 António Liberato |
hama Polarizing Filter I´m uploading a picture of it. What does the "IV" mean on the filter ring? is it the factor? Thanks in advance
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John A. Lind |
António, I don't know what the "(IV)" means. No, it's not the filter factor (or it shouldn't be). In general, a polarizer absorbs about 1-1/3 f-stops of light. This would be a correction for lens aperture (open up by 1-1/3 f-stops). In terms of the commonly used "filter factor" it's about 2.5, which is the amount by which the shutter speed must be increased (multiply shutter speed by 2.5). The correction required varies a little among polarizers of different brands; this is a starting point. If you are metering through the lens with the filter on, you don't need to apply any filter factor. The metering will automatically compensate as it is measuring through the filter. -- John Lind
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António Liberato |
Hi John. Thanks for answering my question. The problem is the camera doesn't meter through the lens... I have to correct this manualy. I've just found the original box where it came and it says there that the filter factor is 3-4x. So this means I should multiply the shutter speed by 3 or 4? I have to test it anyway... but I have to know where to start. :) Thanks again. A. Liberato
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John A. Lind |
António, This may seem a bit cumbersome . . . The old, old, old method used with rangefinders and separate metering (built-in but not TTL or hand held) and one still used when metering using a hand-held meter with a polarizer . . . (a) Align it for the effect you want looking through it. If you're using an SLR, change (a) to "Align for desired effect with it mounted on the lens." There should be a dot on the ring that turns . . . or if it has a handle to use that . . . to keep the same alignment between eye (or initially on the lens You should probably do some testing of it to get a feel for how much the metering changes with alignment of the polarizer. This variance can easily be different from scene to scene . . . it depends on how much polarized light is present that can be blocked out. -- John Lind
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Jon Close |
Since the original box gives the filter factor as 3x-4x then (IV), ie. 4, probably does indicate the filter factor. A filter factor of 4 is equivalent to 2 stops (ISO from 200 to 50, or aperture from f/5.6 to f/2.8, or shutter speed from 1/500 to 1/125).
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António Liberato |
Thanks to both of you. I'll give it some testing. I have a starting point now. I think I'll shoot a slide film, (bracketing and anotating the results) and then I hope I'l conclude something out of it...
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