metering polarized lights

[michaelnotar]

when i shoot painting using the cross polarization technique, after metering the lights with an incident reading (lights have pol's on them before metering) and opening up for the pol on the camera lens, i am still a stop dark.

does this have to do with the meter not being intended to meter polarized light (sekonic 758/358)?

[Doug_Kerr]

Hi, Michael,

Originally Posted by michaelnotar

when i shoot painting using the cross polarization technique, after metering the lights with an incident reading (lights have pol's on them before metering) and opening up for the pol on the camera lens, i am still a stop dark.

does this have to do with the meter not being intended to meter polarized light (sekonic 758/358)?

I have no experience with this, so my thoughts are totally based on theoretical concepts.

I doubt that the meter has any discrimination with respect to axis of polarization, so its reading should be a true reading of the illuminance of the (polarized) light. (This basically will tell us the amount of total luminous flux landing on each unit of area of the surface, never mind that its polarization is now mostly in the same axis.

Assuming that the basic subject surface is diffuse, I would expect the emitted light to be basically randomly polarized, and I would expect the illuminance would be just what one would expect considering the luminance of the incident light and the (diffuse) reflectance of the subject (not affected by the polarization). (That is, considering the luminous flux that lands on each unit of area, the total reflected luminous flux is just the product of that with the reflectance, only now it is distributed among components with random axes of polarization.)

Then the polarizer on the camera would give a little more than one stop of attenuation to randomly polarized light.

So far, I have just reconstructed your own line if reasoning! So beats me why the result seems underexposed.

Have you made test shots with the polarizer on the camera at different orientations? If they all show essentially the same exposure result, then that would confirm the assumption about the reflected light being randomly polarized. And that would, for me, leaves the mystery in place.

Remind me why you are using this polarized light setup?

I'll do a little poking around in the literature to see if I have missed some important photometric principle!

Thanks.

[Dennis_Vied]

I would guess that the metering does not take into account the effect that the cross polarization has on the exposure. Opening one stop for the pol on the lens would account for one axis of pol, (i.e. with normal illumination), but not the fact that the illumination is also polarized, so perhaps one should open two stops for cross polarization. (i.e., If you could meter from the film plane, it might indicate correctly).

[BobSmith]

The only thing I use a meter for in such a circumstance is to check that light is absolutely even across the area that the painting will cover. Beyond that, I shoot tethered and look at the file in ACR examine the values and adjust exposure accordingly. Why bother with the meter's prediction of what the exposure *should* be when you can see the actual results of your exposure in seconds. Once the exposure is established I shoot everything at exactly the same setting. I always put the art and lights in exactly the same position. Every piece of flat art I've photographed for the past couple of years has received the same exposure... 1/60 @ f/7.1. Shooting and processing is a breeze.

Bob Smith

[Dennis_Vied]

What kind of lighting do you use, Bob?

[David_Buzzard]

You can't get an accurate incident light meter reading while using a polarizer. The exposure factor changes as you rotate the filter, sometimes as much as 1.5 stops. If you have a spot meter setting on your camera, use an 18% gray card to get an accurate in-camera reading.

If you're copying artwork, make sure to get a shot with the gray card in it so you can check the colour balance as well. Polarizers can cause colour shifts as well.

David Buzzard

[michaelnotar]

yes, bob smith, i do just that, use the meter to see that the light is even and then check the computer. however, i am going to run some 4x5 film EI tests, i know that strobes have a different EI than Sunny 16 light. i have dialed the internal calibration on my light meters for a certain histogram on my digital (namely OE +.5) and and curious to see what results i will get now.

i will be testing provia 100F under regular strobe and outside light, as i also want to shoot some landscapes etc with it. it was my main film in the film day. also doing the test with 120 film and in which i will test it under outside, strobe unpolarized and strobe polarized. i am thinking of taking the difference of its unpolarized and polarized EI and applying to the 64T. 64T is spendy, and i have many rolls of provia 100f 120 already around.

and i will test kodak 64T for a accurate color test art repro under tungsten unpolarized.