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How do filters such as the Singh-Ray LB Polarizer transmit more light then other circular polarizers? I am particularly interested in this technology because they claim to only add 1-1/3 stop to the exposure, where as most CPL's that I am familiar with eat up somewhere around 2 stops. I would think that they would all transmit an equal amount of light if they are achieving an equal amount of reflection reduction and so on.

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Hoya HD polarizers adds only one stop and are extremely high-quality which is why I'm only buying those from now on! Most are 2 1/3 stops by my measurements. I have NO idea what the difference is, so good question! – Itai Mar 14 '13 at 18:16
@Itai - Wow, a 1 1/3 stop savings if you go with the Hoya. That is a pretty big deal. – dpollitt Mar 14 '13 at 18:21
Hoya HD. The standard non-HD, Super-SMC, SMC or simple all reduce by 2 1/3 EV. So those two hip letters make all the difference :) – Itai Mar 14 '13 at 18:25
My newest sunglasses are basically CPL filters on my eyes. Everything is insta awesome. Tilt head 90 degrees and you can see LCD screens and reflection, no tilt. can't see things! Good fun. – NULLZ Mar 14 '13 at 22:31

Hopefully someone else can give a more detailed answer, but a circular polarizing filter is a compound filter produced by several stacked elements. Additionally, there are multiple different techniques that can be used to make each of these layers.

Presumably, some light of the desired polarization is either absorbed or reflected during the process, however some processes and/or material qualities result in less absorption and/or reflection of the desired light.

I'm not sure what the most likely direct cause is though or if there is even a most likely one. It would probably take a detailed read of the specification for the filter to understand what is different about the optical properties. Also, I don't personally know which techniques would allow more or less overall light. I just know that there are multiple options for how to do it.

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They could also simply be less restrictive/effective as polarizers, letting light through that is polarized at a somewhat broader range of angles. Since we're usually more concerned with excluding stuff that's polarized 90 degrees out rather than, say, what's a mere 15-20 degrees out, widening the acceptance range would not have a deleterious effect on most images, but it would let more total light through. – user2719 Mar 14 '13 at 20:39

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