Adding extension is one of the easiest ways to increase your magnification. All lenses will focus closer as you move them away from the sensor. The extension can be built-in extension as is the case with many macro lenses, or extension tubes or bellows.
Extension tubes and bellows have no optics in them and don’t affect the quality of your image directly, but they do absorb light and will lengthen you exposure time, sometimes by several stops. This can make shooting from a tripod or other fixed camera position mandatory. The extension tube mounts between your camera and lens so it does need to be specific for your camera brand. While there are no optics in an extension tube some of the major brand name ones contain electronic circuitry compatible with the camera’s system. So they may allow auto focusing (usually up to a certain f-stop) and TTL metering, while others will make you operate the camera in a strictly manual mode.
Bellows offer the advantage of being able to vary the amount of your extension. Some of the more expensive bellows by manufactures like Novoflex offer electronic circuitry that allow for metering, most bellows like the one pictured above require complete manual operation.
Magnification provided by extension
A good rule of thumb for calculating your magnification is:
Magnification = Total extension / focal length
While this is actually a fair exact calculation, I offer it as an approximation because the total extension and to a lesser degree, the focal length are numbers that in practice can be hard to determine. Most lenses have a certain amount of built-in extension, macro lenses have enough built-in extension to get to 1x all by themselves. So in reality the Total Extension is the sum of Internal Extension and Added Extension. While it is easy to know how much extension you’ve added, it is difficult to know how much internal extension your lens has at any give focus distance.
Continue with next section: Close-up Lenses