I own both the EF 300/4L IS and the EF 100/2.8L macro IS. They are really very different lenses, in my experience. The maximum magnification of the 300L is 0.24x, or just a bit under 1:4. The maximum magnification of the 100L is, of course, 1:1, so that is a 4x increase in magnification. You can't really factor in the sensor size, since you can apply the same logic to both lenses equally, but even so you would have at best 0.38x "magnification," not anywhere near 1:1.


There are plenty of times when I can get a shot with the 100L that I could not get with the 300L, in so far as close-up shooting of small subjects. But interestingly, there are also times when I can't get close enough to my subject to warrant using the shorter focal length of the 100L. I was shooting little crabs hiding in tidepools last week and for having a rather primitive nervous system, they are awfully good at sensing motion from a distance. The only way I could get shots of them with the 100L was by parking my lens right outside a crevice, and sit super still until they ventured out again. Not easy. On the other hand, I could sit at a distance with the 300L and get a reasonable shot. It's a tradeoff; the 100L is easily sharper in the macro regime, but the 300L has a much more comfortable working distance, provided you don't need more than 1:4.


As I don't have experience with macro lenses @ 180mm, I can't really recommend one way or another. But keep in mind that the 180mm macros are much more difficult to handhold. You will find that you need really good light, a very small aperture, and a very fast shutter speed. That's the reason why I went straight to the 100L macro, starting out. I knew I didn't have the technique to handhold a macro near MFD and get sharp shots. The 100L has about 1-1.5 stops stabilization near 1:1, has a maximum aperture of f/2.8 instead of f/3.5, and is lighter and less front-heavy. You might think that you need a tripod and macro rail, and those would be fine for still subjects, but the experienced macro shooters really need to move quickly and many of them handhold. Some even manage to focus stack with the MP-E 65mm, which to me is a skill that seems to verge on magic.


Here is a recent photo of a spider that I caught in my house. It barely moved; it must have been old or something?


5D Mark II, 100/2.8L macro IS @ 100mm @ f/10 @ 1/500s, manual focus, manual flash.


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100% crop of the above image.


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