How Focal Length Affects Your Composition
A scene basically is comprised of your photo subject and the environment that surrounds it, and when composing a photograph you have to consider how you want to fit your subject into that environment. The focal length that you choose to shoot with determines how much of the background your composition includes when your subject is a particular size in your frame.
Consider the figure, which shows a pencil photographed at a 1:2 ratio. The pencil is represented at half-size on the camera’s digital sensor. The image on the left was taken with a 50mm macro lens (the wider of the two lenses) and the one on the right with a 100mm macro lens (the longer of the two lenses). Notice how the image taken with the 50mm macro lens includes more of the background and surrounding environment than the image taken with the 100mm macro lens.
50mm, 1.3 sec, f/16, 320 100mm, 1.3 sec, f/16, 320
Fixed macro lenses for 35mm DSLR photography typically offer normal to telephoto focal lengths (45mm to 200mm). The smaller the focal length, the closer you have to get to your subject in order to achieve a 1:1 ratio, and the wider your angle of view. The larger the focal length, the farther you are from your subject when achieving a 1:1 ratio, and the narrower your angle of view.
If you like to shoot with wide-angle lenses in order to include as many details of the scene as possible, or if you prefer to maximize the amount of sharp detail in a scene, you will likely benefit most from a macro lens with a normal focal length. If you favor creating compositions that minimize how much of the background is included in your frame and have a very shallow depth of field, a telephoto macro lens will work best for you.