How much for a half decent polarizer

[Dave Humphries]

Most polarizers sold today are circular polarizers however I have seen el-cheapo polarizers sold on eBay which I am sure are linear models. If I were you, I would ensure that my polarizer was a circular model, not linear...

Do you mean they may be sold as CPL, but when delivered, say PL on the rim, or that they may have been printed with "CPL" on the rim, but still in fact be PL (linear)?

Which begs the question; does anyone know of a simple test that could be performed to differentiate?
(e.g. with a pair of polarising sunglasses or an LCD screen)

[dem]

Which begs the question; does anyone know of a simple test that could be performed to differentiate?

Sure. Stand in front of a mirror and look through the polarizer at its reflection. Then flip it back to front and look again. If you have not noticed any difference, then it is a linear polarizer.

EDIT. While looking through the polarizer with one eye, make sure to close the other eye. Otherwise you might miss the difference.

[dem]

Polarizers have two separate elements and moisture can get in between the two layers, especially in humid an moist environments, and I have definitely had one polarizer get ruined by moisture getting into it.

+1 to that.

It is worth mentioning that, unlike some "photography tutorials" tell, the two elements in the CPL DO NOT rotate with respect to each other when you are adjusting the angle. They are glued together in a specific orientation, with the polirizer grid at 45 degrees to the principal axis of the birefringent quarter-wave plate. Hence the problem with moisture dissolving the glue (manufactures prefer a fancier word "adhesive") and spoiling the CPL. As one would expect, this always takes place at the rim.

When I inspected the polarizer, I noticed a small area near the center that had some sort of distortion that was impervious to being cleaned.

It is probably a mechanical fault such as a knock on a table corner, a tripod or something hard. The front of CPLs is usually tougher than the back. The back can be easily damaged inside a bag if the polariser is not stored properly. I managed to ruin a CPL by throwing two filters in a side pocket. The metal rim of one filter ended up rubbing against the glass of the CPL and left a mark.

[Mike Buckley]

It is probably a mechanical fault such as a knock on a table corner, a tripod or something hard.

I suppose anything is possible. Even so, I'm very careful with my equipment and don't remember any incident like that ever happening. The distortion on the filter also didn't have the look of anything being knocked. It had the look of something very thin that was on the surface and could be cleaned, though cleaning both surfaces changed nothing.

[Tringa]

Sure. Stand in front of a mirror and look through the polarizer at its reflection. Then flip it back to front and look again. If you have not noticed any difference, then it is a linear polarizer.

This is interesting Dem. Just tried it with my polariser and saw no difference which suggests it is a linear polariser. In one respect this is not surprising as this B&W polariser is between 40 and 50 years old, when I think linear polarisers were more common. However, I use it with my DSLR without any problems, which suggests it is a circular polariser.

Dave

[Mike Buckley]

Dave,

Most but not all digital cameras are adversely affected by a linear polarizer. When they are affected the exposure and the auto focus could be off. If the exposure is off and if you review the histogram, you could compensate for the exposure. If you tend to use manual focusing, you eliminate the potential problem of the auto focus being affected. In short, it's possible (and in your case probable) that indeed you are using a linear polarizer.

The following is another method of determining whether a polarizer is circular or linear: "Hold the polarizer about 2 or 3 inches in front of your eye, and look at your reflection in a mirror. Looking from the camera's viewpoint (i.e. with the filter threads pointing towards you) you should see the image of your eye in the mirror (as if looking through a neutral density filter). Now turn the filter around so that the filter threads point at the mirror. If the polarizer is linear you should see the same thing that you saw before, but if it is circular it will appear black and you will not see the reflected image of your eye." That's excerpted from this.

[Manfred M]

Dave - the reason that circular polarizers are required in many modern cameras is that they employ technology that they can use technology that is quite similar to what a polarizing filter works.

On my Nikon cameras, light is directed to the phase detect autofocus mechanism using a hinged secondary mirror that sits behind the primary mirror. This means that parts of the primary mirror are semi-transparent to let some of the light through to the secondary mirror. One of the ways that this is accomplished is to etch very fine lines on the primary mirror to let the light through, in effect this is a polarizing device, so if you add a linear polarizer into the optical path, limited light will get through to the autofocus mechanism. If any other areas of the camera use linear polarizing elements, the camera will be fooled.

This type of technology has been used in SLR cameras for many years, as early as the 1960s. Leica used this type of secondary mirror design in the Leicaflex SL with the light metering mechanism. I believe Canon used a semi-silver fixed mirror in one of its designs in around the same time period, so circular polarizers have been required for quite a few decades.

[DavidAllen]

Just remember to post up some of the results on here!!

Hah! I've seen many of the photos posted here & believe me, they are way out of my league.
I don't want you all to desperately search for something positive to say about any photo I post - I know they're all pretty poor.
As I say, I don't aim for competition winners or compliments, I just enjoy taking pics and post processing for my own amusement.

[Donald]

Hah! I've seen many of the photos posted here & believe me, they are way out of my league.
I don't want you all to desperately search for something positive to say about any photo I post - I know they're all pretty poor.
As I say, I don't aim for competition winners or compliments, I just enjoy taking pics and post processing for my own amusement.

I look back at the first photos I posted on here back in 2009 and ... oh dear.

Posting up that first one is by way the hardest. First pic I ever posted (not on here) got no responses whatsoever. Was ready to give up after that.

People searching for something positive to say won't do anything to help you. Nor will people who just negatively criticise without offering any suggestion as to how it might be improved (thankfully, we rarely see the latter on here). Giving and receiving constructive criticism can be hard, but it is one of the most valuable ways of learning.