Originally posted by Dubesor * Crop factor / shooting distance - Is the 100mm long for the APS-C crop factor? I forget which exact camera my dad has, but I think it's a K20D or K200D. The works he'll be digitizing are in the 20x20 inch range. I'm having a hard time visualizing what the shooting distance would be to fit a work like that in one shot on 100mm APS-C (or 50mm or 35mm).
I have the 50 & 100 and an aps-c camera. For this sized object, I'd pick the 50mm for a working distance of a bit over 5 feet, but it depends on the space you have available. The 35mm would probably be a bit more convenient distance-wise. I'd only plan on stitching if there's a specific need for higher resolution outputs.
Originally posted by Dubesor * Distortion / Sharpness - Among the macro choices, would the 100/2.8 have the least distortion, be the most flat-field, and have the best sharpness, etc?
Between the 100 and 50, I'd pick the 50 but I'd also say they're both excellent to the point it's not much of a choice. The biggest flaw of the 100 is purple fringing on high contrast edges wide-open, which won't be a problem for reproduction work.
As mentioned above though, there are plenty of 'non-macro' lenses that could do the job very well.
Originally posted by Dubesor * Other applications - A side consideration is other uses of the lens. I think my dad could use the lens I get for other things, for example the original macro purpose, portraiture, landscapes. It seems for actual macro work, 100mm is a better choice because you can shoot from a little further away (especially for small moving subjects)? And reports say DFA 100 2.8 macro is a strong portrait performer. What situations do the DFA 50/2.8 Macro and the DA 35/2.8 Macro shine at?
Absolutely right about the minimum focusing distance, the 50mm is close enough at 1:1 that it gets tough to position lights or reflectors, the 35mm would be even worse. I usually opt for the 100mm if I know I'm going to be working at higher magnifications, but the 50mm makes a great 'walk-around' for photographing non-moving objects that aren't too tiny. It would depend on what else he has in his bag, how you prioritize the secondary purposes, and what subjects he's likely to aim at.