Extension tubes are probably the most popular option for increasing a lens’s maximum reproduction ratio. They are pretty much just hollow tubes that push the lens’s rear optical element away from the camera’s image sensor. They don’t have any optical elements, so you don’t have to worry about image degradation. I’m not terribly familiar with how they work, but the end result is a shorter working distance and increased magnification. And if you’re ok not having electronic aperture control (or use manual lenses and don’t need it), they can be very cheap.
Before you go out and buy a set of extension tubes, there are number of things you need to consider:
1.) You will lose light. This is problematic if you want to use natural light–particularly if you’re shooting at low ISO, high shutter speed, and small apertures. If you will commonly be photographing subjects or in environments that don’t require high shutters speeds, the light loss isn’t a huge problem. You can always get around the light loss by using a flash, but unless you pay careful attention to the backgrounds you shoot against, you will oftentimes wind up with black backgrounds when using a flash for macro photography.
2). You will lose working distance. There are formulas to calculate approximately how much you will lose, but they require very specific and sometimes obscure information about the lens you will be using the tube(s) on, so I won’t get into the math here. If you’re super curious and need to scratch that itch, your best bet is to Google it.
3.) You will lose infinity focus. One of the side effects of extension tubes is that your lens will lose the ability to focus to infinity. If you want to retain the lens’s ability to focus to infinity without having to unmount the lens first, you’ll want to look at other options for increasing magnification.
4.) They can be pricey. If you are not using manual lenses, low-cost extension tubes will not allow electronic aperture control–they don’t have the electrical contacts. If cost is an issue, there are work-arounds that will allow you to use tubes without the contacts. I shoot with manual lenses, so I’m actually not familiar with what these work-arounds are, only that they exist. The DPReview forums (macro or camera-specific) are a good place to look if you need an answer. If cost is not an issue, Kenko makes extension tubes with electronic couplings for popular lens mounts–B&H Photo Video sells them for $200
Another option to consider if you’re thinking about extension tubes is a bellows. They are essentially variable length extension tubes that extend like an accordion. I don’t have any personal experience with them but, from their looks, I think that they are best suited to tripod work. They look a little cumbersome to use if you’re shooting hand-held.
On to diopters…