You can purchase many different types of cameras and lenses. To get the best picture for your money and the type of scene you intend to capture, knowledge of the numbers and symbols on the camera lens is helpful. These numbers tell you how your camera functions and information about the lens itself, and can make or break a great photo opportunity.
MM
The letters "mm" on a camera refer to the focal length in millimeters. Focal length is how far away the lens is from the sensor inside of the camera. Larger numbers mean that you will have a telephoto shot, which is good if you are taking a lot of pictures from a long distance away. The smaller the number, the closer the view. If your camera has an mm range, you can adjust the view within the range to get the best result.
USM
USM refers to autofocus. If your lens has "USM" written on it, the camera can focus automatically when you press the shutter button to take a picture. You don't need to focus manually.
Null Symbol
The null symbol on a camera lens is a symbol for a screw. When this symbol has a number behind it, a screw of that mm number fits the lens if you ever need to replace it.
Series Numbers
Series numbers typically precede the USM letters. Series numbers tell you that the lens is a particular model in a series. For instance, "3USM" would mean that the lens was the third version of that lens.
Ratio Numbers
Each camera has an aperture, or opening where light comes in. This opening can have a range of size for the lens you are using in your camera. The range is expressed as a ratio. The ratio affects how your camera is able to handle light, since the aperture size determines how much light enters the camera sensor.
Tags: your camera, that lens, camera lens, camera range, mean that, numbers tell
