100/2 USM

I can't say why, but once I saw this lens, I had to have it. But Alex, I said to myself, you've already got a 70-210mm zoom! What do you need with a 100mm prime? For low light portraits, dumbass, I replied. Plus, the 70-210 is big and kinda heavy and slow and generally a mediocre performer. And lord knows, I didn't want optical mediocrity interfering with what I secretly hoped would become the 100/2's primary purpose: photographing naked women! So far, the lens hasn't quite come into its own with regards to the naked women bit, but otherwise it's been great.

It's a pretty compact and lightweight lens, no heavier or longer than my 24-85, and coincidentally, it takes the same hood as my 70-210, so yay for component commonality. I really like the perspective of this lens; I find myself using it a lot for general shooting (not just naked women), but I'm still coming to grips with the super-shallow depth of field that it has at maximum aperture. It's also deceptively difficult to handhold at slow shutter speeds--I'm pretty confident holding my 24-85 at 85mm down to about 1/15 of a second, but doing that with the 100 results in blurry images. I know, I know, the old rule is that you can't handhold lower than the reciprocal of the focal length, but we all know that you can break that rule if you're steady. The shake of your hands is really noticeable when using the 70-210 zoomed all the way out, but on the 100 it doesn't really appear in the viewfinder.

It also has a wonderful, wide manual focus ring, with the added benefit (thanks to the miracle of ring USM) of full-time manual focus. It's mighty sharp, though as I said before, I'm hardly a sharpness nazi, so I can't tell you what its lpm resolution or MTF curves are. BTW, this was another successful Ebay purchase, and I got a great price on it. I love this lens.