Thanks for the suggestion. I checked out the G-Claron on bhphoto.com. Their desription:
"The G-Claron is a lens of symmetrical design with six elements in four groups, optimized for 1:1 reproduction. The normally used range of linear magnifications is 5:1 to 1:5. The G-Claron may also be used for distances up to infinity by stopping down to f/22 or less. For photographic work the G-Claron can be used up to an angle of 64 degrees and is free from mechanical vignetting at f/16 and smaller apertures."
I usually photograph macro subjects at very wide apertures, so I'm wondering whether or not I could use a G-Claron, based on the above quote. I'd probably want to use it at f/9 to f/16, but not much beyond that. How much "mechanical vignetting" are they talking about?