Circular Polarizer effects on water

Eruditass · 11-05-2009, 09:08 AM

Originally posted by mikeSF

my understanding is that a circ pol IS a linear polarizer to which another element has been added (called a quarter wave plate). The effects of the quarter wave plate are needed by DSLRs for proper metering and even auto focusing. other than that, with both, you rotate them to achieve the desired degree of effect, from minimal to maximum (90 degrees).

Yep,

and of course linear is a lot cheaper

Wheatfield · 11-05-2009, 11:38 AM

Originally posted by mikeSF

uh, "pentax beginner's corner"...hello?

Break em in fast, that's my philosophy

Wheatfield · 11-05-2009, 11:40 AM

Originally posted by mikeSF

my understanding is that a circ pol IS a linear polarizer to which another element has been added (called a quarter wave plate). The effects of the quarter wave plate are needed by DSLRs for proper metering and even auto focusing. other than that, with both, you rotate them to achieve the desired degree of effect, from minimal to maximum (90 degrees).

Pentax DSLRs do not need a circular polarizer for correct exposure metering, and won't need one for AF unless the light levels are iffy and the angle of incidence of the filter is such that there is significant interference.

Eruditass · 11-05-2009, 11:48 AM

Originally posted by Wheatfield

Pentax DSLRs do not need a circular polarizer for correct exposure metering, and won't need one for AF unless the light levels are iffy and the angle of incidence of the filter is such that there is significant interference.

AF will work with a linear polarizer? I'd only use one in bright light anyway - this is great news!

Wheatfield · 11-05-2009, 11:24 PM

Originally posted by Eruditass

AF will work with a linear polarizer? I'd only use one in bright light anyway - this is great news!

It should work most of the time. The science behind what is happening is that light coming in the lens goes through a beam splitter. This is a partially silvered section of the main mirror, Most of the light is reflected up into the prism and then on to the viewfinder. The metering cell is located in the prism, is not affected by the beam splitter.
The AF sensor is located below the mirror box just behind the lens mount. The beam splitter allows some light to pass, and also polarizes it. This is just the nature of the beast. Once past the beam splitter, the light is reflected by a mirror mounted under the main mirror and is directed to the AF sensor.
Since the light going to the AF sensor is polarized, a linear polarizer can play havoc via cross polarization.
A linear polarizer that is 90º off the beam splitter can do a pretty effective job of blinding the AF, but it is unlikely to happen often in practice.
I've found that when I am using a polarizer I am tending to be somewhat more methodical about my shooting, and as such am not using AF anyway.
If you find the AF to be hunting when you have a linear polarizer on, you have probably run into the rare situation where you have cross polarized the light and have blinded the AF sensor to the point of making it ineffective.

Eruditass · 11-05-2009, 11:27 PM

Awesome, thanks for the info! Makes sense, and I plan to be very deliberate with my filter as well. Now to find a good cheap linear polarizer

Igilligan · 11-06-2009, 01:27 PM

Originally posted by audiobomber

A polarizer does what "fishermen's sunglasses" do. What you see with your naked eye is the glary surface of the water. The polarizer takes away the glare and allows you to see into the water. So the image with the sun reflecting off the surface is truer to life, and also the more interesting to me. If there was a big trout under the ducks, I might prefer the CPL version.

Forget the trout... if there was an alligator under the ducks that would be cool!~

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11-05-2009, 09:08 AM	#16
Eruditass Veteran Member Join Date: Jul 2009 Photos: Gallery Posts: 2,207	Originally posted by mikeSF my understanding is that a circ pol IS a linear polarizer to which another element has been added (called a quarter wave plate). The effects of the quarter wave plate are needed by DSLRs for proper metering and even auto focusing. other than that, with both, you rotate them to achieve the desired degree of effect, from minimal to maximum (90 degrees). Yep, and of course linear is a lot cheaper

11-05-2009, 11:38 AM	#17
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	Originally posted by mikeSF uh, "pentax beginner's corner"...hello? Break em in fast, that's my philosophy

11-05-2009, 11:40 AM	#18
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	Originally posted by mikeSF my understanding is that a circ pol IS a linear polarizer to which another element has been added (called a quarter wave plate). The effects of the quarter wave plate are needed by DSLRs for proper metering and even auto focusing. other than that, with both, you rotate them to achieve the desired degree of effect, from minimal to maximum (90 degrees). Pentax DSLRs do not need a circular polarizer for correct exposure metering, and won't need one for AF unless the light levels are iffy and the angle of incidence of the filter is such that there is significant interference.

11-05-2009, 11:48 AM	#19
Eruditass Veteran Member Join Date: Jul 2009 Photos: Gallery Posts: 2,207	Originally posted by Wheatfield Pentax DSLRs do not need a circular polarizer for correct exposure metering, and won't need one for AF unless the light levels are iffy and the angle of incidence of the filter is such that there is significant interference. AF will work with a linear polarizer? I'd only use one in bright light anyway - this is great news!

11-05-2009, 11:24 PM	#20
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	Originally posted by Eruditass AF will work with a linear polarizer? I'd only use one in bright light anyway - this is great news! It should work most of the time. The science behind what is happening is that light coming in the lens goes through a beam splitter. This is a partially silvered section of the main mirror, Most of the light is reflected up into the prism and then on to the viewfinder. The metering cell is located in the prism, is not affected by the beam splitter. The AF sensor is located below the mirror box just behind the lens mount. The beam splitter allows some light to pass, and also polarizes it. This is just the nature of the beast. Once past the beam splitter, the light is reflected by a mirror mounted under the main mirror and is directed to the AF sensor. Since the light going to the AF sensor is polarized, a linear polarizer can play havoc via cross polarization. A linear polarizer that is 90º off the beam splitter can do a pretty effective job of blinding the AF, but it is unlikely to happen often in practice. I've found that when I am using a polarizer I am tending to be somewhat more methodical about my shooting, and as such am not using AF anyway. If you find the AF to be hunting when you have a linear polarizer on, you have probably run into the rare situation where you have cross polarized the light and have blinded the AF sensor to the point of making it ineffective.

11-05-2009, 11:27 PM	#21
Eruditass Veteran Member Join Date: Jul 2009 Photos: Gallery Posts: 2,207	Awesome, thanks for the info! Makes sense, and I plan to be very deliberate with my filter as well. Now to find a good cheap linear polarizer

11-06-2009, 01:27 PM	#22
Igilligan Guest	Originally posted by audiobomber A polarizer does what "fishermen's sunglasses" do. What you see with your naked eye is the glary surface of the water. The polarizer takes away the glare and allows you to see into the water. So the image with the sun reflecting off the surface is truer to life, and also the more interesting to me. If there was a big trout under the ducks, I might prefer the CPL version. Forget the trout... if there was an alligator under the ducks that would be cool!~