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04-30-2010, 07:12 PM   #16
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KxBlaze - in a nutshell LP(linear polarizers) only adsorb roughly 1 stop of light Vs a CPL(circular polarizer) which will eat up two stops, and LP tend to be more efficient at removing reflections and enhancing saturation.

04-30-2010, 07:26 PM   #17
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Thanks for the advice guys. So I'm looking at Hoya Top Linear Polarizer Filters, Marumi DHG Circular Polarizers, and Nikon PL2s. They are all about the same price except for the last which I'm trying to get through auction. Anyway, does anyone have an opinion on the Marumi DHG vs. Hoyas? I guess a better comparison would be the Marumi DHG vs. the Hoya Pro1.
04-30-2010, 09:24 PM   #18
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QuoteOriginally posted by Digitalis Quote
KxBlaze - in a nutshell LP(linear polarizers) only adsorb roughly 1 stop of light Vs a CPL(circular polarizer) which will eat up two stops, and LP tend to be more efficient at removing reflections and enhancing saturation.
Thanks for the info, looks like I'll be getting a Linear instead of CPL.
04-30-2010, 09:37 PM   #19
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What made me fall in love with LPs is not only the performance, but the costs are so low compared with CPLs that it isn't even funny.

QuoteQuote:
Kxblaze:Oh, I did not know that. So what do they do differently exactly?
Stretch your LBA budget.

04-30-2010, 10:15 PM   #20
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QuoteOriginally posted by Digitalis Quote
KxBlaze - in a nutshell LP(linear polarizers) only adsorb roughly 1 stop of light Vs a CPL(circular polarizer) which will eat up two stops, and LP tend to be more efficient at removing reflections and enhancing saturation.
Looking at BH technical sheets, CPLs usually seem to lose 1.3 stops, not 2:

QuoteQuote:
Filter Factor Between 2.3 and 2.8 (approx. +1.3 stops)
Taken from the spec of a regular Hoya CPL.

Wikipedia is invaluable: filter factor.
04-30-2010, 11:06 PM   #21
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"Looking at BH technical sheets, CPLs usually seem to lose 1.3 stops, not 2"

- manufacturers use different techniques of applying the polarising foil, some waste more light than others..2 stops is typical light loss for a polariser. Remember underexposing is detrimental to image quality on digital so when i'm using a polarizer I compensate with -3 in AE modes because reflected light meter readings are more likely to be misleading when you are using polarizers..because the meter will see the subject is reflecting less light therefore it will increase exposure when in fact, the exposure hasn't changed much at all. hence the reason why incident light meter readings are so useful, and with most modern electronic meters you can add a filter factor which makes life simpler.
05-01-2010, 11:41 PM   #22
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Just to add to the discussion re Linear vs Circular polarisers, I just came across this, in the UK "Landscape Photography" magazine:

==== START QUOTE ====

Polariser Choice: Linear or Circular?

There are two types of polarising filter on the market - Linear and Circular. Only the Circular will work properly with your Digital SLR. Although both varieties are physically circular and similar in appearance, the Linear variety will affect the accuracy of your camera's metering system.

This is because digital SLRs polarise some light inside the camera. If this light has already been polarised by a Linear polariser, a false meter reading is given. Circular polarisers are constructed with a wave-retardation plate, allowing the light waves passing through to rotate and appear unpolarised to the camera's metering system.

Buy Circular.

==== END QUOTE ====

Don't know how accurate or legitimate all that is, but I just thought I'd throw it into the mix....
05-02-2010, 04:09 AM - 1 Like   #23
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QuoteOriginally posted by Derridale Quote
Just to add to the discussion re Linear vs Circular polarisers, I just came across this, in the UK "Landscape Photography" magazine:

==== START QUOTE ====

Polariser Choice: Linear or Circular?

There are two types of polarising filter on the market - Linear and Circular. Only the Circular will work properly with your Digital SLR. Although both varieties are physically circular and similar in appearance, the Linear variety will affect the accuracy of your camera's metering system.

This is because digital SLRs polarise some light inside the camera. If this light has already been polarised by a Linear polariser, a false meter reading is given. Circular polarisers are constructed with a wave-retardation plate, allowing the light waves passing through to rotate and appear unpolarised to the camera's metering system.

Buy Circular.

==== END QUOTE ====

Don't know how accurate or legitimate all that is, but I just thought I'd throw it into the mix....
This generalisation is the nonsense the manufacturers distribute freely, as they make more money with the CPLs.

Though the description of the quarter-wave-plate is correct, the conclusion is not. There are basically very few DSLRs that use a metering system with a polarizing element in front of the metering sensor. Only a handful of film SLRs had that in the past (Pentax LX being the most important example here).

Only AF sensors are place behind these semi-transparent mirrors, which polarize the incoming light. The effect is simple, the AF sensor gets a bit less light, than it would get with a CPL. In practice this is of small or no importance, as you usually would use a polarizer under bright light conditions (blue skies will only appear blue, when the sky is clear), where as small additional light loss will have no consequence for the AF.

