Originally posted by Lloyd_Christmas I assumed that I could just move the bellows in and out until I find a sweet spot?
In theory this would be true. However in order to cover the entire slide frame you need to move the slide further away from the lens on a crop sensor vs full frame. There practical limitation however. the rail with slide attachment is too short to move the camera and lens far enough away. And even if you are able to extend the rail the bellows on the slide copier is not long enough to reach the lens.
There are several threads on work around for these practical limitations. Some are quite clever.
From my calculations one needs 0.66× magnification to cover an entire slide frame on a crop sensor. According to my Bellows II manual the following lenses work with that magnifcation (lens followed by film-to-subject distance cm/in):
Super-Takumar 55mm f/1.8 - 22.9/9.02
Super-Takumar 85mm f/1.9 - between 36.6/14.41 and 33.9/13.42 (0.55 and 0.8×)
*Takumar Macro 50mm f/4 (0.72×) - 21.3/8.39
*Note: this is the 1× version that does not require an extension tube to reach this magnification. However you will not get the entire slide frame. The lens is somewhat rare or much sought so prices are higher.
I have been unable to find complete close-up tables for any of the 0.5× 50mm macro lenses, e.g. SMC Takumar Macro 50 f/4, Pentax 50mm f/4 Macro K,M or A. However Murray O'Neill's Pentax Macro and Close-up Accessories eBook has some partial tables with the K-mount lenses and extension tubes. Unfortunately the tables for the K-mount bellows don't include the minimum bellows extension of 38mm.
According to the tables for extension tubes (I'm unsure what the values in parenthesis mean):
A 50mm f/2.8, M 50mm f/4 macro with tube 1 = 12mm: 0.72(0.68)× - 21.6(21.8)/8.50(8.58)
K 50mm f/4 macro: 0.73(0.68)× - 21.3(21.5)
However, these tables are with the lens set at the minimum distance of 0.245 meters so it might be possible to use these lenses set at infinity. Maybe I can be motivated enough today to test the SMC Takumar at infinity. Update: see edit note below.
The minimum extension on the Bellows II is 37mm and the K-mount bellows is 38mm. Adding adapters and extension tubes would increase the minimum extension.
If the sensor to subject (slide) in any of these combinations exceeds the maximum of the bellows and slide copier than they won't work without modification.
Edit: Well so much for that. With the macro set to infinity I was still unable to fit the entire frame on the sensor.