Moose Filter

Started Dec 5, 2003 | Discussions thread
Ching-Kuang Shene
Veteran MemberPosts: 5,055
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Re: Moose Filter
In reply to phily_guy, Dec 6, 2003

phily_guy wrote:

a linear polarizer will mess with the camera's sensor which sets
the exposure/shutter speed.

There are two problems here. FIRST, camera's sensor DOES NOT sets the exposure/shutter speed. SECOND, linear polarizers WILL NOT mess with camera's sensors. The answer to the first question is simple. The light meter determines exposure (i.e., the EV - exposure value), which, in turn, determines the aperture and shutter speed to be used for the next exposure. The answer to the second ques is more complicated. Let us not get into too much technical issues.

Virtually all SLR has a flipping mirror behind/below it there is a beamspliter or similar mechanism that splits the incoming light into multiple beams for light meter, AF system, and viewfinder. The meter reads in the split incoming light and determines an EV, while the AF system reads the incoming light and focuses the lens. In general, the ratio of the split light has to be a constant (e.g., 50% for viewfinder, %30 for AF system and 20% for the meter system - just an example). With a linear polarizer is used and being turned, the polarized light cannot guarantee the light split ratio to be constant and actually becomes a variable that depends on the rotation angle of the linear polarizer. As a result, viewfinder may get 60%, AF 25% and meter 30%. Because the meter was designed to use 20% of the incoming light, if now 30% arrives, the meter will assume the lighting condition is 10% more, which it is not the case, and instruct the exposure system to use smaller aperture and/or faster shutter speed. This is, of course, in correct because the scene is still illuminated the same way. By the same reason, if turning a linear polarizer causes 15% of the incoming light to reach the AF system, AF may fail because every AF system requires a minimal amount of light for focusing, usually F5.6. This is the reason that linear polarizers do not work well with SLR/DSLR and cameras that have beam splitter. Fortunately, most consumer digicams do not have beamspliter, and, hence, the use of linear polarizer is OK.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500 User Guide

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