 BetterPhoto Member |
Filter Question: The Way Polarizers Work Although a stupid question... could you please tell me the practical difference between a linear polarizer and a circular one... Thanks
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Donna R. Moratelli |
Hi Elsa, the circular polarizer is for an autofocus lens and the linear for a manual lens.
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Jeff S. Kennedy |
There is no practical difference between the two other than a linear polarizer can mess up an autofocus cameras ability to focus. IOW you can use a circular polarizer on an AF camera and on a manual focus camera but the linear polarizer can only be used on a manual focus camera.
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robert G. Fately |
The linear polarizer (which was the only kind at first) is like a 'picket fence' with extremely small gaps between the pickets, which thus allows only light vibrating in one orientation to pass through. When you twist the polarizer about its axis, you can see the sky darkening at certain positions. Autofocus mechanisms are often hidden behind half-silvered sections of the SLR mirror; if you inadvertently oriented a linear polarizer in the unfortunately wrong position, the AF would not work. Circular polarizers have concentric circles, rather than straight lines, as the 'pickets'. They thus don't completely eliminate light in one orientation, and therefor don't confuse the AF mechanism. If you twist a circular polarizing filter, the difference in the sky's darkness (to use the same example) is minimal while you turn it.
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Charles W. Craft |
The previous description of the linear polarizer and its ability to confuse autofocus (and sometimes autoexposure) is correct. However, a circular polarizer is actually a sandwich of two components. The first is an ordinary linear polazizer which works exactly like in the previous description. The second component is a quarter-wave plate. What this does is effectively depolarize (or circularly polarize) the light after it goes through the linear polarizer. So the way a circular polarizer works is that first the linear polarizer filters the light and removes the glare, darkens the sky, etc. Then the quarter wave plate changes the polarized light to something the AF and AE sensors can use. The effect on the picture is the same. If you hold a linear and a circular polazizer to the sky side by side and rotate them, you will see about the same changes. But, if you look through a circular polarizer backwards, there will be no effect since the quarter wave plate depolarizes the light before it can get to the linear polarizer, so there's nothing for the linear portion to work with.
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Jordan |
I have a Sigma 28-80mm lens and I want to get a polarizer filter for it. The problem I see is that the thing that you hook the filter to is back more towards the camera than where the lens pops out and in when you zoom or focus. Thus, it appears that the filter would obstruct the zooming and focusing. Would this be a problem? Thanks Jordan
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