Correct filter size, or step up/down rings?

[mikule]

I'm looking at adding a few filters to my current setup, mostly to experiment with b&w photography, but also wouldn't mind getting a polarising filter for use with colour.

Just wondering,do many of you just look around until you find the right size filter for your lens, for example a 40.5mm? or find a more "common" thread size and step up/down to the lens you want to use it on?

Also, does any one have any experiance with 'filter kits' where you use the square filter sheets as opposed to the screw on ones? I'm guessing most people wouldn't favour those, especially for street photography, as they seem a little cumbersome and intrusive.

Any feedback would be great,

cheers
Michael

[rover]

The only size I have had trouble finding is 40mm for Canon lenses. 39 and 40.5mm are not uncommon, though you may not find a great selection of used ones out there. I have bought screw in filters for my lenses.

[laptoprob]

I have a Minolta red and UV in 40,5. Not sure of an orange one... Not using it since I sold my Rokkor 40mm.

[Pherdinand]

Step UP is okay, sometimes convenient. Step down? never.