[vandamro]

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This is a really newbie question. I've never used a circular polarizer. I generally understand their physics. But other than reducing glare off of shiny surfaces, where else would you use them? What effect would you get?

Are there any good rules of thumb for their use? (e.g. always on a sunny day, always outdoors) Or is it a purely a case by case basis?

I'm just trying to figure out it I should start thinking about making a CPL filter investment.

[Parallax]

A polarizer will serve as a neutral density filter. It allows using a larger aperture in in bright light than would be used for a given shutter speed/ISO setting without the filter.

[vandamro]

Do they increase contrast in images?

[Nass]

I'm not sure if I would describe their effect as just increasing contrast as such, but they will make certain colours stronger. For example, if you have blue sky, then the blue does tend to be stronger/deeper if polarised, that's what I tend to use mine for. This istockphoto gives you a bit of an idea

[vandamro]

So I have been doing my reading on CP filters.

The big question I have now is about these step up and step down adapter rings. Certainly, that would save money from having to buy so many filters.

I have a lens with an 82mm thread size. Can I use something smaller? How much smaller can I do without causing vignetting?

Also is there a limit on how much you can step down?

Any advice would be appreciated.

[Parallax]

So I have been doing my reading on CP filters.

The big question I have now is about these step up and step down adapter rings. Certainly, that would save money from having to buy so many filters.

I have a lens with an 82mm thread size. Can I use something smaller? How much smaller can I do without causing vignetting?

Also is there a limit on how much you can step down?

Any advice would be appreciated.

How much smaller you can go would depend on several things, including the FL of the lens and whether it is a full frame coverage lens or not.Try cutting some different size holes in cardboard and shooting through them.
If it is a concern, I would buy an 82mm filter and rings to allow its use on smaller filter size lenses. An 82mm polarizer will be pricey but it should be usable on just about anything with the proper ring.

[Nass]

So I have been doing my reading on CP filters. The big question I have now is about these step up and step down adapter rings. Certainly, that would save money from having to buy so many filters.

I have a lens with an 82mm thread size. Can I use something smaller? How much smaller can I do without causing vignetting? Also is there a limit on how much you can step down?

Any advice would be appreciated.

You don't want to step down with filters, you want to step up . If you step down then it's like the photo above, you're putting on a filter that only works for part of the frame. What I have, is step up rings and expensive filters in my largest filter thread. By having it in my largest filter diameter thread it covers the biggest lens I have, and by using this cheap step up set from China, I can use that same filter on all my other lenses by just adding this.

A "but" is if you need to focus very close to the lens, for example macro. Having that set on your lens adds a bit of an inverted cone onto the end and that might be an issue if you're trying to focus superclose. The slight caveat I can foresee here might be full frame lenses on APS-c (digital) bodies. Since you're only using part of the full frame 'width' in theory you can use a step down with that without the coverage suffering. I havn't had to do this myself though so I can't honestly give you a reliable answer on that.

[Ben_Edict]

Do they increase contrast in images?

No and yes! No, actually the decrease excessive contrast, as they absorb the polarized part of the light. I.e. thy large amount of polarized light, you'll find in a clear sunny sky. In that case, the sky brightness is reduced and the overall brightness contrast of the image (bright sky versus darker forground) goes down.

At the same time the colour contrast increases as the sky's blue gets more saturated.

The same is true for foliage, where all the typical reflections off the leaves can be reduced with the polarizer. Also, reflections on water surfaces will be reduced or eliminated, bringing down brightness etc.

This effect is the same with linear and circular polarizers and both varieties can be used with Pentax DSLRs without problems.

Ben

[RioRico]

and by using this cheap step up set from China, I can use that same filter on all my other lenses by just adding this.

Thanks for the link to eBay.uk for those rings, priced at £7.99 -- the best price for a comparable set at eBay.us is US$12.99 -- but with today's dollar/pound rate of 1.53 the eBay.uk price is US$12.29 (all with free shipping from HK) so I've just saved over 5% (and yes I went for it). I guess that's called arbitrage. But I digress...

"A penny saved is a penny."
--anon.