Ben

05-02-2010, 05:37 AM   #24
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Thanks for clearing that up, Ben My knowledge of circular vs linear polarisers is rudimentary, especially when it comes to how they interact with exposure and AF systems.
05-02-2010, 12:35 PM   #25
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QuoteOriginally posted by Ben_Edict Quote
This generalisation is the nonsense the manufacturers distribute freely, as they make more money with the CPLs.

Though the description of the quarter-wave-plate is correct, the conclusion is not. There are basically very few DSLRs that use a metering system with a polarizing element in front of the metering sensor. Only a handful of film SLRs had that in the past (Pentax LX being the most important example here).

Only AF sensors are place behind these semi-transparent mirrors, which polarize the incoming light. The effect is simple, the AF sensor gets a bit less light, than it would get with a CPL. In practice this is of small or no importance, as you usually would use a polarizer under bright light conditions (blue skies will only appear blue, when the sky is clear), where as small additional light loss will have no consequence for the AF.

Ben
Although as pointed out there is not an optical element in front of the metering sysyem which is designed to polarize light, the semi-transparent flip mirror which diverts light to the view finder will polarize both the light to the view finder and the light to the AF sensor (transmitted + reflected + absorbed light = total light entering the system from conservation of energy and absprbed light should be small). The information posted by Derridale is correct although Pentax cameras appear to have been designed so that the effect is not very pronounced. It is easy enough to explain from the Fresnel equations - Wikipedia, the free encyclopedia (See the graph showing the difference in reflectivity between S (perpendicular) and P (parallel to plane of insidence) polarised light about half way down the page).
05-02-2010, 03:21 PM   #26
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QuoteOriginally posted by MattGunn Quote
Although as pointed out there is not an optical element in front of the metering sysyem which is designed to polarize light, the semi-transparent flip mirror which diverts light to the view finder will polarize both the light to the view finder and the light to the AF sensor (transmitted + reflected + absorbed light = total light entering the system from conservation of energy and absprbed light should be small). The information posted by Derridale is correct although Pentax cameras appear to have been designed so that the effect is not very pronounced. It is easy enough to explain from the Fresnel equations - Wikipedia, the free encyclopedia (See the graph showing the difference in reflectivity between S (perpendicular) and P (parallel to plane of insidence) polarised light about half way down the page).
Matt I did not question the polarizing properties of a semi-transparent mirror and I did not write, that Derridale is wrong. But the manufacturer's quoted conclusion is simply wrong. I think that one or two Canon 1xx modells really need a CPL - at least I remember reading that - but no other DSLR (and that would be 95% then does need a CPL.

The point is, that semi-transparent mirrors, as used in DSLR, are made by metall deposition on the mirror glass. As we know reflections on a metallic surface will not alter the polarisation of light, so the the main part of the light, which goes to the viewfinder and lightmeter won't be affected as much (as that is simply reflected and does not go through the mirror), as some maufacturer's want us to believe.

I also think, that the real world user experiences posted in several threads here, do justify my pronounced scepticism against manufacturer's marketing messages.

Ben
05-02-2010, 03:48 PM   #27
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Whoah, guys - don't blame those comments on moi - I was just passing them along as messenger. It was in the Landscape Photography magazine

Interesting discussion though - there's something to learn every day.
05-02-2010, 05:12 PM   #28
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QuoteOriginally posted by MattGunn Quote
Although as pointed out there is not an optical element in front of the metering sysyem which is designed to polarize light, the semi-transparent flip mirror which diverts light to the view finder will polarize both the light to the view finder and the light to the AF sensor (transmitted + reflected + absorbed light = total light entering the system from conservation of energy and absprbed light should be small). The information posted by Derridale is correct although Pentax cameras appear to have been designed so that the effect is not very pronounced. It is easy enough to explain from the Fresnel equations - Wikipedia, the free encyclopedia (See the graph showing the difference in reflectivity between S (perpendicular) and P (parallel to plane of insidence) polarised light about half way down the page).
Any polarization that light going to the exposure sensor picks up is negligible, certainly not enough to cause any issues with the light meter.
This is because the metering is done off the focusing screen, and with reflected light, not light which may be somewhat polarized by passing through the main mirror.
The AF is a different situation, since the pellicle portion of the mirror does have somewhat of a polarizing effect.
Cross polarization happens when two linear polarizers are set at an angle to each other.
When that happens, some light transmission is obstructed.
At some point, it is possible to obstruct light transmission enough that the AF sensor will no longer have enough light to work with, but this is a rare situation indeed.
05-03-2010, 02:30 AM   #29
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QuoteOriginally posted by Derridale Quote
Whoah, guys - don't blame those comments on moi - I was just passing them along as messenger. It was in the Landscape Photography magazine

Interesting discussion though - there's something to learn every day.
Pete, that's clear, I think.Nobody blamed you. Indeed it is interesting to read the quote you posted, as it shows nicely, how the industry treis to increase profit with the free distribution of half-true messages.

Ben
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