greybeard
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One thing not mentioned about copper piping is that water plumbing is normally sweat-soldered using 50/50 tin-lead. Industrially, copper tubing for compressed air joined with a low-melting braze alloy. The reason is that lead-tin solder is not good for cyclic stress, and there is a lot more stored energy in a line full of compressed air than there is in a line full of domestic water. (For different reasons, PVC pipe is also a bad idea for compressed air.)
That having been said, if you keep the line pressure down and essentially all of the pipe is inside the wall, you are unlikely to have any dramatic experiences with the setup. The simplest transition from rigid or flexible copper to pipe threads at point of use is the compression style with ferrules, which avoids the solder/braze issue entirely.
From a safety standpoint, then, you are probably all right. However: for photographic use, the air has to be really clean, and condensation in the supply line will dictate a point-of-use filter/regulator assembly. Expansion and cooling will deposit water (and possibly also oil) in the line, and the only practical place to take it out is just before use. By the time you have priced the copper line, fittings, and filter/regulator, you may find that a small oilless "pancake" compressor complete with regulator and filter will actually be cheaper, as well as portable.
That having been said, if you keep the line pressure down and essentially all of the pipe is inside the wall, you are unlikely to have any dramatic experiences with the setup. The simplest transition from rigid or flexible copper to pipe threads at point of use is the compression style with ferrules, which avoids the solder/braze issue entirely.
From a safety standpoint, then, you are probably all right. However: for photographic use, the air has to be really clean, and condensation in the supply line will dictate a point-of-use filter/regulator assembly. Expansion and cooling will deposit water (and possibly also oil) in the line, and the only practical place to take it out is just before use. By the time you have priced the copper line, fittings, and filter/regulator, you may find that a small oilless "pancake" compressor complete with regulator and filter will actually be cheaper, as well as portable.