Any lens can be a macro lens with extension tubes.

When I bought my first DSLR camera (a Canon 50D) a few years ago, the first thing I wanted to do was shoot macro/closeup photography of flowers.  I love flowers and am obsessively attracted to ridiculously bright colors.  It didn't take me long to decide that I couldn't get adequate results with the equipment I already had.   Because I didn't want to spend a whole lot of money, I researched how macro photography worked and found a few important points:

1. Macro photography obviously requires getting as close as possible to your subject, which means your lens has to be capable of focusing at a close distance to the subject.
2. Most lenses cannot focus if they're too close to a subject.  The typical focusing distance is a few feet.
3. The best way to enable close focusing of a lens is to find a way to "move" the lens a significant distance away from the camera (while still keeping it attached to the camera to take the picture). 

Photographers use many ways to accomplish my #3 above.  One of the most amusing looking and maybe oldest ways of doing is is with bellows:

 



You can see how the accordion-shaped bellows could dynamically expand or contract the distance between the camera in the back and the lens in the front, allowing for variable levels of close focusing.  Bellows are still used, sometimes on extremely high end projects.  They can be relatively EXPENSIVE if you buy them new.

Another device that serves a similar purpose is a set of "extension tubes," which literally ARE tubes that move the lens further away from the camera, while still providing electronics so that the camera can control the lens.  Extension tubes come in several different sizes, so you can add or remove them to expand or contract the distance between lens and camera.  Shown below are 3 tubes that can be taken apart and used individually or put together as they are in the picture.

Kenko Extension tubes

Extension tubes look like a lens, but they have no glass in them, no optics at all.  They are just air and electronics (via the pins on the lower left of the tubes in the picture shown below).  Some people like to call extension tubes "expensive air".  The electronics are the costliest part.


Looking through the extension tubes at the picture above.

Extension tubes will allow any lens to focus closer than it ordinarily can, meaning that they turn ANY lens into a pseudo-macro lens.   How much closer you can focus depends on the lens, but the tubes often enable focusing at about half the distance to the subject that the lens without the tubes can focus.  Note that super wide angle lenses (e.g. a Canon 10-22mm) won't effectively work with tubes, because with tubes added, the focusing distance is so close that the maximum focusing distance is INSIDE the front glass element of the lens!  The only way you could focus on anything with such a lens and tube combination is if you could somehow place the subject inside your lens!  You don't want to do that, ha.

Whatever lens you choose, you'll want to choose the sharpest lens you have, because sharpness can be even more critical at close focusing distances.  The tubes have NO EFFECT on lens sharpness, because they have no optics.

Of course, this closer focusing with extension tubes comes with a down side, which is that you "lose infinity".  To lose infinity means that when the tubes are attached between the camera and lens, the lens can no longer focus on anything that is more than a foot or so away.  Say, if you were shooting flowers in the garden you could not focus -- manual or otherwise -- on that bird sitting on the fencepost a few feet away.    The laws of physics just won't allow it.  But to solve that problem you would remove the tubes and attach the lens to the camera as you normally would.  Of course, by the time you do this, the bird will have flown away ;-).  The other down side is that the tubes cause you to lose light and focus speed, so you need a brighter environment to shoot.  I typically use a tripod and focus manually when doing macro, so focusing speed isn't a huge issue for me.

My first macro setup was the Canon EF 50mm f/1.8 II Camera Lens (aka "the plastic fantastic") and the Kenko DG Auto Extension Tube Set for the Canon EOS AF Mount.  The Kenko tubes have a fully functioning electronic pass-through, meaning that with the tubes attached, the camera can: (1) autofocus the lens (which often comes in handy with macro, but not always); and (2) adjust lens aperture.  The latter ability is highly necessary with macro photography, because aperture controls depth of field, thus how much of your image is in focus.  You can find less expensive extension tubes, but they either don't have electronic controls at all, or are poorly functioning in other ways.   I've even heard of people taping cardboard tubes between camera and lens, but I don't suggest this! You can also buy Canon-branded extension tubes, but they are ridiculously expensive, and only slightly better in build quality than the Kenko tubes.  The Kenko tubes are a happy medium.

Note:  If you buy Canon Kenko extension tubes, make sure and get the ones that allow focusing with EF-S lenses.  Ask whoever you buy them from to confirm this. They will know what this means.  I can explain the reasoning for this in detail if  you like, but just know that it allows use of the tubes with more lenses. Also, make sure to get tubes that fit your make of camera.  Kenko makes tubes for both Canon and Nikon.  For other manufacturers, I don't know.

