LF exposure compensation when using a polarizing filter

[paul_kerfoot]

I don't understand how exposure compensation is accomplished when using a

polarizer on a LF camera. Fixed filters, bellows compensation and film

reciprocity compensation all make sense to me but using a polarizing filter on

a LF camera would seem to have the same problem as using it on a rangefinder.

 

Paul

[gallery on flickr]

Paul, because the amount of light varies depending on the rotation of the filter, it can be a bit tricky when not using TTL metering. I have placed the filter over my spot meter and rotated it until I was happy with the effect of the filter and then taken a reading with the meter through the filter. You might be able to also hold the filter over an incident meter and take a reading, I am unsure of the accuracy of the second method.

 

Chris

[leonard_evens]

I've also read through the filter using my Pentax Digital Spotmeter, and it seems to work okay. But I think you may have problems using a linear polarizing filter that way, at least if the spotmeter uses a mirror as I think the Pentax does.

[trw]

Black and White, C-41, or E-6?

 

If it's anything but E-6 you can probably just allow 2 stops and take care of any varience from that in printing or scanning. This works for me in 35mm*.

 

If you're using the zone system it'll change (possibly completely remove) some of the highlights you've metered though, if they're highlights because of the specular component, not the difuse component.

 

 

 

 

* This works for me in 35mm, but I don't have the right size adapter to use my polarizer on my press camera.

[brian_ellis16]

I use polarizers quite often with my 4x5 camera and it's no different than any other filter. You just use the factor for a polarizer, i.e. 2 1/2 stops or so. The filter factor remains contstant regardless of the amount of polarized light.

[mike butler]

Brian's got it.

 

It's a common misconception that a polarizer has a variable exposure compensation. It's

essentially a neutral density filter and has the same exposure correction at its nonpolarizing

angle as its maximum polarization.

[paul_hoyt]

I have always approached the problem using the following logic. The filter has a filter factor that is independent of the polarization, usually 2.5 if I remember correctly. That means the exposure needs to be corrected 1.3 f/stops. If I am polarizing the sky, the degree of polarization should be independent of the exposure. I am removing light from the sky to separate the clouds and sky but my filter factor remains the same. The reflectance values of non-polarized images will be reduced by the filter factor only and the film should be exposed to render them where I feel they should fall in the exposure scale. If the polarized sky drops 1 or 2 f/stops it is because you are removing light from the sky (and only the sky) and you should not try to compensate by increasing your exposure above the filter factor of the polarizing lens. Some people use the Polarizer at a non-polarizing angle instead of using a skylight or UV/haze filter.

 

Paul

[pvp]

As others mentioned, a polarizing filter has a fixed filter factor, since it will act like a neutral density filter on areas with no polarized component in the light. The actual filter factor will vary from one manufacturer's filter to another, so it's not a bad idea to test each polarizer in your collection. All you need to do is meter a non-polarized subject without the filter, and again <I>through the filter</I> (just hold the filter in front of the meter.) The difference between the values the meter sees, will be the factor for that filter and will remain constant. Make sure the meter is reading the same area each time; a good subject would be something like a rough concrete wall.

[paul_kerfoot]

Thank you all, especially Brian & Mike. I did not realize that the polarizers compensation factor is the same no mater what the degree of polarization. On 35mm, its taken care of for you by the in-camera metering so you don't notice. I am not yet shooting LF but will be retiring in six months and am going to give it a go. I plan to shoot Provia, Velvia and Astia. Chromes tend to be quite picky about correct exposure; hence my question.

[brian_kennedy]

Take a reading through the filter. For my B&W circular polarizer, the factor is consistently 1.3 stops (which means 2.5x the exposure) -- this is much less than the 2 to 2.5 stops you often hear.

[deleteaccount]

The filter factor varies according to how the filter is rotated and its orientation to the sun. The B+W Filter factor I use is between 2.3 and 2.8 (approx. +1.3 stops).

 

Hope this helps.

[bob_salomon]

"Paul, because the amount of light varies depending on the rotation of the filter,"

 

No it does not! The density of a polarizer remains the same regardless of the angle of rotation. It always passes the

same amount of light. That is why it only has one filter factor on it. Don't believe it? Lay it flat on a light box and rotate it

flat on the light box. You will easily see that the density remains the same at all times.

 

" but using a polarizing filter on a LF camera would seem to have the same problem as using it on a rangefinder."

 

What problem? If you are thinking about previewing the effect of the polarizer then Heliopan polarizers have calibrated

rims so you can hold them to your eye, rotate till you see the desired effect, note the position on any number on the rim

and place on the camera and rotate the rim so that number is in the same relative position on the rim.

 

If you are talking about metering and exposure correction either divide the film speed by the filter factor or increase

exposure by the exposure correction (about 1 1/3 stops) or meter the scene through the filter. All will work.

 

Since filter factors and exposure compensation numbers are starting guides you may find that you prefer to fine tune

them for your working techniques. Once you do they will remain the same for every shot.

[Troll]

According to St. Ansel's Basic Series, he recommends a standardized FF = 2.5.

[bob_salomon]

Bill,

 

The filter factor might vary between Pol filters from different manufacturers as there are several different polarizing foils that

the manufacturer has a choice of when making the filter. Also within a manufacturers range of filters the factor can vary

depending on the type of polarizer; ie green or yellow polarizer, Kaesmann vs non Kaesmann, etc.

[Troll]

Bob Salomon , Jul 19, 2008; 10:00 a.m.

 

"Bill, <P>

The filter factor might vary between Pol filters from different manufacturers as there are several different polarizing foils that the manufacturer has a choice of when making the filter. Also within a manufacturers range of filters the factor can vary depending on the type of polarizer; ie green or yellow polarizer, Kaesmann vs non Kaesmann, etc."<P>Bob, you'll have to take it up with Ansel, not with me.

[keith_lubow]

<p>"The filter factor varies according to how the filter is rotated and its orientation to the sun. The B+W Filter factor I use is between 2.3 and 2.8 (approx. +1.3 stops).</p>

 

<p>Hope this helps."</p>

<p>It doesn't help at all. In fact, it hurts, because it is wrong. As many of the other responders have stated, each polarizing filter has a fixed factor. Turning it changes its effect on certain subjects within the picture, but does not give one variable neutral density.</p>