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.
Do you know what causes that? Is the meter, for example, not equally sensitive to all polarizations (as if it had a polarizer in front of its detector)? Do we see a change as we rotate the meter?
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
well i did some testing with polarizing...
using a polarized LED flashlight, when i used another pol over the reflected reading lens of the meter at 90 degrees of the other one i got a reading of 2.4 stops less light than when i metered it without a pol on the meter but one on the light. voila!
i still dont know why this is tho....
i will contact my BIP instructor whos the head of such department and see whats up.
when i used the incident meter with a pol'd light and put a pol on the meter i could cancel out all the light as the meter said error not enough light to operate.
4x5 film takes me back. What you need to do is find some Polaroid film to do test shots to nail your exposure. Once you have the exposure more or less down, shoot a test sheet that you can send in to the lab ahead of the main batch of film. You'll be able to tell from this whether to do a push or pull when you process the rest of the film.
Another thing you have take into account is that you have to deal with the bellows extension factor. It's not an issue when doing landscapes and whatnot where the camera is at infinity, but when you crank out the bellows to shoot something in a cramped studio, the light has farther to travel through the camera, and that throws the camera's f/stop settings off. On a camera with TTL metering, it's not an issue as the camera automatically corrects for it.
I don't know why polaroid filters change their exposure factor as you rotate them, but they do. Any easy way to check this out is take one out on a sunny day, crank the filter ring around, and watch the light meter go crazy. You'll also see the same thing if you pan the camera around the horizon (not into the Sun, do 3:00 to 9:00, and you'll really see it shift).
well i did some testing with polarizing...
using a polarized LED flashlight, when i used another pol over the reflected reading lens of the meter at 90 degrees of the other one i got a reading of 2.4 stops less light than when i metered it without a pol on the meter but one on the light. voila!
I will assume you mean you had a flashlight with a polarizer on it and you shone it right on the acceptance dome of the meter, and then either put a polarizer in front of the meter or not.
And when there was a polarizer in front of the meter, you oriented its axis of polarization at 90° to that of the one on the light. Right so far?
In that case, I would expect a difference when the polarizer was put in front of the meter of substantially greater than 2.4 stops. Ideally, that arrangement would provide "complete attenuation" of the light actually striking the meter. So this suggests that one polarizer or the other was substantially "less than perfect". (And for all I know, this degree of attenuation is typical of the polarizers we typically use for photography).
This is quite different than having the polarized light from the light source strike a diffuse reflecting surface and then "observing" the surface with a camera with a polarizer on it.
In that case, the light reflected from the surface should not be polarized. I have so far assumed that the normal rules of diffuse reflection would apply, but it seems not. But in any case, your test doesn't really shed any light on this discrepancy.
One question (likely not relevant) is was the polarizer on the camera a circular type? If not, and if the camera is an SLR with a semi-refelctive mirror, and with the exposure metering detectors in the finder, then there can be a discrepancy in expsure matering. But perhaps metering in the camera wasn't in the picture (since you used an external indicent light meter).
I will try and set up some tests as soon as I can to further probe this curiosity.
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
I don't know why polaroid filters change their exposure factor as you rotate them, but they do. Any easy way to check this out is take one out on a sunny day, crank the filter ring around, and watch the light meter go crazy. You'll also see the same thing if you pan the camera around the horizon (not into the Sun, do 3:00 to 9:00, and you'll really see it shift).
That is because the light from the sky is partially polarized. Thus, a polarizer on the camera will attenuate the light to differing degrees depending on the orientation of the axis of polarization of the polarizer.
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
also i checked it with a ambient meter and it does not change as i rotate the meter.
I will assume you mean you had a flashlight with a polarizer on it and you shone it right on the acceptance dome of the meter, and then either put a polarizer in front of the meter or not.
And when there was a polarizer in front of the meter, you oriented its axis of polarization at 90° to that of the one on the light. Right so far?
-correct.
with the reflected meter reading with a pol on the meter and light, the pol on the meter was held at 90 degrees, i metered when it was the darkest through the viewfinder of the hand held meter. when i did the test on ambient mode of the meter, i could get the meter to say that all light had been cut out when they two pols where at 90 degrees from another.
each filter was a linear 72 or 77mm b+w pol. not using an SLR, using a 4x5 and digital back, no bellows draw. im doing manual metering anyways.
Linear Mode
