What is a prime lens?
Most amateur videographers and photographers use zoom lenses for their Nikon or Canon DSLR Camera exclusively. And most videographers limit themselves to the zoom lens that came with their camera. By manipulating a zoom lens, operators can make an object appear larger in their viewfinder without physically moving the camera.
Professional camera operators who use prime lenses (instead of zoom lenses) do not have this option. If they want to make an object appear larger in their viewfinder while using a prime lens, they must physically move the camera closer towards that object.
Why use a prime lens?
A professional photographer will use the best tool for a specific job. It is possible to bang in a nail using the handle of a Swiss Army knife, but a professional carpenter knows to use a hammer. A zoom lens is like a Swiss Army knife, it can be used at many distances for many situations. A prime lens is designedfor a single focal distance.
In earlier days of photography, zoom lenses were very primative. Zoom lenses compromised quality for versatility. As zoom lens technology improved, the quality of images produced by a zoom lens attained enough quality to be acceptable for most photographers.
However, a 50mm prime lens will produce a better image than a zoom lens adjusted to 50mm. While modern zoom lenses may produce an image over 95% as good as the prime lens, professional camera operators know that zoom lenses will always be "almost as good" as a prime lens specialized for a specific task. Zoom lenses are complicated constructions with many glass elements cemented together. Even the best glass does not transmit 100% of the light that goes through it. So the more glass elements, the more light that is lost through the glass lens. A 50mm prime lens does not have the extra glass that lets the zoom lens also focus from 28-200mm. This simpler prime lens does not lose as much light, and so allows for a better image.
Cameras with an interchangeable lens system are more expensive, and they require more skill to use for best advantage. Professional Hollywood cameras exclusively use cameras with interchangeable lenses for a reason. They allow the camera operator to choose the optimal lens for the task at hand.
