Circular or Linear Polariser?

[TDN169]

I got a set of filters off ebay to play around with, but I never tried out a linear polariser. Is it any worse than a circular polariser, and if so why?

And, for that mater, can anyone explain the difference?

[fotogmarc]

I've never used a linear filter, but have read that it hinders your auto focus and metering.

[Prospero]

I have heard that too.
dSLR cameras cannot focus with a linear polarizer, but only with a circular one.
Many point and shoot cameras can focus with both.

From what I have heard, a linear polarizer has more effect than a circular polarizer, and it is cheaper.

[Rhys]

AF won't work with linear.

[JLV]

I use a linear with my FZ20, No problem.

[TDN169]

Ah, so it's probably the ultrasonic motors which need CPL.

Can anyone give the scientific difference? Apparently CPl filters 'corkscrew' light waves.

[Prospero]

No, it's not the difference in the motor that matters, but the difference in the autofocus system.
The autofocus of SLR cameras often use beamsplitters, which are semi transparant mirrors. Part of the light will pass through this mirror to the autofocus system, while the other part will go to the viewfinder for metering and focussing.
When the light is linearly polarized, the amount of light that passes through this mirror is much lower, which is why the autofocus won't work properly and the metering system will not receive the right amount of light (hence giving you a wrong exposure). Apparantly it does work fine with a light that is circularly polarized.

Point and shoot cameras often do not have a system with beamsplitters and therefore can focus with both.

I don't know the scientific differences between the two filters (and don't have time to look it up). Perhaps someone else knows.

[AlexMonro]

Not sure if this counts as a scientific explaination of the difference, but basically, a circular polariser is a linear polariser with a quarter wave plate on the back, which scrambles the polarisation of the light before it reaches the lens, and any beam splitters.

This means that if you ever dismantle a circular polariser (or drop it and it springs apart), it's important you get the glass the right way round when you put it back together. Fortunately, I got it right first time after I dropped my Hoya! :) (Even luckier that it landed in soft mud and didn't break the glass too!)

[TDN169]

Ah, yes, I had to take my Tamron filter to pieces to clean the edge of the glass and put it in the wrong way. It was only after about 100 shots I realised my mistake!

So. basically, a CPl polarises light to filter it, and then returns it to 'look like' normal light.