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None-vignetting Cokin polarizer
Posted By: fried_bread, 05-18-2014, 01:04 PM

The Cokin P series circular polarizer vignettes badly on sigma 10-20mm f4/5.6 at 12mm and below . ( Well to be honest a chinese version - The design is pretty much the same ) . This is due to the filter being 6.5mm thick with the smallest diameter aperture ( 72mm ) being furthest from the lens . - More intrusive than the holder .

Some initial experiments showed that the K-r Auto-focus and exposure seemed to work OK fine with a linear polarizer . So a sheet of Linear polarizing material was purchased ( sheets are sold for testing glass for stress etc on ebay etc ) and cut down to an 87mm square . This will then slip into the rear slot of the cokin holder ( A bit of tape will help prevent it slipping out ) - A holder like Cokin P W/A HOLDER ( BPW700A) minimizes vignetting .
The filter can be rotated with the cokin holder . This may produce vignetting depending on the rotation angle . This can be helped by the modification below...

Read more at: https://www.pentaxforums.com/forums/62-do-yourself/262479-reducing-cokin-p-vi...#ixzz32645J000

The effects on image quality appear small ( possibly because the material is thin ) and because there is only a thin item in the rear slot an additional standard cokin filter can be used in the front slot , ( but a graduate filter is not likely to be compatible with the holder rotation!)

(Also the rear slot could be used to hold any suitably sized colored sheet or clear acetate with gel etc and an additional filter in the front slot ! )

I've only tested this on a Kr - other models may suffer problems with auto-focus / exposure with a linear polarizer!




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