Generally speaking, the Canon EF 100mm is a great lens. The bokeh is fine, if blur is your thing. The lens is pretty fast. f/2.8 to f/22 is useful as a creative range. The optics are sharp and the all-time focusing is a boon.
So what's not to like? Well, here's the thing. The Canon EOS50D, which I was using at the time of this review, has the ability to use the viewfinder in "live preview" mode, and when doing so, will allow you to zoom in on your focus point (or anywhere else, but that's irrelevant to my point here) such that you can see extremely fine detail. At which point you can manually focus the lens so that it is exactly right. Marvelous, right?
It would be. But the lens has some mechanical backlash problems. Let me explain backlash; if you're not familiar with it, it takes a bit of describing.
Backlash is a problem that occurs in gear trains or arrangements that causes the finest adjustments to repeatedly over or under adjust, depending on which way you are going, because the gears don't (can't, really) fit perfectly together.
In a standard gear train, the gear teeth have a little bit of freedom to move before they make contact with the teeth of the next gear in the gear train, and this causes movement of the gear to be without resistance.
The gear turns relatively easily, because it isn't actually engaged, and then when the teeth actually make contact a fraction of a turn later, they "slam" together and more of an adjustment is made than one would expect.
Which in turn requires adjusting the other way, but now the gear has to move back across the non-engaged space and slams into the tooth behind in just the same manner. This can result in a very frustrating experience.
So now we get to the issue with the lens. The focus adjustment has a noticeable backlash problem which becomes evident at the very highest magnification of my camera's live preview.
Using the 100mm, I undertook to capture an image of a small creature (about 50 thousandths of an inch across) on the wall of my salt water aquarium. I used live preview, max magnification, and manual focus:
I got it done as you can see above, but this was an experience of great disagreement between the lens and me. I spent literally ten minutes trying to get the lens to the best possible focus.
I would touch the lens ring, and the focus would get sharper, and then frustratingly, slide right past that setting to slightly blurry again. I'd then adjust the other way, and the same thing would happen again the other way.
The camera was on a tripod, it was orthogonal to the plane the object I was trying to focus on was attached to, the object was agreeably motionless — all the recipe ingredients for an easy to focus situation. Except we are talking about a very small object, and so very fine focus control is required.
Is this going to bother you in your application? Probably not, unless you shoot very tiny things, as I do from time to time. Even then, we're talking about trying to squeeze the very last wisps of focus accuracy the lens is capable of out of it.
But after spending almost $500 on a lens, would you not want to get all you can out of the optics? I know I do. If what you're shooting is moving, you could easily lose the best possible shot because the focus is just about uncontrollable in that last percentile or so.
It is possible that I got a bad lens; perhaps others would be so good to check this out before they post their reviews. But I have to review the lens I have, because that's the experience I've had.