is really a circular polarizer:
Find a mirror. Close one eye and hold the filter in front of your other eye, looking into the front of the filter. (male threads pointing at mirror). Look through the filter and observe it's reflection in the mirror. If the reflection of the filter glass looks black, it is a circular. If its looks clear, it is a linear filter. The direction you look through the filter is important, male threads point to the mirror. My OEC filter fails the test. I have a Hoya PL_CIR filter of a different size. It passes. Also no focus or exposure problems with the Hoya.
Don't buy a cheap filter, or at least test it out in front of a mirror.
Fast forward to a rainy day today. I grabbed the filter and took a few shots out the back door. Ugh. Terrible autofocus and underexposed. I wondered if it was not really a circular filter.
I never really learned what the difference was between a circular and linear polarizer filter so I did some googling. I found that a circular polarizer has an additional layer (on the camera side). In overly simplified terms, it re-scrambles the light so our meters and autofocus don't get confused. So the first two layers do the polarizing job, the final layer mixes things up a bit for the camera sensors.
I found this test to see if a filter is really a circular polarizer:
Find a mirror. Close one eye and hold the filter in front of your other eye, looking into the front of the filter. (male threads pointing at mirror). Look through the filter and observe it's reflection in the mirror. If the reflection of the filter glass looks black, it is a circular. If its looks clear, it is a linear filter. The direction you look through the filter is important, male threads point to the mirror. My OEC filter fails the test. I have a Hoya PL_CIR filter of a different size. It passes. Also no focus or exposure problems with the Hoya.
Don't buy a cheap filter, or at least test it out in front of a mirror.