at 100mm, you dun have to be as close as the 50mm to get the same size in your VF... This is why the insect shooters tend to use longer lens so as not to spook the insects away....
I know coz I had a spider jump onto a 60mm Nikon micro when I was trying to take it's pic. :bsmilie: You probably won't have such a problem with a 105 or a 180mm..