Linear v. Circular Polarisers

Arpe · 02-20-2007, 04:33 PM

When i bought my first film SLR, manual focus etc. I got a linear polariser with it (it is definitely linear, have done test to prove it). I then upgraded to an autofocus film SLR, and now my autofocus istDs. I have never had a problem with this linear polariser.

So what is it that is supposed to be wrong with them for autofocus SLRs (and why haven't I had a problem do you think)? AND in usage and results, is there any difference?

Trawlerman · 02-20-2007, 05:58 PM

Here's what Luminous Landscape have to say:

Circular Vs. Linear Polarizers

There are two types of polarizing filters available — linear or circular. Linear polarizers are more effective and less expensive than circular ones. But circular polarizers are needed with just about any camera that has a through-the-lens metering system, or autofocus.

The reason for this is that both of these systems use semi-silvered mirrors to siphon off some of the light coming though the lens. If that light is linearly polarized it renders either the metering or the autofocus ineffective. This means that you're going to have to buy circular polarizers unless you're shooting with a pre-1970's camera, or a view camera.

Polarizers

Arpe · 02-21-2007, 02:17 AM

And yet I've never had a problem with my linear

How come?

*dave kitson* · 02-21-2007, 04:45 AM

Originally Posted by Arpe

And yet I've never had a problem with my linear How come?

I think it's a case of you will potentially have a problem with a linear polarizer (depending on several variables such as angle of polarization etc.), rather than you are guaranteed to have a problem.

There are lots of posts on the net saying people have had no problems with linear polarizers.

dave

Trawlerman · 02-21-2007, 11:14 AM

I guess it may be worth trying them out (if you can) before committing yourself to a purchase.

I just got an 86mm HOYA Circ-POL ands it cost me a whole chunk of money. If i'd been able to get a linear one the cost would have bee a lot less.

FWIW, I have a 72mm Hoya Linear-POL on a Vivitar superzoom and have yet to come across any problems with that one. Guess it's the luck of the draw and down to individual camera's.

*dave kitson* · 02-21-2007, 01:43 PM

Originally Posted by Trawlerman

I just got an 86mm HOYA Circ-POL ands it cost me a whole chunk of money. If i'd been able to get a linear one the cost would have bee a lot less.
F

Yes, big filters cost big bucks...

BTW. what ship is that in your signature? Is it a R-Class? these old eyes need a bigger image these days..

dave

Arpe · 02-21-2007, 05:04 PM

Okay so it's a "maybe" thing. It just seems that whenever anyone is giving advice about this they say something like, "a linear polariser won't work on an autofocus camera...", which is why I asked. I didn't realise that it worked for so many others as well! Thanks.

*barondla* · 02-21-2007, 09:32 PM

I have never heard that linear were more effective than circular (assuming it is working properly with the camera). Circular is linear with one other added element to put a spin on the light. Don't they both polarize the same amount?

thanks
barondla

Arpe · 02-22-2007, 01:13 AM

I have no idea!!

gorfmai · 02-22-2007, 01:24 PM

Both do polarize the same amount. I have seen some AF cameras that cannot focus / shoot for anything with a linear polarizer, others don't have such problems. As someone said, there's a lot of variables involved.

For whoever asked, how do they work -

Polarized light travels in "lines" - it is a linear wave form from ricocheting off metal, water, the ground, etc.

A linear polarizer has 2 peices of glass that are linearly polarized. When the lines on both elements are parallel (match up), the polarization effect is minimal, as lines of light can still slip through. When you rotate one element so the 2 peices are now perpendicular, you are basically cutting off every angle that polarized light can pass through the filter, hence the reduction in light. Only polarized light is affected, which is why at no time does your filter cut off ALL light (unless you happen to be surrounded by only polarized light waves!)

A circular polarizer does the same thing, except instead of lines, you have 2 slightly offset sets of coincentric circles.

You can do the same effect at home by finding 2 pairs of polarized sunglasses. Hold them both horizontal, minimal effect. But as soon as you start to rotate one...

Yes, I know I have oversimplified some of this, and its been a while since college physics. So please, no need to nit pick...

Arpe · 02-22-2007, 04:54 PM

So you get exactly the same result?

**Adam** · 02-22-2007, 04:58 PM

Originally Posted by gorfmai

Both do polarize the same amount. I have seen some AF cameras that cannot focus / shoot for anything with a linear polarizer, others don't have such problems. As someone said, there's a lot of variables involved.

For whoever asked, how do they work -

Polarized light travels in "lines" - it is a linear wave form from ricocheting off metal, water, the ground, etc.

A linear polarizer has 2 peices of glass that are linearly polarized. When the lines on both elements are parallel (match up), the polarization effect is minimal, as lines of light can still slip through. When you rotate one element so the 2 peices are now perpendicular, you are basically cutting off every angle that polarized light can pass through the filter, hence the reduction in light. Only polarized light is affected, which is why at no time does your filter cut off ALL light (unless you happen to be surrounded by only polarized light waves!)

A circular polarizer does the same thing, except instead of lines, you have 2 slightly offset sets of coincentric circles.

You can do the same effect at home by finding 2 pairs of polarized sunglasses. Hold them both horizontal, minimal effect. But as soon as you start to rotate one...

Yes, I know I have oversimplified some of this, and its been a while since college physics. So please, no need to nit pick...

Can circular polarizers be used with mirror lenses?

gorfmai · 02-23-2007, 11:39 AM

Originally Posted by Mo

Can circular polarizers be used with mirror lenses?

Best of my knowledge they can, but I don't have an AF mirrir lens currently at my disposal to verify. I can't think of any reason off the top of my head why it wouldn't work... anyone else?

*jfdavis58* · 02-23-2007, 02:20 PM

More like over complicated.

The polarizer is the same in each filter and there is only one polarizing layer. Think of it like a grating or venetian blind. Light waves pass between the lines when they parallel the lines of the grating but are trapped/stopped when moving across the lines.

The circular polarizer has a second layer-a half-wave refractor which effectively re-scrambles the light waves.

Strictly speaking the problems are only a hassle for the metering system because you can usually focus manually. And they only occur with camera technology which employs a beam-splitter to move some of the light to the AF/AE system.

The jury is still out on if the K-D cameras still use beam splitters. Somebody should probably ask Pentax, but getting an informed answer might be problematic. One could also experiment-wild over-exposure should result if interference is present.

Cideway · 02-24-2007, 07:47 AM

Originally Posted by jfdavis58

More like over complicated.

The polarizer is the same in each filter and there is only one polarizing layer. Think of it like a grating or venetian blind. Light waves pass between the lines when they parallel the lines of the grating but are trapped/stopped when moving across the lines.

The circular polarizer has a second layer-a half-wave refractor which effectively re-scrambles the light waves.

Strictly speaking the problems are only a hassle for the metering system because you can usually focus manually. And they only occur with camera technology which employs a beam-splitter to move some of the light to the AF/AE system.

The jury is still out on if the K-D cameras still use beam splitters. Somebody should probably ask Pentax, but getting an informed answer might be problematic. One could also experiment-wild over-exposure should result if interference is present.

Pentax Cameras use beam splitters. I don't know why Pentax Cameras aren't as affected by linear PLs but i have used a canon with a linear and the exposurs was completely unpredictable and the AF would be inconsistent.