revup67 said:
Neuroanatomist: I had a hunch you were going to say all of that. And follow each of your points / reasons for the 58C and 67C though one question remains to be answered: with the 67C stepping down to the 58mm Macro 24 EX Flash..any vignetting or other issues that might occur while stepping down? As I've got the 100mm IS USM L 2.8 and don't see much need for a flash (at least at this point) unless you feel its imperative.
No vignetting on the 100 L Macro. If you look at the front of that lens, you'll see that (much like the MP-E 65mm) the front element is much smaller than the filter threads - all that 'dead space' leaves plenty of buffer against vignetting.
Depending on the subject, I find flash to be helpful with the 100 L macro as well, though not as 'necessary' as with the MP-E 65mm. Yes, the 100mm L has IS - but, keep in mind that IS is less effective at macro distances - that 4-stop IS only functions as a 2-stop IS at 1:1. Also, there's often a need to stop down for more DoF and at 1:1 the apparent aperture loses 2 stops. So, if you're taking a 1:1 shot at f/11, you're getting the amount of light you'd get at f/22 at 'normal' subject distances, and you've only got two stops of IS to help with handholding, and I prefer to keep the ISO as low as possible. The other factor is that subjects can move, and IS works by allowing slower shutter speeds - taking a shot of flowers on your dining table is fine with a long exposure, but shooting a flower moving in the wind, or a moving insect, mean longer shutter speeds result in subject motion blur.
Here's a shot from the 100mm L with flash. It was outdoors on a breezy overcast day, and I was under a canopy of trees - not enough light for a fast enough shutter, short of going to ISO 6400 (not somewhere I ever go!).
EOS 7D, EF 100mm f/2.8L Macro IS USM, 1/60 s, f/8, ISO 400