I mentioned before (for Table Sugar) that a reversed lens on the front of another lens makes a macro (close-up) lens. The problem is that with all that glass in the way the pictures aren't that sharp and vignetting is pretty horrible.
You can also do decent macro work with single reversed lens on the front of your camera. The most common lens for this is a 50 mm prime because they are cheap and have good optical quality. The advantages are decent macro magnification (about 1:1), good sharpness, and good brightness because you only have a single lens with a pretty big aperture. The downside, unless you shell out a lot of money for an adapter, is you lose ability to control aperture. (Obviously, a dedicated macro lens will tend to be a bit sharper and much brighter, but cost a heck of a lot more than using a lens you already own)
Before I bought anything, I decided to try it out just holding a lens up (backwards) to the camera and seeing how well it worked.
I decided to go ahead and get set up for a reversed Canon 50 mm f/1.8 prime. This is one lens pretty much everyone has because it is so cheap yet so good for portraits on a digital body with a less-than full frame sensor. The set up includes the lens ($80), a $11 52 mm reversing ring (which screws into the front of the 50 mm lens and fits into the 350D body like a lens), and a $18 extension tube set (fits between body and lens (or reverser) for a total extension of up to 65 mm). I bought the reversing ring and extension tubes from the same seller on eBay.
The sensor on the Rebel XT (350D) is 22.2 mm wide, so to get a 1:1 macro we'd need a field of view of 22.2 mm. Using the extension tubes and reversing ring, I found the following:
- Unreversed lens: 150 mm field of view (1:6.76)
- Unreversed lens w/ full 65 mm extension tube: 15 mm FOV (1.47:1) but only a 3-4 cm working distance from the object.
- Reversed lens, no tubes: 30 mm FOV (1:1.36), 10 cm working distance
- Reversed lens, min tube (14 mm extension): 21 mm FOV (1.04:1), 8.5 cm working distance
- Reversed lens, full tube (65 mm extension): 11 mm (2:1), 6.5 cm working distance
Also, to set the aperture, put the business end of the 50 mm prime back on the camera, set the aperture in Av mode, push the depth of field preview (little button underneath the lens release button) and while holding it down, remove the lens. This causes the lens to have the aperture stuck even though it is off the camera. Obviously, this is not a recommended way to use the lens, but it works. Even better might be finding an old 50 mm prime at a flea market that has aperture options.
This set up tends to have a really small field of view (common for all macro work, actually). In particular, it is hard to get a decent field of view without stopping down the lens too much and getting a really dark viewfinder. But... putting the camera on a tripod helps a lot in terms of setting up shots and preventing camera shake. Just make sure you turn mirror lockup on and use a remote release to avoid vibrations.
When reversed, for whatever reason, the focus wheel on the lens does nothing. I haven't figured out why yet.
Also, reversing the lens can be pretty hard on the lens (which is why you want to use a cheapy like the 50 mm f/1.8). After all, you'll be inserting/removing the lens a lot (on both sides), the weight of the lens will be held by the end-cap, and you'll be putting a decent amount of force on the filter threads. But, if I break my 50 mm prime, I won't be too disappointed because they are so cheap.