ND + CPL Stacking

I was thinking of getting a CPL and ND filter (both screw-in type) and I intend to stack the 2 filters for taking long exposures of flowing water. I'm going to be using a T1i and 18-55mm lens. (or a 17-55mm lens sometime later). Let's just say that the filters are non-slim mount. How bad would the vignetting be?

Do you or a friend or a local camera store have 2 filters of equivalent rim height that would fit your lens? Stack them and shoot something solidly light (makes seeing the vignetting easier) like a blue sky using manual exposure to get exposure correct. Open your camera to widest aperture and then take a series of exposures at various focal lengths at BOTH nearest and infinity focus. Download and check on your monitor. You can run them thru Auto Levels if you really want to be exacting about it. Canon typically is pretty good about giving a wide berth for their lenses re filter diameters, even for 2 stacked. However, depending on the rim heights (you say non thin rims) even then its usually not feasible as you get to the real wide angle focal lengths. I'd expect no problems until you get down to low 20's (mm), at least on an APS-C camera.

good idea. thanks. I'll probably go to my local camera store and play around with stacked filters.

When you do your test on images, shoot a set WITHOUT any filters as a reference. Why? Because there is a natural darkening of any lens in the corners NOT due to vignetting by anything external to the lens optics itself. You want to make sure when you see suspected vignetting that it is DIFFERENT than the an image at the equivalent aperture, focal length, and near or far focus distance naked of any filters.

I see. So basically i'm eliminating the possibility that the vignetting is caused by the lens and not the filters right? cool2.