polarizers for manual focus camera

[bernard_korites]

I have a Nikon FM10 which is manual focus but has TTL light metering. I want to

use a polarizing filter with it. The issue is whether or not the linear

polarizer I already have will work ok with it or if I need to get a circular

polarizer. My understanding is a linear polarizer on a camera with an internal

beam splitter will mess up the autofocus (which I don't have) and the light

metering (which I do have). So I guess the question comes down to whether or

not the FM10 uses a beam splitter in its light metering system. Does anyone

know?

[cameradude]

Linear polarizers are for manual focus systems. Circular poloraizers are for AF systems. I just read the meter and adjust accordingly. The meter will show that its a little bit darker with the polarizer on there. Try it out, see if it works.

 

I could be wrong, but this is with the lenses and polarizers I have F3, the manual camera, and D80, and F4. Linear polarizer stays on the lenses when I use the F3, circular stays on the F4 and D80. My experience is not with the FM10. Good luck.

[ShunCheung]

A linear polarizer can interfere with the AF mechanism. That is why we should use circular polarizers on AF bodies, but on manual-focus bodies, you can use either type. Linear polarizers tend to be less expensive.

[bernard_korites]

What got me going on this question is BH lists a circular polarizer as an accessory for the FM10 while Adorama lists a linear one. But then, in the description of the linear polarizer, Adorama says:

 

"Circular -- for all types of cameras: required for beam splitting metering systems commonly found in auto-focus SLR's and in most current TTL Slr's.

 

Linear -- for older metering systems. "

 

Various filter web sites say you need a circular polarizer for any camera that uses a beam splitter for light metering. Apparently the beam splitter causes double polarization with a linear polarizer and light metering may be innacurate.

 

I guess the question boils down to whether or not the FM10 uses a beam splitter for light metering.

 

So far I haven't had many problems other than the occassional under or overexposed shot. Now I'm starting to wonder if that was due to my error or the linear polarizer.

[richard_cochran]

The FM10 doesn't use a beam splitter for light metering. In this respect, it is like most (but not quite all) manual focus cameras that have TTL metering.

<p>

You can verify for yourself that a linear polarizer doesn't distort the meter reading on this camera. Point your camera at a source of light you know to be unpolarized. An ordinary light bulb will do just fine. Mount a linear polarizer and rotate it as you watch the meter. You'll see that the meter reading doesn't change. A tripod may help you hold the camera steady and eliminate changes due to moving the camera.

<p>

If you try that experiment using a typical autofocus camera with a beam splitter in its light metering path, you'll see that the meter reading changes depending on the orientation of the polarizer. But since the source is unpolarized, the film (or digital sensor) will always receive the same amount of light regardless of the polarizer orientation. If the meter indicates widely different exposures based on the polarizer orientation, some of those meter readings are wrong.

[erie_patsellis]

the FM10 doesn't use a beamsplitter, but the F3 does, and a linear will, at certain angles cause metering errors, from what I've read. I use a 72mm B+W POL-C that I've had for years on my F3's and F4, the F too, but it's meterless..

 

erie

[bernard_korites]

Now I'm really confused.

 

I tried Richard's experiment above. I aimed the FM10 with Nikon 45/2.8 lens and linear polarizer at an incadescent goose neck lamp using a tripod. The FM10 uses a 3 diode metering system; over, correct, under exposed. I aimed the camera such that the exposure was correct with the under expose diode just about to come on. When I rotated the linear polarizer, the under expose diode came fully on the went off as I continued to rotate. The diodes seem to be one f stop apart so I think the difference in exposure readings as I rotated the polarizer was a fraction of an f stop, maybe 1/4 or so.

 

I tried it again with a halogen lamp, same results.

 

Thinking the source was perhaps was slightly polarized due to reflections off the lamps bulb shield, I tried it again with a plain incadescent bulb, no shield. Same results.

 

I tried the same experiment with my Sony 717 digital camera for which I have a circular polarizer and the meter reading did not change. The Sony's meter is a lot more sensitive that the FM10's since it can indicate 1/5 of an f stop whereas the FM10's diodes are 1 stop apart.

 

A quarter of an f stop or so isn't much, especially when shooting outdoors, but it does indicate there is some interaction with a linear polarizer and the FM10's metering system. What the metering error would be when shooting polarized light outdoors rather than unpolarized incadescent light is a question for someone who understand more about these metering systems than I do.

 

If you check various web sites of filter makers and filter sellers you will find about as many opinions on this issue as web sites.

 

I even called Nikon's technical support. The gentleman I spoke with said a linear polarizer would be ok with the FM10 even though it clearly isn't.

 

So I would say, if you are shooting slides where exposure is important, and you're using a polarizer, unless you know exactly how your camera's metering system works you would be safer using a circular polarizer.

[cameradude]

A polarizer blocks light.

 

You need to understand that when light is blocked, it will be darker.

 

That is why your camera is telling you to open up your aperture.

 

Sometimes one can analyze why he/she was put here on Earth. Other times he/she can just ignore those thoughts and live.

 

My point is to stop analyzing, shoot a roll of slide film with your polarizer, and see what happens, make a sacrifce and take a risk. You might lose about $20 when you see your slides under/over exposed, who cares, you learned you did it wrong.

 

Carpe Diem!

[bernard_korites]

Robert-

 

<You need to understand that when light is blocked, it will be darker.>

 

I am aware that polarizers change the amount of light entering a camera.

 

This discussion is about a second order effect namely the interaction of linear polarizers with split prism TTL light meters giving false indications via double polarization. Reread the posts above, especially Richard's and my response.

 

Any input you have that would shed light :) on this issue would be welcome.