Using a DSLR is fine for viewing things on the web but it can't touch the raw output of a multiple pass scan from a dedicated negative scanner. A DSLR can't match the bit depth of what a negative scanner can do. The AF module of the camera focuses on the surface of the emulsion, not the grain. It doesn't have that resolution and it is mostly focusing on the acetate. Up close with the best AF macro lens you can find is still not good enough to pull out grain focusing because camera AF sensors don't have that resolution. They probably can't tell the difference between the thickness of one sheet of acetate and two. So you have a built in error which can be the whole film itself. If the camera had active IR AF, it might be able to get a little closer. Then you have the issue of vignetting because there are no true process lenses made which fit a DSLR and have AF. You're still better off focusing by eye. In a nutshell, scanning negatives that way is good enough for the web but if one is wed to a DSLR, it would be best to wet print then take the photo of the print itself. Far more detail is available that way.
Phil Forrest