polarizers

[Mike Kovacs]

Here's what might sound like a really dumb question for some of you. How exactly are people spot metering and calculating exposure when using a polarizer? Should you meter from the area that is highly polarized or not highly polarized? I guess this is the difficulty - the 1-2 stops of polarization filter factor doesn't effect all portions of the scene, and one has to be mindful where the highlights lie.

I feel like I am always guessing to some degree when I use one with the LF or my MF SLRs. I'm largely using colour transparency film for my nature work, so accuracy is necessary. It seems when I do get it slightly wrong, its usually to the err of underexposure.

[Steve Hamley]

Mike,

I usually don't spot meter, but regardless of how I meter, I don't meter through the filter. You should determine the filter factor by test exposures and apply that factor to the unfiltered meter reading. Some folks believe the color sensitivity of the meter cell can cause issues metering through a filter. I rarely have any exposure issues.

Steve

[Terence Spross]

Actually because of the effects of the polarizer on the scene, I would prefer to use a through the lens meter, but alas my LF has no meter. I'm aware that there are ground glass meters or adapters for hand held meters but I've never used one. When using a polarization filter with LF, I first used the same filter on my 35mm and viewed the same scene with it from different angles to quickly see the desired effect, I also metered the subject, after moving in closer to the main subject area and using the cameras center-weighted meter. Then I transfer the settings to the LF shutter, install the polarizer and rotate it the same, and position the tripod accordingly. Having a 35mm carmera and using it for a composition aid and as an exposure meter does not require you to actually take pictures with it.

This is different than how I usually meter, as I usually use an incident meter. I already know how that compares to the neutral density card and camera meter use and take the subject into account.

I rarely bracket exposeres, taking the extra time to measure highlights and shadows, I usually get the correct exposure the first time. Bracketing with LF gets rather expensive. I always try to get it right the first time. The kinds of photos taken with LF usually yield sufficient time to think about exposure in detail. Quick shots with 35mm are often a different story where a second guess, based on thinking after the shot, say about the need for more shadow detail in a particular, sometimes yields the need for a second exposure with a half-stop more open, for example. But by then the scene has changed slightly so the adjusted exposure might be taken from a slightly different position - not what bracketing practicioners would call bracketing.

Using filter factors is all real nice for average subjects, but I actually try not to shoot average subjects , therefore I find filter factors are misleading.

[John Z.]

Mike,
If you meter the sky through the polarizer, you essentially cancel out the effect that you would hope to obtain, which is to darken the sky. In theory you could spot meter a part of the scene (e.g. foreground) which is not polarized, to obtain an idea of the polarizer filter factor. You could also do a bracketed test using the polarizer on a non polarized surface, such as a wall, door, etc, and get an idea of the filter factor required for your polarizer. If you use the polarizer on a non-polarized surface, it becomes simply a neutral density filter.

Most of my polarizers require an exposure adjustment of about 1 and 1/4 to 1 and 1/2 stops.

[Wilbur Wong]

Because of the variety of lenses, that I carry, I also carry two different sizes of polarizing filters. Which ever one isn't on the lens on the camera, I hold by hand in front of my spot meter, and take all my readings through that. (oriented loosely to the same angle as the one on the lens.)

[Juergen Sattler]

I always use an exposure adjustment of one stop and it seems to work just fine for me.

[Brian Vuillemenot]

I use one of those Horseman exposure meters with a 4X5 inch panel that fits into the back of the camera. With that, I don't have to worry about filter factors or bellows compensation when doing closeups. It makes the whole process a lot simpler; I'm surprised that more people don't use these things!

[Mike Kovacs]

So it seems the consensus is to regard it as a neutral density filter for most situations. That should hold true unless the subject one is metering has a small polarized component, in which case one should expose a little more.

Remember too, a polarizer is for more than just blue skies - it can be used to get a handle on unwanted reflections like the sheen of wet folliage.

[Steve Hamley]

Mike,

Your logic has an inherent assumption that the polarized portion is a relatively large portion of the total light - an assumption that I think you will find isn't true in practice.

Steve

[Alan Davenport]

For a subject that is completely unpolarized, a PL filter acts just like a neutral density and will have the same light reduction regardless of how the filter is turned.

So if you meter the non-polarized part of the scene, and apply the appropriate filter factor (I use 2 stops for my PLs, but it varies somewhat between brands) then you'll get a "correct" exposure for that part of the scene, with the polarizer reducing polarized light depending upon how the filter is turned.

Better yet, take an incident reading, apply a 2 stop factor (or whatever your filter needs) and shoot away.

[Brian Ellis]

A polarizer has a constant factor, it doesn't change depending on how much or how little of the light is polarized. Just meter the scene, apply whatever factor you use for polarizers, and fire away. There's a discussion of the common fallacy of thinking that the factor chages as the light changes in Ansel Adams' book "The Camera" (or maybe it's in "The Negative," I forget which).

[Clay Turtle]

What? "A polarizer has a constant factor, it doesn't change depending on how much or how little of the light is polarized" never used one except on a 35mm. But it seems to me that the effective light as seen through the lens varied between lite shading to almost pitch black & so did the prints? As circular polarizers were designed to maintain constant polarization, I would accept that statement about thembut with the plane polarizer I use, hmm I have to check it out?

"exposure meters with a 4X5 inch panel that fits into the back" ya, I happen to run across an ad on that the other day. Started me thinking maybe I could use light reading from the back plate area, I suspect that I may have to use ev instead of f-stop function &/or spot attachment instead of dome but I was going to start logging these reading just to see how they relate to calculated exposure reading of scene.