 monx |
Circular polarizer v/s Linear polarizer I have read that autofocus cameras don't calculate the exposure value correctly with a linear polarizer. Could someone throw some light on the reason(s) behind this and any workaround? Does it have something to do with the filters being mounted on the focussing ring itself (and hence changing the polarizing direction when the focus point is changed)?
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Jon Close |
Most (all?) autofocus SLR's, have a passive autofocus system with sensors behind the lens, so any filters that cut the amount of light entering the lens will also affect autofocus performance. There is an additional problem with a linear polarizing filter. The SLR mirror has a beam splitter prism to direct light to the AF and exposure sensors in the body, and this effectively polarizes that light to the sensors. Adding a linear polarizing filter to the front of the lens works at cross purposes to the beam splitter's polarizing effect and alters or can completely block light to the AF and exposure sensors. So metering and autofocus become unreliable. A circular polarizer organizes the light waves differently, so that it does not affect the light directed to the AF and exposure sensors.
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monx |
Thanks! So, hypothetically speaking, if I mount the LP filter in such a manner that the polarization direction of the light that passes through the filter is the same as the one that is allowed by the prism to the sensor, then the metering by sensor would be the value that falls on the film, isn't it? But in that case, the image in the viewfinder would be slightly different from that on the film, right?
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Jon Close |
I don't know. Possibly. I think that's why use of a linear polarizer is usually described as making AF and metering unreliable, rather than impossible. There may be a filter position/orientation where the metering will be accurate and others where it'll be off. It's best to just use a circular polarizer and not worry about it, since the filter's orientation is dictated by the direction of the light source rather than where AF/metering won't be affected.
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