Polarisation of Light and Camera Light Meters

Why is it that a TTL (Through The Lens) light meter on a camera will not detect the change in the intensity of light when a polarising filter is placed over the lens and yet if no manual adjustment is made, the film will be underexposed? WAG’s will be accepted…

“C’mon, it’s not even tomorrow yet…” - Rupert

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I’m a professional photographer, and in all my experiences a polarizing filter always affects my TTL meter readings about 2 stops.

I do have to say though, that all my polarizing filters are of the old “linear” variety due to the fact that I use them only on my older manual focus cameras. The new autofocus can’t use the linear ones and have to use the circular ones, so the drop in light coming through the lens may not be as great when using those… but I don’t have any experience using circular polarizing filters.

I just wanna know why a light meter won’t notice that a polarising filter has been put on… that’s what my photo teacher says anyway… how does a light meter work, anyway?

“C’mon, it’s not even tomorrow yet…” - Rupert

If you need a graphic solution, http:\ alk.to\Piglet

“I just wanna know why a light meter won’t notice that a polarising filter has been put on… that’s what my photo teacher says anyway”

It should… Any polarizing filter will stop some light from going through the lens, which your light meter should detect. The meter is made to see how much light is coming through the lens, and then tells you what settings you should use to let a pre-determined amount of light into the camera for a correct exposure.

Light meters are made to assume that a normal scene would reflect 18% if the light that hits it. Ask your photo dealer to see a Kodak 18% gray card; shis is what your camera is assuming you are shooting when you meter a scene. Normally there are brighter spots and darker spots, but it averages out to this value. This sometimes doesn’t work if you aren’t shooting a “normal” scene. For example: if you are shooting a scene that is mostly snow (snow reflects WAY more than 18% of the light) the camera will still expose for a normal esposure and underexpose the film. If you got a correctly exposed print from your lab, the snow would be the same shade as the 18% gray card. Same thing would happen if you shot a black wall. The wall would reflect way less than 18% of the light and the camera would overexpose the film. A correctly exposed print would make the wall the same color as the 18% gray card.

The way to get a correct exposure would in those two situations would be to use the 18% gray card. Hold the card so that the sun is hitting the card the exact same way that the sun is lighing the scene. Get close to the card so that the card fills up the entire viewfinder and see how the camera would expose the film. Set your camera for that exposure and take the picture of the scene using that exposure. Now the FILM is correctly exposed, but when the prints are made, the printer assumes the same thing for the prints (that most scenes are 18% gray) and may print your snow and wall scenes to be 18% gray anyway… A way to fix that is to make the first frame of a roll a shot of the 18% gray card. Tell the printer that you want the whole roll printed the same exposure that the first frame calls for. You should get correctly exposed prints then…

Hope this helps…

The circular polarizers on my camera knock off about 1 1/2 f-stops.

And on my camera the TTL light meter does work with the polarizer in place (as well as any other filter I hang on it). Are you sure the camera has TTL? It sounds to me more like the light meter is separate from the lens and thus isn’t looking through the filter.

Sorry to revive this one, but my photograophy teacher insists that you have to manually adjust the exposure when using polarising filters. She says for some reason (and has no idea why) that TTL light meters don’t acknowledge the presence of the filters and therefore the exposure has tomanually adjusted… I can’t see any reaso wy this is, but she has been doing it for a long long time…


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Rory, here’s a bit about polarizers and why you should manually adjust the aperture when using a circular polarizer. I hope this helps. Hell, I even learned something. I just assumed that a TTL would automatically make the correct compensation. But as you will see in this article, that’s not always the case.

[url=http://www.kodak.com/global/en/consumer/pictureTaking/lenses/lensFil7.shtml]Kodak Filter Tutorial**

Happy snaps.

Crap, that’s about 3 links I’ve blown in the last two days. Try this.

Kodak Filter Tutorial

OK, Here’s the deal. Yes, if you use TTL metering, you should not have to make any manual adjustments for filters. The problem is that in many SLR cameras, the light has to reflect off the surface of a non-silvered or semi-silvered glass at an angle to get to the light meter. The reflectivity of a transparent materil depends on the polarization, so the light meter gets different amount of light depending on the direction of polarization. (That’s often reason you use polarizers in the first place - because light reflected off the surface of water is polarized.)

So, you can see that even a fixed correction won’t work. You need to correct the exposure depending on the polrizer direction.

Fortunately there is a handy gadget called a circular polarizer. A circular polarizer is a regulr polarizing filter with a quarter-wave plate behind it, which converts a linearly polarized light into a circularly polarized light. So the light loses the linear polarization, and will be registered correctly by a TTL meter.