Circular VS linear polarizer

Hello there !

 

Quick question: How do you recognize a circular polarizer from a

linear polarizer. A friend of mine gave me a used polarizer but he

couldn't tell me if it was circular or linear.

 

Thanks !

 

Fred

Not sure whether this works universally, but hold it up to your eye. A linear one will polarise whichever side you look through. With a circular one, only when you look through the side that the lens would look through.

Hold it in front of a mirror, and try to see if you can see

light going through the polarizer, bouncing off the mirror,

and coming back through the polarizer again. If the "camera side"

(male threads) is against the mirror, the circular polarizer won't

pass light, though it will pass some light if you flip it so

the camera side is away from the mirror. A linear polarizer

will pass the same amount of light regardless of which

way it's flipped.

Great answer guys ! :-) Looks like it's a circular.

 

Thanks !

 

Fred

I believe that another way of determining the type of polarizer you have is to mount it on a lens attached to a SLR with through the lens metering. If it is a circular polarizer, the exposure determination should not vary as you rotate the polarizer. I am assuming here that the camera is metering an object that does not reflect polarized light in a preferential manner, such as a neutral gray test card, a brick wall, etc. In fact, it is to ensure that through the lens metering on SLRs works accurately that it is recommended that circularly, rather than linearly, polarized filters be utilized.

Bill's method will only work if the camera has a beam splitter

in its metering path, thus requiring a circular polarizer for

accurate metering. In other words, it'll work with most

autofocus cameras, but it'll fail with most manual focus

cameras, though there are exceptions both ways.

<p>

Before about 1980 or so, nobody marketed a circular polarizer

for photographic use; they were all linear. Yet SLRs with

TTL metering have been around since at least the '60s, and

they all worked fine with linear polarizers, regardless of

orientation.

<p>

All of my

cameras' meters are unaffected by the polarization of light

hitting them, whether its linearly polarized or circularly

polarized, so on my cameras, that test would falsely

indicate that every polarizer is circular.

I tried this experiment yesterday with my trusty Canon F1-N, which dates from the early 1980's. Rotating the circularly polarized filter in front of the lens produced no variation in the camera's indicated exposure reading. Rotating the linearly polarized filter did, with the variation being plus or minus one stop, i.e if the correct exposure should have been f11 @ 1/250, the meter would indicate

an exposure reading between f8 @ 1/250 and f16 @ 1/250, depending on how the linearly polarized filter was oriented relative to the camera's beam splitter. The indicated exposure with the circularly polarized filter attached was f11 @ 1/250 with no variation with polarizer rotation.

<cite>depending on how the ... filter was oriented

relative to the camera's <b>beam splitter</b>.</cite>

<p>

Of course. I'm not so familiar with the Canon F1-N, but any camera

with a beam splitter in the metering path will show

variation depending on how a

linear polarizer is oriented. That's the major reason that

circular polarizers were widely marketed to the

photographic community. My only point is that there are many

SLRs with TTL metering which don't have beam splitters, and

which therefore won't show the effect.