neuroanatomist said:
For slight changes in magnification, yes. But a fixed focal length macro lens cannot change magnification 3-fold or more, as in coin vs. lighter. That's going to require moving the camera - re-framing the shot.
This is just a terminology quibble, I think, so I tried to select the term I felt best clarifies what's going on. I still think that "framing" is, in this case, exactly the same as changing the magnification. Even with the MP-E 65mm, the focus and magnification are changed at the same time; there is no stable point of focus in front of the lens while magnification is changed, nor can you move focus in or out arbitrarily when changing the magnification. In my example, when I change the focus that implies moving the camera (relative to the subject - obviously it doesn't matter if you move the subject closer; whatever works best for your tripod or work setup). The MP-E is a poor lens for photography of larger products because of its relatively very high magnification and limited focus range; it is probably best to start with a less-magnifying lens like the TS-E 90mm for larger objects. If your subject is watch dials, or coins, however, it may be far enough (if those subjects are no bigger than a full frame of film, and less for APS-C).
I think that we're in agreement on this point, unless I'm mistaken. I just don't want anybody to be confused, because of me or otherwise.
You're absolutely correct that you can't change magnification three-fold (or whatever it is) with a fixed-length lens easily (ignoring the MP-E 65mm and similar macros, and tons of extension tubes) and even if you had enough extenders the image quality might start to degrade too much. However, some really good MP-E 65mm macro users must shoot at the smallest apertures to maximize DOF; in this case it doesn't really matter if the whole shot "isn't equally sharp, it's all equally blurred" (to paraphrase Bob Atkins talking about pinhole cameras), because we care about control over DOF, which naturally comes at the expense of sharpness, no matter the lens, for macros.
RE: the 135/2 vs the TS-E 90, the 135L achieves 0.19x native and 0.41x with a 25mm extension tube; the TS-E 90 has a much closer MFD and achieves 0.29x native and 0.60x with a 25mm tube.
I am actually getting significantly more than this because the EF-S compatible set of Kenko auto extension tubes allows three extenders for 68mm of extension at essentially the same price as a single Canon 25mm tube. I see I was in error to not mention this, since lately my focus was just on getting the most magnification out of both lenses (with the full 68mm of Kenko air) which of course is not a useful comparison when talking about taking a standard picture with both lenses. The TS-E 90mm could get a bit more working length with less extension tube use, although when you consider its minimum focus distance natively is marked at 0.45 meters, significantly less than the 135mm f/2L, it's possible that it will always offer less working distance for the same magnification. Even with 68mm extension, the working distances feel pretty generous.
I haven't used the MP-E 65mm myself, though, but stopping down even with 68mm of extension on the TS-E 90mm or the 135mm f/2L offers good DOF control for subjects that fill the frame without obviously sacrificing sharpness. With 68mm extension, a single drop of water on a placemat extended 1/3 to the edges of the frame with the 90mm, so f/7.1 was almost enough to get it all sharp. In comparison, I tended to use the 135mm f/2L at its maximum (still quite short) focal length.
All said, of these two lenses the TS-E 90mm is a much more flexible option for product photos: More maximum magnification is available and tilt allows more control. That said, I think it's easy to overestimate how much use it is. Unless your subject fills the frame, you will essentially be rotating DOF through the focal point, especially when wide open, and it doesn't rotate all that much, especially on a crop camera. However, I am not ready to count out the 135mm f/2L, especially for wildlife macro (or near-macro).
I can't be of much help on the subject of lighting because I don't know if the short working distances are too short to light properly, and it's not a central issue for me when I use ambient light and no flash. A ring flash may not be possible on the 135mm f/2L due to its larger size, and with less extension than needed for magnifying individual drops of water it may still be possible to adequately light subjects for the TS-E 90mm with lights slightly to the side.
RE: DoF, that's the main challenge, IMO. With a coin or other object well-represented in 2D (as Mt. Spokane's shot nicely shows), DoF isn't an issue. But with a 3D object like many small products the OP will likely want to shoot, DoF may be a limiting factor. With a macro lens or a telephoto like the 135L, that means soft images at f/22-32 or focus stacking. Or, a TS-E lens where the tilt allows control over DoF.
That's worth reinforcing. Can't disagree with that.