A good polarizing filter is probably the single most useful filter for color photgraphy, and is still useful in black and white work. If used properly, it can reduce reflections from shiny surfaces, heighten sky contrast, improve foliage contrast by removing flare and double as a 2x ND filter for some situations (and two stacked linear polarizers can also make an infinitely variable ND filter).

Polarizers come mostly in two types - linear and circular polarizers.

Most normal light has a mixture of polarizations that is mostly pretty random.

Linear polarizers selectively remove that part of the light that is not polarized in the same plane as the filter. What remains and passes through to the camera lens is mostly polarized in the same plane as the filter is oriented. (that is very similar to an red filter for B/W that removes mostly green light, passing primarily red light on to the lens).

Circular polarizers are really two filters stacked one behind the other. The filter furthest from the lens is a regular linear polarizer. Immediately behind that (in the same mount) is a "quarter wave plate" oriented at 45 degrees from the polarizer. That converts the "linear polarized" light coming back from the front polarizer to "circularly polarized" light that contains all polarizations again. Just how it does it is complicated, but I can post a more in-depth article if interested.

Reflected light is partially polarized in the process of being reflected, whether it is from an object in the image that you are trying to photograph, or reflected from the beam-splitting mirror in your DSLR that allows autofocus/metering. That means, in theory at least, that feeding the camera with linearly polarized light might interfere with light metering and/or autofocus, while feeding it with circular polarized light should not. Because of this, the normal dogma is that you need a circular polarizer on a DSLR to allow autofocus and metering to work properly. In actuality here, I have found little or no problem with linear polarizers (which are often better, cheaper and less prone to other artifacts including color casts). That has been true with istD, istDS, K100D and my current K20D.

The simplest test for whether a polarizing filter is really and truly a circular polarizer is very easy. Look through it into a mirror. This is a Hoya PL-CR filter that is first held in the normal orientation (male threads toward the camera lens or my eye). Light is reflected off the mirror and passes back through. There is some darkening (about 1-2 stops with each pass through the filter) but the camera is clearly visible in the reflection. There is very little color cast visible.



Now reverse the filter. This is the same Hoya PL-CR filter with the male threads pointed to the mirror, away from the camera. Now the mirror image of the filter is very dark, making it harder to read the logo and much harder to read the lens lettering..



(Just why that happens is fairly complicated. Again, if there is interest I can post a more in-depth explanation).

If you try the same test with a good linear polarizer, there is absolutely no difference on reversing it - the image looks identical, and with minimal color cast.


Now look at a cheap Visico CPL circular polarizer bought off ebay. This is in normal orientation, male threads to camera lens.



And this is the Visico CPL reversed, female thread to the lens.



The first thing that is obvious is the lack of darkening when the filter is reversed, but there is also a definite yellow color cast in both orientations. This "circular polarizer" is only circular in shape, not in type!

This holds true for other filters I have acquired over time, some with lenses, some on their own. The Hoya 67 and 77 mm PL-CR filters are true circular polarizers with minimal color cast. The Hoya 49 and 52 mm PL are true linear polarizers, again with very little color cast. In contrast, each of the cheap Asian filters from Visico or no-name from Korea fail this simple test, both obviously as true circular polarizers and also by introducing unwanted color casts.

Try it with your own filters and see which others pass the test!

[gofour3]

That's only true for automatic shutter speeds longer thsn x-sync. At sync speed and above, the camera is setting its exposure via the reading it takes just prior to pushing the button.

Yeah I'm not sure about the flash sync speed 1/75 (x) and a LP, as I've never used a polarizer filter at that shutter dial setting or with a flash.

I don't shoot my LX in manual mode and only "automatic", so the only time I've used a polarizer filter is then. I also don't own a CPL, so all my various sized LP filters work fine on the LX in that mode with no metering issues.

Phil.

[Wheatfield]

Yeah I'm not sure about the flash sync speed 1/75 (x) and a LP, as I've never used a polarizer filter at that shutter dial setting or with a flash.

I don't shoot my LX in manual mode and only "automatic", so the only time I've used a polarizer filter is then. I also don't own a CPL, so all my various sized LP filters work fine on the LX in that mode with no metering issues.

Phil.

It's not just at 1/75 sec, it's from x sync on the dial up to 1/2000 senond. The OTF metering only works below x sync, at least so I recall.

I've finally broken down and have bought new circular polarizer filters for my newer lenses. Up until recently, I used whatever filter I had in the size I needed. Mostly linear, as my large filters had been bought for my 6x7 lenses.
When I'm using a polarizer, I am probably also using a tripod, and shooting a fairly static subject. I honestly can't say if I've ever tried using AF with a polarizer. The way I shoot, manual focus is often required.
When I shot film with the LX, I did use circular polarizing filters. I worked for Astral Photo at the time and used their house brand filters. They had a really well deserved poor reputation, but the company changed vendors at some point and the first run of filters we got were quite good.
Subsequent runs were more typical of them.

[gofour3]

It's not just at 1/75 sec, it's from x sync on the dial up to 1/2000 senond. The OTF metering only works below x sync, at least so I recall.

OTF metering only works in the "AUTOMATIC" aperture priority metering mode (at any shutter speed in the metering range of 125 sec to 1/2000) and that's when the half mirror with the reflective surface flips up when the shutter is fully pressed. (So a linear PL is OK)

In manual mode this half mirror is used to get the metering reading you need to adjust the shutter/aperture, so if you use a linear PL the reading will be messed up.

Phil.

[tibbitts]

I'd also test even name-brand filters, especially if they don't come from a well-know source (Adorama, B&H, etc.) I have one Hoya filter that I believe may not be genuine, and Hoya indicated that they have experienced counterfeit copies.

[Wheatfield]

OTF metering only works in the "AUTOMATIC" aperture priority metering mode (at any shutter speed in the metering range of 125 sec to 1/2000) and that's when the half mirror with the reflective surface flips up when the shutter is fully pressed. (So a linear PL is OK)

In manual mode this half mirror is used to get the metering reading you need to adjust the shutter/aperture, so if you use a linear PL the reading will be messed up.

Phil.

Yup, that's correct. Above the sync speed the OTF metering is not able to adjust for changes in lighting during exposure. That function is disabled at speeds where the shutter curtains are not fully open. At speeds above sync, the exposure time is set by reading off the front curtain only.