Gustavo Mazzarollo (a fellow macro shooter with some mad skills) recently asked me how I would rate Canon's 300mm F4 L and how it compares to the 180mm macro. At first I wasn't going to do a blog post on it -I'm not all that and a bag of chips, and there are other sites out there that review lenses much better than I could. But I realized that I could answer Gustavo's question in a way that other sites might not. You see I spent a lot of money on Canon's 180mm macro only to realize that I couldn't really use it for any of the images that I wanted to take, and if I can save you from making the same mistake that I did then it's worth writing about.
Before I start a post where it seems like I'm bashing the 180mm macro let me start out by saying that it's an awesome lens from a technical point of view -great color, contrast, and plenty sharp. So no angry letters please :) But it does have some "limitations" that you need to be aware of...
All too often I hear people say that for bug photography you need a lot of working distance to keep from scaring the critters, and so a long focal length lens is a good choice. More experienced macro shooters either know how to get close to skittish subjects, or they know when to go looking for them when they are not so active (late evening / early morning) so working distance is a non issue. You can, with practice, get close to just about anything.
Canon 65mm F2.8 macro lens at almost 4x. Single frame (I don't focus stack).
For flash photography the increased working distance of a long lens works against you. Due to the Apparent Light Size Principle it's easier to get good diffusion the closer your diffuser is to the subject. But if you get your flash out past the end of the lens then you've just lost the gain in working distance that comes with long glass. I took most of my macro shots with the 180 L using either natural light, or a mix of natural light and flash.
For shooting closeups the 180 L just doesn't have the "reach" of a long prime like the 300mm L, and hard to reach subjects were impossible to shoot at the magnifications that I wanted to shoot them at. Also keep in mind that, shooting the same scene, the bokeh with the 300mm L (or any long focal length prime) is going to be better than the bokeh of the 180mm L because there is more distance between the lens and the subject with longer glass (so more distance between the lens and the background as well).
180mm F3.5 L
300mm F4 L (taken in the middle of the day so the light is pretty harsh)
I know this is gonna sound kinda of odd coming from a macro shooter, but I just can't recommend a long focal length macro lens. They don't work well for macro unless you want to use natural light and a tripod, and they just don't have the reach and bokeh of a long focal length prime. I currently carry a Canon MPE-65mm macro lens and the 300mm F4 L and those two cover all of the macro and closeup shooting that I do.