Linear Polarizer vs Circular Polarizer?

mike_d wrote in post #13856518
I thought the problem with a linear polarizer was that it interfered with the autofocus system.

Modern autofocus systems use mirrors that act as beam-splitters: most of the light is reflected to the viewfinder for metering and viewing, while the rest (typically 25%) is transmitted and then reflected by a secondary mirror to the autofocus sensor, which is in the camera body. With a polarizing filter attached, the ratio of reflected/transmitted light (fixed for non-polarized light) varies with the polarization plane orientation of polarized light, relative to the beam-splitter, causing the amount of light reaching the autofocus sensor to vary. This effect can be particularly troublesome if your front element moves with the lens, as it would cause the amount of light reaching the sensor to change while the lens focuses.

The same loss of light principles that affect the autofocus system can also affect a camera's built-in meter, and obviously this is never good. What use is a light meter if it receives the wrong amount of light?

Do you need a Circular Polarizer?

So, do you need a circular polarizer? If you have a modern AF SLR the answer is almost certainly "yes". yes. Check your instruction manual. Some older manual focus SLRs also require circular polarizers. The camera manufacturers should be able to tell you if you don't have an instruction manual, or the manual doesn't say. What happens if you use a linear polarizer on a camera that really needs a circular polarizer? Basically you run the risk of exposure errors ( +/- 1 stop might be typical). This may not be a problem for print film, but can be a disaster for slide film. Not all exposures may be wrong, but some will be. There can also be small autofocus errors in some cases if any of the lenses in the AF system are birefringent (polarization sensitive), which can happen if plastic lenses are used and they are under some stress. If you care about your pictures and your camera needs one, spend the few extra dollars and get a circular polarizer.

If you have an older camera and you are unsure, there is an easy test which can often indicate whether or not a camera needs a circular polarizer. Attach a linear polarizer to the camera and take a TTL meter reading off a blank wall illuminated by indoor (tungsten) lighting. Don't use any kind of shiny (reflective) surface. A carpet might also make a good target. Rotate the polarizer and see if the indicated exposure changes. If it does, you need a circular polarizer. The only problem with this test is that some cameras read out exposure only in full stop increments, which may not be enough to detect the effect of the polarizer on exposure. Though the camera may actually be changing it's exposure settings by almost a full f stop, the readout might not show it. Cameras with 1/2 or 1/3 stop readouts should clearly show

BTW if your canera doesn't need a circular polarizer, you can still use one and it will act just like a linear polarizer. Any camera can use a circular polarizer. The only downside to using a circular polarizer where only a linear polarizer is needed is that circular polarizers are a little more expensive than the linear variety.