Originally posted by WPRESTO A longer lens will not work = it will not be possible to position the lens & slide to get the latter in focus.
Actually, I use (on a A bellows) an cheap old Takumar-F 28-80 zoom. I can fill the APS-C sensor and focus with the lens at 80, but I usually shoot it back a bit ~70 to give me the space to rotate and crop later, and it works pretty well.
I don't know the features on the K10, but I'd strongly recommend not using the viewfinder, but using live mode with focus peaking if it has it. I have one of the user modes on my K-3ii set for digitizing, including turning on the 2 second delay so I get mirror lockup and my hands far from the camera so I don't create any shake. In live mode the button in the middle of the directional pad zooms in and I always zoom it in 10x, then pan around to find an area and will show the focus peaking, and adjust focus on at least each negative strip, though I will adjust mid strip if its a shot I want to be sure comes out well. I have exposure set to full manual, but I use the green button to pick the shutter speed.
Remember that the original design of the bellows involved a double cable that would stop down the lens and trigger the shutter. Since I don't use that I have the aperture locked down permanently. That is the "auto" par of auto bellows. If thats what you have, and you're not locking down the aperture before firing, you're shooting at wide open, no matter where the lens is set. If you bellows isn't an "auto bellows" its not an issue. I have the lens set to ƒ8 and have gotten sharp photos even if the shutter time is as long as 2 seconds. My light source is an open window. Its less than ideal but works.
Just an example of a scan done with this setup: