Having purchased Extension Tubes, I wish someone had told me not to. They are extremely impractical, and if you have the time and patience to fiddle with them all day, they are ok, but if you're always in a rush or are in a time and composition sensitive application, they are very impractical.

The issue is although they shorten your MFD (which is good to get more magnification) they also significantly shorten your MaxFD. This gets worse the shorter the lens. For instance, a 70mm lens with a 25mm Extension tube has a focusing range of 3cm. Imagine using a Prime Lens where you can't foot zoom. That's what an ET feels like on a normal lens. You can't change your composition to any great extent because you are virtually locked at a certain FOV.

Quote Originally Posted by davesmith View Post
Having never used either, what's the difference or advantage of using extension tubes compared to teleconverters, particularly away from macro?
They both have their uses for Macro. Though TCs degrade IQ more, they make the background more blurry though which is very useful for flower photography or when you need a lot of DOF but a blurry background. Some Flower Photographers stack a 1.4X and 2X on a Macro lens for flowers to get nice blurry backgrounds at small apertures like f/16


Quote Originally Posted by Tannin View Post
IQ does not suffer in the slightest. In fact, I believe that extension tubes increase your IQ. This is just my own gut feeling, I've never read anything scientific or official saying this, but it stands to reason that a lens will be better optimised further towards the middle of its focal range, and my results seem to confirm it. (I'm talking here about the situation where, for example, your subject is 3.4 or 3.6 metres away. In theory, it's within the 3.5m MFD of the 400/5.6, but in reality the lens is only just barely able to focus on it. With a tube, however, the lens is well inside its focal distance range. Be all that as it may, tubes certainly don't reduce IQ.

Other effects? You already know the main one: loss of infinity focus. You lose some light as well. I've never measured this and can't tell you how much exactly, but you do sacrifice a small amount of shutter speed for any given light level and aperture/ISO setting. You probably won't notice this in practice. Finally, in theory, you get some vignetting, but that won't apply to your 40D - it doesn't seem to apply on the (larger sensor) ID III either, doubtless it's only if you go full frame that you see it.

Short answer: go for it, they are very, very useful!
I used to think so as well, in theory it's stated that infinity focus is lost and IQ is not affected, but I find in practice the reality is much different

With IQ, ETs magnify optical defects, especially CA and even worse, spherical aberrations. Many lenses perform very poorly at MFD, especially the 35L, some 17-55 IS and 70-200 f/4L IS. They have some residual spherical aberrations which really rear their ugly heads at MFD, and these only look worse with an ET.

With infinity focus, the literature suggests losing focus at infinity, but imo it is a spectacular understatement. For my 70-200 for example, with a 25mm ET, it can't focus beyond about 2 metres at the long end, and beyond about 1 metre at the short end. If >1 metre is considered "infinity" I need to relearn my english