A neat trick I learned last year was the ability to take macro photos with an interchangeable lens camera, using just the kit lens. By detaching the lens and flipping it around so that the front element was facing the camera sensor, a mid-range telephoto zoom lens functioned like a macro lens, allowing me to take extremely close-up photos of everyday objects. The trick worked well with mirrorless cameras, which have their sensors exposed (ie. not shielded by a mirror), but it was still finicky. You're leaving both the sensor and inside of the lens exposed, and getting a sharp focus was very tricky. But with some practice, the results can be very pretty.
So after getting the 6D, I wanted to continue dabbling in macro photography. But as it turns out, the "reverse lens" trick doesn't work out nearly as well with a DSLR. That's primarily due to the DLR's use of a flipping mirror for viewfinding. With the mirror flipped down, I can use the optical viewfinder to find framing and focus, but it has to flip up and reveal the sensor for the camera to take a photo. That mechanical flipping proved too jarring for me to keep a steady focus while keeping one hand on the detached lens--I couldn't hold both the camera and lens steady enough to get a sharp shot. One option was to keep the mirror flipped up and use the LCD in "live view" mode, but I didn't want to leave the precious full-frame sensor exposed.
Without being able to use the reserve lens technique, macro photography can be achieved a few other ways. Of course, you can always buy a legitimately classified macro lens. These are lenses that grant you the ability to get right up to the subject. When I wrote about lens specs a few months ago, one of the traits that I called out as being important was the minimum focal distance. This is the shortest amount of space you have to put between the subject and the "focal plane mark" of the camera--typically the sensor. Non-macro lenses require that you subject at least several inches (or over a feet, in some cases) between the camera and the subject, meaning you can't get close up. A macro lens will let you get right up to a subject so that there is at least a 1:1 magnification ratio between the real physical subject and its representative size on the camera sensor. You're capturing more up-close detail because the subject--typically something that is small--is taking up more physical space on the sensor.
But without a dedicated macro lens, you can also shoot macro photos with the help of a lens extension tube. Here's how those work.
A lens extension tube can be a pretty simple piece of equipment. It's just a cylindrical barrel with the appropriate mounts on each side to connect to your lens and camera, sitting in between the two. There are no optics on the inside--its sole purpose is to artificially create more distance between the camera sensor and the lens elements. But that extra distance has a dramatic effect on what images the sensor sees from the lens.
To explain this, let's quickly recap how lenses work. The most defining attribute of a lens is its principal focal length, denoted in mm. That number indicates the theoretical distance from the sensor at which light converges on an invisible plane and can be seen clearly by the sensor. A short focal length is associated with a wider field of view, while a longer focal length (eg. telephoto) is associated with a narrow field of view. A narrow field of view means you can see things that are far away, so that is one form of magnification. But because you need a long focal length to achieve that, you can't magnify up-close subjects with a telephoto lens. Even telephoto "macro" lenses still have a high minimum focal distance.
But by putting extra distance between the lens and the sensor, you are affecting the magnification capability of the lens while using its native focal length. Here's where some funky math comes in.
Tube extenders, like Fotodiox's $12 kit, come with several extension sizes which can be mixed and matched. What extension distance to use is best determined through experimentation, because the magnification is affected by both the extension distance and the lens focal length. In fact, the magnification gained is the extension tube's length divided by the focal length. For example, putting a 21mm tube on a 40mm lens grants .5X more magnification (21 divided by 40 is approximately .5). That doesn't mean that the magnification of that combination is .5X, since this is additional magnification added on top of the len's native magnification strength. For 1:1 macro photography, the goal is to find the right extension so that the combined magnification totals 1X or higher. Confusing, right?
$12 isn't a lot of money to spend to have fun with macro extenders, but you do want to buy one with sturdy construction and reliable ring connectors. With an extender attached, you're basically only able to focus close-in on subjects, so you'll have to remove them when you want to shoot anything at medium distance. They're not something you're going to keep permanently attached to your camera.
Another consideration is that cheap macro extenders don't have electronics, so you have no on-the-spot aperture control unless your lens has a manual aperture ring. The lens will always be wide open, which acceptable in most cases since macro extenders also reduce the amount of light the sensor sees.
I've been testing both cheap macro extenders and also pricier ones that have electronics built in. Fotodiox's Pro extender for Canon lenses is $60, but worth it for the aperture control. I actually liked closing the lens up a little bit when shooting macro photos, since too shallow a depth-of-field makes it difficult to get everything you want in focus. This is especially the case when shooting macro subjects at an angle, which I had to do just to let enough light in from the sides when cramming the glass right up to the subject.
Another really fun thing to do with macro extenders is take photos of LCD screens, like on phones. Since those LCDs are backlit, you don't need another light source and can put the lens right on the display, as I found myself doing with my phone. It's kind of like using the camera as a microscope, since 20 megapixel photos show a ton of detail at the macro level. In the photo below, you can even see the sub-pixels that make up the app icons--in this case, the tiny grid icons inside an iOS folder.
Finding focus is still the most challenging part of using a macro tube extender, and my preferred technique is to put the tube on a wide angle zoom lens (in my case the 17-40mm). The zoom lens has two variables of control--the amount of zoom and the focal distance. I set the lens on manual focus and set it to as close as possible, and then actually use the amount of zoom to adjust for focus. With that technique, I've been able to take some really fun photos of LEGO minifigs:
I'd love to hear about your experiences with macro photography--whether you use a tube extender or not. Chime off in the comments below!