Polarisers

Not really a problem...

I've been under the impression that linear polarising filters fool AF into not working.

I have a polarising filter that I've always believed is a linear one; it says, on the side: "Hoya 72mm PL Japan".

I've had it for many years, and it's done its job very well on my ME Super, with my MF Sigma 28-200.

So just out of curiosity, I put it on a newly acquired AF lens (Pentax SMC FA28-200), on my K10D, and am surprised to discover that the AF seems to work, at least with a few indoors test shots.

I've obviously got at least one misconception here - could someone put me straight?

A circular polariser can be rotated. When attached to your lens, can you rotate it?

You need a circular polariser. All polarisers rotate, the circular refers to the direction of light polarisation.

A linear polarisier is suitable for older film cameras, pre-AF.

Duh, of course.

What, do you want me to post a couple of pics taken through it, to prove that I know what a polarising filter does?

Look, I know that many of you guys know far more than I do about this stuff. But trust me, I haven't been using a polarising filter on and off for the last twenty-odd years, in the belief that it has a fun rotaty thing on the end just for the hell of it, and also just happens to annoyingly make the viewfinder go a bit dark.

However, I've never had an AF-capable camera before the K10D, so I've tended not to pay too much attention to new-fangled gadgetry until now.

The point is, I've screwed it on to the front; it does its usual polarising thing.

Also, the AF works (trust me on this one too - I can tell, because I half-press the button, it makes a whizzy whirry noise, the picture in the viewfinder becomes sharp, and I get this cool red square in the middle).

So, out of the following possibilities:

- is it definitely a circular polariser (even though I thought it was a linear one)?
- is there something wrong with my belief that linear polarisers bugger up the AF
- or something else?

TIA

Ok, Captain's log, supplemental...

Just to assault received wisdom even further, I've had a rummage in my camera bag, and extracted my old Cokin filters.

Amongst which is one labelled 'P 160 Pola'

On its box it says "Linear Pola".. and to confirm this, a moment on the Cokin web site yields this:

http://www.cokin.com/ico3-p1-7.html

Clearly, the 160 is the linear, and the 164 is the circular.

With the 160 on the front, the AF still works.

So. What is the issue with the two different types, and AF as opposed to MF cameras?

Chris,

Like you I've been struggling with this one. From what I can find out the auto-focus issue is less of a problem than exposure. Linear filters can cause 2-3 stops under exposure in some cases depending on the exact nature of the metering system.

What's happpening with your exposures? Like you I have just moved to K10D (but from Spotties) and find autofocus OK when playing, but I haven't fired in anger yet so don't know about exposure.

Michael

Thanks. I'll have a look tomorrow sometime and report back...

I have several polarisers, all of them linear

I also have several Pentax autofocus cameras

my experience is that not one Pentax AF camera is actually upset by the use of a linear polariser. I have used ME-F, SFX, Z50p, MZ7, *istDL2 and K10D with success.

your mileage may vary.

Look through it into a mirror.

If it's linear it will look the same no matter which side you look through it.

If it's circular, one way it will look dark and if you flip it over it will look lighter.

Here's a link to everything you may want to know about polarisers.
http://www.luminous-landscape.com/tutorials/polarizers.shtml


Cheers
Drew

Here is a link to a more technical explanation.

http://www.jr-worldwi.de/photo/polarizer.html

Couple on interesting points from one site.

"LPs are simpler, less expensive and theoretically optically cleaner than their circular counterparts. If you know for a fact that your camera meters and auto-focuses properly with an LP, feel free to go linear. Otherwise, play it safe with a CP."

"Testing for Linear Polarizer Compatibility
It's easy to test your camera's LP compatibility, at least with regard to metering:

Mount the LP and fill the viewfinder with a large dull light-colored surface like an indoor wall.

Make sure no specular reflections, atmospheric scatter or other polarized light sources reach the camera during the test.

Under constant lighting, turn on the LCD and watch the exposure settings as you rotate the polarizer.
If the exposure settings change significantly over any 180° rotation of the polarizer ring, your camera requires a CP."

Also remember that if your lens is not of IF type then as you focus the degree of polarization will change and have to be re-adjusted after focusing. This is due to the front focusing ring rotating.

Thanks for noting this. And thanks to Galoot for posting the link (at which is also www.dpfwiw.com/polarizer.htm#circular, which has the LP test on it that RF mentions). Very pleased to report that my K10D passes this test. So I'll stick with my linear polarisers

Ah yes indeed.

The Sigma 28-200 I've used for years has a rotating front end, and its a right pain to constantly readjust the polariser. The Pentax one I've just acquired is IF, so another bonus!

A quick check showed no exposure problems with a linear polariser on my K10D. Admittedly that was in the back garden rather than the sort of picture I would be most likely to use a polariser on so I will reserve final judgment until then, but I wouldn't envisage problems.

Michael

All this does beg the question as to why it is that the K10D, and, it would appear, other Pentax AF cameras, seem to be able to use LPs perfectly well.

It seems that many others must not be able to, given that so many references are unequivocal in their insistence that CPs are required for AF cameras.

I think it's called "received wisdom", in other words something that's repeated so sften that it becomes one of the basic facts.

Of course "experts" can be utterly and dramatically wrong...fortunately, not most of the time!