Polarised filters

I'm having a quick rummage thro' t'internet for a circular polariser and, as with most things, there's a huge difference in prices. Presumably, this reflects a huge difference in quality.

So, from a £10 "Kood" to a £62 "Hoya Pro 1", is there really £52 of a difference between them?

Any recommendations?

Ta much in advance.

If you want to maintain maximum quality then buy the best you can afford. Not all filters are made to the same standards.

The most common problem with polarisers is a colour shift and expensive ones will minimise that.

Having said that, sometimes end of line or bulk purchase bargains can change the usual maxim that you get what you pay for.

As I've posted on a number of occasions, first establish that you actually need a circular polariser.

You haven't said which camera you're using, but linear polarisers work perfectly well with the K10D, and maybe with the later Pentax dSLRs too.

They're a lot cheaper than the CPs.

Someone here tested that and it seems not to be true...

Tested what, specifically?

With the K10D, or with the later ones?

Point it to a matte wall or object, white paper for example and turn the filter.
Since no reflection change, light stays the same but with a linear filter the camera changed the shutterspeed at around 1 stop when set to Av while turning, with circular filter it doesn't changed at all.

Twas me! I tried linear, can't remember exactly what the results were but I ended up with a circular Hoya at £39 from Warehouse Express. Results are champion.

Bottom line is linear polarisers don't allow light metering to work correctly. With CPL, the more you pay the better you get basically. Better in terms of colour rendition / lack of colour cast.

And as I have said before the instruction leaflet that comes with Pentax CPLs explains why it is necessary to use them since the linear ones DO affect metering.

I think there was a rumour that they were necessary because they affected autofocus which isn't true with Pentax

Well, well... you learn something new every day. All this time I've thought that the exposure is supposed to change as you rotate the filter.

Naively, I always assumed that since less light was getting in as you approach the ideal angle, you'd need more exposure, but it seems that it's not supposed to be dependent on the angle of the (linearly) polarised light.

I've done a bit of digging, and this seems to be quite a good article.

Here's how I'm now understanding it - perhaps anyone that really understands the detail would tell me if I've now got this right:

- A LP allows only light through if it's polarised at a particular angle.
- Reflected light tends also to be polarised at a particular angle
- So if you rotate the polariser so that its 'allowed' angle is at right angles to the angle of polarisation of the reflected light, you cut out most of that reflected light.

So far so good, hopefully.

- The light meter measures the intensity of light which is itself polarised, since it's reflected off a partially reflecting mirror.
- If the light hitting the mirror is polarised only at one angle, then the intensity of the light hitting the meter will depend on what that angle was - if it's same, it'll be unchanged, but if it's at right angles, it'll be less.
- Since the sensor doesn't care what angle the polarised light hitting it is at, it's not subject to this variation in polarisation angle.
- So you can get variable metering results, for an amount of light that would be the same by the time it hit the sensor.

Ok. There's one thing that I still don't understand - why is the light coming off the mirror polarised? I thought reflections from metal surfaces retained whatever polarisation they already had. Is it because the mirror is only partially reflecting? If so, why does that make the difference?


Anyway, all very interesting though it all is, if the potential error is only a stop, as suggested in the Bob Atkins article, given all the other variables that contribute to getting the exposure right, I'm still in no hurry to go out and replace my LPs with CPs.

I've used my LPs ever since getting my K10D, and never once noticed a significant problem. Which isn't to say that it's not there, just that it's not large compared with the other things I get wrong

Something else I just noticed...

Point a polariser at your computer screen (assuming it's an LCD monitor, and if it's a CP, ensure that you look through it in the same direction as the camera would).

Rotate the filter.

The screen goes almost completely black.

I never knew that the light coming off an LCD was polarised. But I guess it is.

You are right about that, it's not a huge problem but it's there. now that you know you can deal with it.
Linear filters are also more effective than circular so there is not a big reason to throw them away at all

Thanks for the advice, and for the link, ChrisA. I now have a K-X with a couple of zooms.

I used to keep a simple and cheap UV filter on all my lenses to protect the front element from damage, dirt, fingers etc. I reckon a polariser (although not quite as cheap) will do the same but also cut down on reflections and improve colour saturation.

I'm off to buy a couple of "Hoyas" now - but not the 62 quid ones.

Fine in bright light but loosing ?2 stops of light might make you want to take them off fairly often too!

Interesting about CPLs not being as effective as LPLs. I thought my CPLs weren't as good as I remember the LPLs being. As I use the CPL for static subjects the exposure error is not a problem on digital. More so with slides I guess but I don't remember having problems with my ME Super. Oh well...

K.

Be careful if you go for ones advertised on ebay - there are some imitations out there, they look exactly the same and packaged correctly but are cheap and nasty being offered at high but below the real Hoya price.