Here is a sample from one of my first attempts with my original macro setup.  These are Japanese Andromeda flowers, an early spring bloomer.  As you can see, composition was not my forte back then, but the droplet on the right is pretty sharp and that is a tribute to the lens!

Taken with Canon 50mm f/1.8 and Kenko Extension Tubes

As my lens acquisition syndrome became more severe ;-) and as I moved on in macro photography, I bought an actual macro lens.  My first macro lens was a Canon 100mm f/2.8, and then my generous boyfriend surprised me at Christmas 2012 with a Canon 100mm f/2.8 L macro lens.  These two lenses are very similar.  The "L-glass" version has image stabilization.  In addition, it may be a tiny bit sharper and have slightly better contrast than the "non-L-glass" lens.  It also has 9 aperture blades, which make for prettier starbursts (see my starburst post).  The jury is out on whether or not its improvements warrant the price.  Suffice to say, the new lens was a luxury, not a necessity for macro.  If not for the better starbursts ;-), I think I'd have buyer's remorse.

The main benefit of using a macro lens is that it has a built-in ability to focus closely.  Whereas the closest I can focus with my regular zoom lens (without tubes attached) is about 4-5 feet, with my macro lens I can focus on subjects that are about a foot away. This means I can completely fill the frame of my camera with a flower. Another benefit of macro lenses is that they are typically very, very sharp lenses.  Unfortunately, the tradeoff for optimizing macro lenses for close-up photography is that they are horribly slow focusing.   You don't "lose infinity" as I described with the extension tubes, but you would not want to use a macro lens for wildlife photography on a regular basis, at least not with moving subjects....If you have to catch that bird on the fence while you're shooting flowers, you MIGHT be able to focus on it with your macro lens before it flies away.  No promises.  If you do happen to catch it, you'll get a nice sharp image.

Now that I have both a macro lens and extension tubes, I sometimes like to use both for the uber-macro effect.  Here's my macro setup with just the macro lens attached.  As you can see, I can shoot subjects that are up to about a foot away from my lens front.




Macro lens alone

And here's what the resulting picture looks like (totally non-scientific, I may have cropped a little):

Taken with macro lens alone

And now here's what the camera setup looks like with all three extension tubes added between camera and lens.  Note that the lens front can now focus at about 3-4 inches from the subject, rather than the one-foot or so distance of the macro lens alone.

Camera with lens and extension tubes attached

And a resulting image. 

Taken with macro lens and extension tubes

Most of the time, a macro lens is more than sufficient for obtaining a close, sharp image, but if you want to get an almost microscopic image, the macro lens plus extension tubes can often give you that.

Takeaway

The bottom line is that you can really do macro with either a macro lens or extension tubes and a non-macro lens -- or you can combine tubes and a macro lens.  The macro lens is better, generally, because it is a bit faster and more accurate at autofocusing, works a bit better in lower light, and because you don't "lose infinity" (as I described above).  It is also great for shooting portraits and other non-moving subjects, especially if you want SHARP pictures.   The tubes work well too, and if you only do macro work occasionally, they are a great choice.   They also are light and travel well, so you can throw them in your bag just in case you want to do macro at some point during a trip, but don't want to carry an additional lens.  And they are a great starter device, to see if you even like macro.  Macro can be back-breaking and sometimes a test of patience.  Because it isn't for everyone, the $200 investment in tubes might be a safer bet than the $500-900 for a macro lens.  Resale value of tubes on the used market is high because I think people tend to keep them.  Thus if you decide you want to sell them either to upgrade to a macro lens or because you find that you don't enjoy macro photography, it won't cost you much to do so.  Or, if you're anything like me, you'll want to keep the tubes even if you also buy the lens.

My writing here is certainly not intended as exhaustive coverage on macro equipment.  There are other tools you can use to achieve fairly good macro results.  For instance, you can buy a close-up filter, which is just a magnifying glass that you attach to your lens' filter threads to magnify whatever subject hits the lens. The downside is that the filter degrades image quality.  You can also use reversing rings to mount a lens backwards to another lens that is attached to your camera.  The result is super-magnification.  But the only way to focus with this setup is to move the subject back and forth toward the lens, which can sometimes be frustrating.  My setup is less frustrating.

I hope this is helpful.  Let me know in comments if you have questions.