The way I understood it stopping down increased the image circle
because it improved the quality of the image. But then I found this quote

"4. Why does the size of the image circle in a Large Format lens change as
the lens is stopped down?

Well . . . it doesn't actually. What actually happens is that when the
lens is wide open, the center of the image is very bright and the corners of
the image are VERY dark. So dark in fact that they do not expose the film
(When the film is properly exposed for the center of the image). As the lens
is stopped down, uniformity of illumination improves and therefore the
circle of usable illumination increases."

That's from here

http://www.schneideroptics.com/info/...mat_lenses/#q3

So if I understand this. You could use a lens wide open and get the same
image circle if you could use something like a centre filter? In other words
the edges shouldn't be soft. Does this make sense to anybody?

Nick