There seem to be some confusions about what and how a polarizer can affect a camera's operation. A simplistic short course on the subject ...
What is polarized light? Polarized light is light in which all the light rays are vibrating in parallel planes. What is a polarizing filter? A polarizing filter is an optical grate that allows passage of light rays which are vibrating in parallel alignment to the grate. Light rays that are at an angle to the grate are absorbed in proportion to how far out of alignment they are, ideally until if the light ray is at 90 degrees to the grate it is entirely absorbed. There are several kinds of things that "naturally" polarize light, most common of which are flat sheets of glass and mirrors, prisms, and like specular reflecting surfaces (like the surface of still water in a pond). The use of a polarizing filter to remove a great deal of specular reflection from objects like this is dependent upon this natural property of light - polarization through reflection. What is a linear polarizer? A linear polarizer is a simple optical polarizing grate. Light passing in either direction through it is polarized on exit. What is a circular polarizer? A circular polarizer is a filter assembly comprised of an optical grate followed by a quarter wave plate. The notion is that the grate polarizes the light on entry, then the resulting polarized light is depolarized on exit so as to be non-polarized on exit *but minus all the light rays that were not parallel to the optical grate on entry*. Circular polarizers are directional: light rays passing in the correct direction through the filter is not polarized on exit. Light rays passing through the filter in the wrong direction is polarized on exit. Modern SLR and other cameras often use mirrors and prisms in the mirror box and elsewhere as 'light pipes' to metering and focus sensors. In such cameras, the natural polarization through reflection can cause the sensors to have incorrect inputs which causes aberrant behavior. For this reason the circular polarizer was invented: it does the job that was desired of a polarizing filter ... to remove the depolarized component of a light source in a scene ... without leaving the light polarized as it entered the light pipes to AF and metering sensors behind a lens. Most Pentax SLRs that I'm familiar with (MX, *ist D through K10D) site all their exposure sensors behind the focusing screen. This means that linear polarizers generally do not affect their exposure readings because the focusing screen acts as a scattering diffuser, automatically depolarizing the light that passed through it. However, most AF SLRs (and I think this includes the Pentax DSLRs) have a light pipe which includes a secondary mirror along with a semisilvered part of the main mirror to direct some of the incoming light to a contrast comparator sensor ... if you used a linear polarizer on these bodies and AF operation, the combination of polarized light and the light pipe can cause aberrations in the AF operation. IN the Luminous Landscape discussion, I believe what they are referring to when they say "manual metering" cameras are cameras for which you are making exposure meter readings external to the lens/ viewfinder system and setting a set of exposure values manually. In this case, your meter readings may be off because, if you are using a polarizing filter, the polarizing filter can absorb a variable amount of light based on its minimum absorbtion density and the amount of depolarized light in the scene that it is absorbing. This can vary from 1.5 to 4 stops worth of light, depending on the scene and the orientation of the polarizing filter combined. Most older cameras, particularly manual focus models, do not require circular polarizer filters. But as a point of convenience and to take care of possibilities that might arise, it is probably sensible to only buy circular polarizers these days. The premium add-on cost is relatively small and that way you know you won't have to worry about buying another filter if you buy another camera which requires it for proper operation. Godfrey On Sep 2, 2007, at 2:36 PM, Rebekah wrote: > You know, I'm not really the person to ask. As to personal > preference, I just bought a linear polarizer (the rotating kind) > mostly because both of my lenses are manually focused but also because > I like the idea of being able control the polarization. ($.99 on Ebay > woohoo!) > > I don't know which cameras are through the lens or manually metered, > so I'm sure everyone else can fill you in on that point. > > rg2 > > > On 9/2/07, Glen Tortorella <[EMAIL PROTECTED]> wrote: >> Rebekah, >> >> Thank you for the link. What, specifically, is meant by the term >> "manual metering camera"? All of my cameras are "manual" cameras, >> but all have light metering systems--i.e. center weighted or the >> newer TTL multi-segment (as in my ZX-M). Thus, the author's caveat >> about compensating when using a "manually metered camera" is somewhat >> unclear to me. >> >> What filter or filters did you decide upon? Do you tend to prefer >> ones that can rotate or ones that are non-rotating? >> >> Glen >> >> On Sep 2, 2007, at 4:53 PM, Rebekah wrote: >> >>> Glen, >>> >>> I looked this up the other day when I was trying to decide on >>> circular >>> or linear, and there's mention at the bottom of the page that you do >>> indeed need to compensate, but only if you are using a manually >>> metering camera: >>> >>> http://www.luminous-landscape.com/tutorials/polarizers.shtml >>> >>> rg2 >>> >>> >>> On 9/2/07, Glen Tortorella <[EMAIL PROTECTED]> wrote: >>>> Can anyone confirm what Igor suspects here? Is the metering in the >>>> ZM-M affected by a linear polarizer? With the A-series lenses, the >>>> ZX-M uses TTL multi (2)-segment metering. Prior to Igor's >>>> comment, I >>>> had never heard that metering could be affected by a linear >>>> polarizer. >>>> >>>> Thanks, >>>> Glen >>>> >>>> On Aug 29, 2007, at 1:24 PM, Igor Roshchin wrote: >>>> >>>>> >>>>> Glen, >>>>> >>>>> In general, it is not just the autofocus but also metering >>>>> that may be affected by a linear polarizer. >>>>> I know that on a ZX-5n, the metering IS affected by a linear >>>>> polarizer, >>>>> and I suspect it is the same story with ZX-M. >>>>> I hope somebody on the list will be able to confirm this. >>>>> I do not know about the other, older cameras. >>>>> >>>>> Having been looking for polarizers myself, I found a shop that >>>>> seems to have a reasonable price for various filters: >>>>> http://hvstar.net/ aka besteastern on e-bay. >>>>> I was not able to find much of additional external feedback about >>>>> this >>>>> store beyond that on e-bay. >>>>> >>>>> Igor >>>>> >>>>> >>>>> >>>>> Wed Aug 29 13:03:17 EDT 2007 >>>>> Glen Tortorella wrote: >>>>> >>>>> Hi All, >>>>> >>>>> I am looking for a polarizing filter for my cameras. I have a >>>>> ZX-M, >>>>> a KX, and a Super Program. Would a linear polarizing filter be a >>>>> good choice for these? The circulars tend to be more >>>>> expensive, and >>>>> it does not seem I would need this type since I am using older, >>>>> manual focus cameras. I am thinking of purchasing the Hoya Purple >>>>> Label linear... -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net
