Originally posted by gmans My apology about reading the OP, did not mean to sound harsh. I have the 50mm F4 macro very nice little lens, it is I believe a flat field copy lens designed for copy work. the distortion would possibly be greater given the frog is not flat and the depth field that is required? Anyhow interesting debate. Comparsion between the 50mm or 100mm Macro versus the 18-55. Anyone?
No worries-- I wasn't actually aware that the 50mm f/4 Macro was a flat-field type lens, either. I had a 55mm Micro-Nikkor in my Nikon days, but found that short of a macro to be pretty much useless in the field. the 100mm f/4 macro seems quite suited to field work from what I've seen and used. If you need more DOF, stop down and use flash.
Also consider the OP will be using in the field to take pictures of living things. For example, something in the middle of his size range (50mm) would give him a 4-6" working range @ 55mm. Getting a big lens that close to something alive will be challenging.
Another thing to consider, is that since you're on APS-C, close-up filters work quite well. With a +3 Hoya Close-up on an SMC-M 50mm f/1.7, this is what I got straight out of camera:
Sharp corner to corner. Since the close-up filters are worst at the edges, it isn't a problem with larger image circles-- and if you rack out an APS-C lens for zoom and focus, most will do OK. They seem to preserve working distance, too.
I've got an extra set of the Hoyas over in the marketplace if you're interested in playing with them. $5, so no risk really, vs. the $40 new Hoya is charging these days.