More newbie questions (large magnification macro)

The relevant equations are: 1/o + 1/b = 1/f, m = 1 / (o/f - 1), m = b/f - 1, where m is magnification, f is focal length, o is object distance (lens to object) and b is bellows length (lens to film). Be sure to measure f, o and b in the same units.

For m=3, f=120 mm, one gets b=480 mm (19 in) and o=160 mm (6 in). This bellows length exceeds the capability of some cameras. The object distance might be inconvenient, e.g., lighting the object without the camera getting in the way. For m=3, f=180 mm, one gets b=720mm (28 in) and o=240 mm (9 in). Very few cameras will permit such an extension.

For this one of work you will need a solid (heavy and/or precise) setup because of the long bellows extension and the long exposures. Otherwise vibration will obviate the advantages of going to large format. Exposures will be long, from the light being spread out because of the long bellows extension, the need to stop down for depth of field, and the resulting reciprocity correction. You might not even need a shutter! Unless you really pour the light on or use flash, exposures will probably be well over 1 second.

Other options: Schneider sells shorter focal length lenses (enlarginging lenses) in shutter. These would reduce b and o. They might make o too small. Or you could use a view camera to shoot medium format using a roll film back. These lenses have correspondingly smaller coverage. But at magnifications >= 1 this won't be a problem: the coverage of a lens (the largest image it can project) is double at 1:1 compared to the coverage at infinity.

Nikon, Rodenstock, Schneider all make macro lenses. The Rodenstock one calls for switching the elements depending on magnification. Most macro lenses are optimized for 1:1 but will perform well over a range of magnifications. Many use the G-Claron (which is optimized for 1:1) for infinity work. But with your high magnifications, you probably wouldn't want the G-Claron because it is slow (f9), versus one of the modern faster macro lenses.

In terms of choosing a lens, the boundary for designs for infinity work vs macro work is magnifications of roughly 0.1 or larger. In many cases one lens will work adequately at all ranges. But many macro lenses have smaller coverages than standard lenses, and so might not cover the film for infinity work, or if they do, not allow much movements. For architecture work you may find yourself wanting to use large movements. Ultimately, you might need two lenses. Start with one and see how things work...

Good luck.