Stacking Filters

I generally use one filter per lens, but when I want to use my cp I find myself hassling over getting the UV off before mounting the cp. My concern with stacking the cp is that the effect degrades the further it is from the front element of the lens, say with a UV in between.

I have heard of shooters using multiple nd filters for slow shutters though.

What's the collective wisdom on this?

Depends also on what lens you are using. Stacking on a wide angle would probably cause vignetting. And yes, stacking increases your chances of degrading your photo, especially if you're using cheap filters.

the vignetting problem woulnd' be too apparent on a crop camera, right? For example a 20D and a 17-40L?

As far as vignetting goes, its best to experiment and see how many filters you can add before it becomes a problem at the different focal lengths.
I use square filters and have stacked as many as 3 at one time. 2 is not at all unusual for me. Yes, of course there will be some degradation of quality. The more glass you have ..... If the filters are very good quality then the images should still be acceptable.

Stacking filters is something to avoid at all costs, because of the liklihood of degradation of the image.

There's really no reason to leave a UV filter on a lens all the time anyhow. Use a properly designed rigid lens hood for day-to-day protection of the lens (and, of course, for blocking stray light) and have a UV filter for the really nasty situations such as blowing sand or salt water.

Well I see that SkipD has pitched in with his typical and frequent "anti-UV" rant, but the use of a protective filter is your choice.

As mentioned, stacking 2 filters on a full frame lens while mounted on a crop camera should not cause vignetting. And the quality of the filter can play a role in the image quality.

I almost always leave my UV on when using my CP. I don't ever recall seeing an issue with image quality.

For an EF-S lens, keep in mind that Canon usually designs their lenses to allow one filter without vignetting, so you may not be as lucky with them.

Have Fun,

Wow, that's a very sweeping statement to make. Pros like Joe Cornish and Guy Edwardes stack filters occasionally - Sometimes circumstances are such that stacking is desirable for some of us. As I already mentioned, there will be some reduction in quality, but to get the shot and with good quality filters, it can be acceptable.

As Rick mentioned, using a UV filter is down to personal preference.

It depends on the lens; My EF-S 18-55 has a tendentcy to vignette wide open even without a filter, which is one reason I upgraded to a 24-105L as my walk-around lens.