I just read this thread, and since I saw my name come up, I'll try to answer a few of the questions, and add my insight with a few examples. I'll only be talking about outdoor (natural habitat) macros here, since that's what I know more about.
There are two main lighting techniques for animal* macros: flash and ambient light. Each has its own advantages and disadvantages, and your results will vary greatly depending on several factors, the most important ones being amount of light available, DOF required and animal you're taking pics of.
* by "animal" I mean any small animal you'd like to take pics of, although my examples will be mostly small arthropods, since that's my field of research. The same techniques apply to pretty much any small thing you want to photograph.
Getting a good exposure using ambient light gives you a picture that is practically identical to the one you see through the viewfinder. The results with flash photography are not always what you see through the viewfinder, since the light source is different. Let's have a brief look at the pros and cons of each technique. Keep in mind that this is based on my own experience, and I have worked far more with flash than with ambient light.
Ambient light: Like I mentioned above, the results of getting a good exposure are very similar to what you see right before taking the pic. If you choose your light (time of day, shade/sun) correctly, you avoid those pesky highlights of flashes. Some bugs, like Sphinx Moths, suffer from red eye when you take pictures using a flash from a close distance. You won't have that problem with ambient light. It's usually a soft, nice light.
Of course, not all is perfect, and ambient light has very important cons. One is the shutter speed necessary to achieve a decent exposure. Since in macros you need at least the eyes (and preferably the head) of the bug in focus, you need to increase your DOF as much as you can. At 1:1 we're talking barely 4-5mm @f10, so your shutter speed has to be really slow to get a decent exposure*. This means that for fast moving critters, you're screwed. You'll get a blurry picture even if you have a solid tripod, because bugs, no matter how slow they may be, will move. Also, you don't always have the time to set up a tripod (the bug may run/fly away), and even if you do, a sturdy tripod is always at least 30 cm above ground. Considering that many bugs are at ground level, getting the same perspective on all your shots will get boring really quick.
*You could always use a high ISO, but in my experience, it's not worth it. Granted, I have a lowly XTi and I try to avoid anything beyond ISO 400, but still, I doubt you'll want to use ISO 3200-6400 on a regular basis.
Flash: Using a flash for macro lets you freeze the moment, and achieve the highest sync speed your camera can handle. You can take pics of fast bugs and get a much, much higher keepers rate than with ambient light. It's a constant light source, that gives consistent results regardless of being used on a sunny day or at midnight. It's easily modifiable, regarding light output/exposure, and you can use 1, 2 or as many light sources as you want/can. Nevertheless, it's an added bulk, and it's an extra set of settings you have to fiddle with to get the shot. Sure, you can just set your flash to 0 EV and let the ETTL run the photo shoot, but that's like setting your DSLR on full auto. Also, you'll soon find that many of your shots will have a pitch black background. I can go on, but this is a thread about the pros and cons of lens IS, not a flash/ambient light debate.
Nevertheless, I find it important to point this out, since I hope it will help the OP decide if the lens with IS fits his/her needs better than a lens and a flash unit.
Today was actually a very good day for pics regarding this subject (ambient vs flash light). These next two pics show the difference between ambient light (1st pic) and a ring flash (2nd pic).
The difference is subtle, but there's more contrast in the flash pic, not to mention that it was taken @f10 and ISO 200, while the other one was taken @f6.3 and ISO 400 on a very bright, sunny afternoon. The DOF difference is noticeable in an 8x10 pic (which I print at very often).
I am thinking mostly doing outside, like bugs and dragon fly etc.
In general terms, bugs and (particularly) dragonflies are very active, fast-moving critters. Yes, you might find a slow beetle or a perched bee, but in my experience (here in the tropics, where the temperatures are usually warm), those moments are few and far between. Even if you do take advantage of them, you'll soon find that the bugs you can take slow-shutter pics of are always going to be the same, and you'll get bored of always taking the same shots.
For example, take this pic of a recently emerged butterfly. It was almost still on the tree, but the wings were moving slowly. An ambient light pic would have called for at least a 1s exposure, rendering the body sharp but the wing a blur.
The IS should be a very nice feature for those who shoot spiders, frogs, and such in the forest where often one can't use a tripod. Some spiders and frogs won't stand still waiting for you to set a tripod.
IS would help in getting a slower shutter speed, which would in turn mean more ambient light, which is great if you don't want a black background every time, but it also means more possibility for camera shake, which leads to a blurry pic. And it's very true, most critters will not stand still for more than a second or two. This little lizard barely let me grab this shot before scurrying off. I was also flat on my belly; there's no way I could have set up a tripod, even if I had the time.
[*]@AlaskaMoose: I believe that dllavaneras frequently uses both a ringlight *and* an off-camera speedlite
via Stroboframe bracket on a Rebel to get sufficient light and shutter speeds for his
amazing (!) bug shots. That starts to move into heavier rig territory; YMMV.
First of all, thank you for your comment! I'm glad (and flattered!) that you like my pics. I was using the dual flash setup a while back, but lately I've been favoring just the ring flash, mostly because I have to chase bugs in tight spaces. It's not the weight I mind as much as the size. Nevertheless, I've been practicing different lighting techniques using only the ring flash, and thus avoiding the black backgrounds. That way I went from this:
To this:
The first picture was taken on a sunny afternoon, in the shade. The second was taken on a midnight hike last month.
None of those are natural light macros. There all flash (or mostly all) and with a flash you don't need IS or a tripod.
Exactly. Only recently I've started incorporating ambient light into my macro photography techniques, but it simply does not work as an only light source for me. I need a consistent light source to freeze moving insects, and thus I've only rarely felt the need for IS. Even then, it's only for framing purposes
(hanging upside down from a branch while holding out your camera setup with one hand introduces shake. IS would be great in those situations).
Here is a pic that uses both ambient and flash light:
As you can see, the ambient light creates much more defined shadows below the skipper, while the flash avoids having one side horribly underexposed (and allows a fast shutter speed).
I will continue to prefer flash for my macros (I use a ring flash) over ambient light, since I shoot in adverse light situations. Also, because I can freeze movement, I can get shots like this:
The dragonfly devoured the leafhopper in less than 10 seconds. There's no way I could have gotten that shot using only ambient light.
This very long reply was done with one thing in mind: let you know about the main limitations in macrophotography regarding your light source. IS will help you get more ambient light (and thus more uniformly lit pics), but it will not freeze motion. I'd rather have a tack sharp shot with a black background than a evenly lit blur.
If you feel that IS will help you get the shots you plan to take, then go ahead and purchase it. It will help a great deal with portraits and macros from a long (50cm-1m) distance, if the light is good. If what you want is to freeze running bugs and capture fast-paced action, then I suggest you get the USM non-IS version of the lens and a flash. I got a ring flash and the old USM 100mm macro lens, and you can see what you can achieve with this setup in my gallery.
The clarity , if you are just starting, is indistinguishable from the L version
And what if you're not just starting? Is the difference noticeable in print/screen, or is it only a slight difference at 100% noticeable only by pixel-peepers? I was just wondering if it's enough to get me to upgrade. IS alone is not enough to make me get it. It would have to be at least twice as good.
