I'd like to challenge the view that 6x9 folders give soft pictures
 

RolandRB wrote:
On Jul 8, 8:38 am, RolandRB wrote:
snip

The same camera worked less well at 1/200 shutter speed than 1/100.
There was very noticeable shake in nearly all the shots. I would have
thought it would be the other way round.

I'm not saying your explanation is wrong, but you might consider another
explanation that at 1/100 you were using a smaller f stop, thereby
increasing the "depth of focus", and thereby compensating for the film
not being flat!

It is possible to distinguish unflat-film softness from camera-moved
softness in some test images but maybe not all.

It seems to be well known that roll-film backs (e.g. for 6X9
Graflex),vary in ability to hold the film flat (lever vs. knob) but I
think you can experiment and look at the flatness by assembling a fake
roll of film with (exposed/processed) film taped in place, and just
looking at it to see if it visibly buckles. It probably will be visible.
You may be able to experiment to try out these ideas about whether to
advance the film just before exposure or not, to avoid some curl or fold
or whatever.