Circular vs Linear Polarizer on m4/3 Camera

Charley123 wrote:

I know (or think I know) that a mirrorless camera can use either type of polarizer. Is that correct?

Correct. Linear PL filters have a stronger polarization effect too. But they are hard to get nowdays, because DSLR's are the vast majority of cameras out there and DSLR's need circular PL filters. Often you find a better quality CPL off the shelf at a lower price than a PL. If you really want a linear PL, you likely have to have it ordered it in. If you have to order it, you can of course choose more than only PL or CPL, there are several options like a less strong PL that has less neutral density (better to shoot indoors in poor light)..

I personally use CPL on my M43 cameras, simply because they are easier to get (you never get a discount on a filter that has to be ordered in for you). Also, I suspect many brand name linear CP filters offered by small retailers are very old stock left over from the film SLR days.

Is it the linear or circular that is rotated to polarize? I don't recall.

Both of course need to be rotated. By rotating you adjust the strength of the polarizing effect.

Which would do a better job for image quality on a mirrorless camera?

- PL filters do not last forever. They perish. Moisture enters from the edges between the two glass plates and saturates the embedded plastic polarizer film, which then becomes "cloudy" (=poor contrast and grey veil). Even the clear glue used to bond the film to the glass plates may detach. Kasemann filters have weather sealed edges and will last longer. Having the sealed Kasemann edges increases filter life and makes storage in humid climate easier. For this reason, I would not recommend to buy a PL used or a very old stock bargain. I have recently seen on ebay new Heliopan PL filters at a third what they normally cost - I was tempted but passed, because it could be either fakes, or badly stored perished old stock. By the way, unlike other filters, PL filters can fade too - the film does not like being exposed to very strong light for prolonged periods.

- Anti reflective coating is important on a PL filter. A PL filter has a total of 6 surfaces (two glass-to-air, two glass to embedded film, plus 2 surfaces of the film itself). Of all filters, PL filters deteriorate the IQ the most (and all pola filters do to some extent deteriorate IQ, even the best, but cheap ones can be really really bad - the worst ones for IQ are variable ND filters, which are nothing else tan a PL and a CPL in tandem). That is why you should spend the most you can afford on a PL filter. A PL filter costs considerably more to manufacture than a normal filter. If you compare its price to any other filter, it is perfectly reasonable for it to cost about 3 times more for the same quality (a normal filter is just one optically flat and coated glass disc and a plain metal ring, but a PL filter is two optically flat glass discs plus a film disc plus the extra work to embed the film, plus a rotating metal ring preferably made of brass so it rotates smoothly).

- some PL filters have a color cast (they are not a neutral grey, but for example brownish). It depends on quality of polarizer foil used. You usually get what you pay for.

Could I rotate a polarizer on a lens that has a bayonet mount lens hood?

Sometimes yes, but on long cylindrical shaped hoods you cannot reach inside. You take the hood off to rotate the filter, then put it on again if really needed (I mostly use PL without the hood for convenience).