Focal Length: The first part of a lens’ description is its focal length, which is measured in millimeters. This number describes how close or far you will feel to your subject when looking through the lens. The lower the number, the farther you will feel, while the higher the number, the closer you will feel.
Aperture: The wider a lens opens, the more light it lets in. When lenses are labeled with low F numbers (such as 1.4, 1.8, or 2.8) they have the ability to open wide. In plain terms, this means that the lens will let more light in and allow you to use a higher shutter speed as well achieving a blurrier background.
Fixed vs. Zoom: When a lens is described by two focal lengths (ex: 24-70mm), it is considered a “zoom” lens. This means you will feel closer or farther to your subject depending on how you turn the zoom ring. Lenses with a single focal length (ex: 50mm) do not have the ability to change. However, fixed lenses tend to be sharper than zoom lenses and offer wider apertures. These are called “prime” lenses.
Starting Out: The best lenses for those starting out are the 50mm f1.8 prime lenses for both Canon and Nikon.