Re: Circulars versus linears Polarising filters

["Peter Jon White" <pjwhite@tiac.net>]

> It is my understanding that some cameras (Leica R's) have to use circular
> because of their design in relation to metering, where others use linear because
> of their design.
 
> teg grant

Greetings,

A circular polarizer filters light of a certain polarity, (depending 
on it's orientation), and then randomizes the polarity of the light 
that hasn't been filtered. So the light that makes it through the 
filter isn't polarized, but light that entered the filter of a 
certain polarity has been removed.

A linear polarizer filters light by it's polarity and passes the 
light through as is. The light passing through is polarized light.

Light reflected off of a glass surface is polarized. The surface 
reflects more light of one polarity than the other. It acts in the 
same fashion as a linear filter. So any internal meter than measures 
light reflected off a mirror is reading polarized light, whether or 
not you have a polarizing filter on the lens. 

For instance, my Canon F-1 has an angled semi-reflective mirror in 
the focusing screen to divert light to the meter. That light is 
highly polarized. It has bounced off the reflex mirror, and the 
mirror in the focusing screen.

If I use a linear polarizer on the Canon F-1, point the camera at an 
evenly lit white wall and take a reading while rotating the filter, 
the reading will change, even though the total amount of light 
passing through the lens isn't changing at all. The meter is only 
receiving light of one polarity, because of the mirrors in the camera 
body. When the polarizer on the lens is filtering light of the same 
polarity that the mirrors are passing through, I get a very low light 
reading. When the polarizer is filtering light of the opposite 
polarity to light that is passed by the camera's mirrors, (in other 
words, when the filter and the mirrors are both passing through the 
same light rays), the meter is able to make an accurate reading of 
the light that would reach the film.

So when the filter is removing one polarity of light, and the 
camera's mirrors are removing the other, there's not much left to go 
around!

An M6 doesn't need a circular polarizer, because the light reflected 
off the painted white disk on the shutter curtain isn't polarized.

Peter Jon White