I have two 4x5 cameras, a Speed Graphic and a Sinar. But try as I might, with a range of different lenses, I can't get any images that are truly sharp.

This image illustrates the problem. On the left, a crop from a 4x5 negative (scanned on an Epson V600). On the right, a crop of the same scene from my Nikon D800 (focal length equivalent on my Nikon was twice that of the lens on the Sinar, so it's not 100% fair, but it does illustrate the problem clearly).

Click image for larger version.  Name: compare.jpg  Views: 163  Size: 56.8 KB  ID: 202020

This lens was a Schneider Kreuznach 135mm at f/16, but it doesn't seem to make any difference what lens I use. This particular negative was Fomapan 100, but Ilford HP5 is no better.

I was expecting resolution to spare, but I actually have a lot less than I do on my Nikon. What's going on?

The only constant is my film holders - these are plastic Riteway models. But could the registration be so far off that nothing is sharp, anywhere in the image?

Any help would be appreciated, because I'm at a bit of a loss!