Determine exposure using polarizergreenspun.com : LUSENET : Large format photography : One Thread
I could not determine the amount of exposure correction on a sunny day with a polarizer. Looking back, I guess I should have used my incident meter with a short piece of toilet paper core, gotten a reading with the tube open, and then a reading with the polarizer covering the end of the tube, rotating the polarizer, and noting the difference in illumination (e.g., 1 stop difference, 1 and a half stop difference, etc.). Any suggestions?
-- Terry Lorch (Terry725@aol.com), February 27, 1998
With mine I read the light & open 2 stops. With tungsten lighting in the studio, I open up 1 1/2 stops. Do a controlled experiment on it to find your factors as they do differ with polarizers from different companies.
-- Dan Smith (firstname.lastname@example.org), February 27, 1998.
I did a test with my hand meter in reflective mode by reading a scene without the polorizer on front, and again with the polorizer in front, and got a difference of 1 1/3 stops. In my chromes, they still looked too dark. I went to 1 1/2 stops, and they still looked too dark. I have finally come to a 2 stop difference, and this is what I am using now. I recommend you try a few variations yourself. You can probably work this out using 35mm, shooting the same scene with variations on the same roll. Use transparency film. Now, this just compensates for the neutral density portion of the polorizer. You DO NOT want to correct for the polorization (usually). Why? Lets take a common example, to darken the sky in a landscape shot. If you have used a polorizer, you know that as you rotate one element, you can vary the amount of polorization, and the sky begins to darken, but you notice that not everything in the field of view is affected. The white clouds, for instance, pop out beautifully against this dark sky. They were not affected by the polorizer. So, if you compensated for the polorization, you would simply bring the sky back up to where is was (whats the purpose of the polorizer?), and you would also blow out all of your highlights as well! You just want to compensate for the neutral density factor of the polorizer, not the effects of polorization, because a polorizer adjusts the relative contrast of some objects relative to other objects. Now, you can ALWAYS find exceptions to this, and you can come up with an example where the effects of the polorizer will affect exposure (such as glare from a window, now eleminated with the polorizer), but for most nature and landscape shots, you probably do not want to compensate for polorization. Practice a bit and gain some experience with it. The polorizer is the most important filter for color work, IMHO.
-- Ron Shaw (email@example.com), March 02, 1998.
Use a spotmeter and read the exposure holding the filter in front of the meter in the position that you want to use when you take your picture. I use this method with all of my filters and it works every time. Don't shoot your finger.
-- Jerry C. Hubbard (firstname.lastname@example.org), April 21, 1998.