I am interested in both lenses. I think they are great lenses but cannot decide between the two. The 100mm can go down to f2.8. However, the 180mm has got more focal lenth. On the other hand, I could use extensions with the 100mm.
Does anyone have any experience with both lenses and could share a few thoughts?
The main difference is that the 100 f/2.8 is more suited to do double duty as a portrait lens due to the shorter focal length IS and aperture, whilst the 180 f/3.5 offers considerably more working distance for macro photography. It will focus to infinity but is a bit long and heavy for general use.
If you want greater than 1:1 magnification then a reversing ring will give you more mileage than tubes on a 100mm. Ultimately it comes down to how serious you are about macro photography as the 100 is a far better all rounder but the 180 makes a better pure macro lens.
The huge difference here (obviously) is focal length and Image Stabilization, both of which play into how to pick which lens will work best for you.
If you plan on photographing subjects that would benefit from a greater working distance (insects, animals, etc) or lighting is an issue such that being further back helps, then the 180L obviously has a huge advantage here. If, however, that isn't a relevant issue for you then it can actually be a PITA to have to be ~2x further away from your subject to get the same frame, this was my experience with it at least.
The other thing to consider is with ~2x the focal length camera shake becomes a greater issue, you pretty much have to work on a sturdy tripod with the 180L but with the 100L you can get away with hand holding or less sturdy supports, adding IS to that equation helps the 100L even more.
You could also throw focal compression into consideration but the other issues seem more pressing. Also consider what else you could use these focal lengths for, I find I have several lenses in the ~100mm area but only 1 (70-200) that gets into the ~200mm range.
