What defines a prime lens in photography?

A prime lens is characterized by having a fixed focal length, meaning it does not zoom in or out. This fixed focal length often allows for optimized optical design, resulting in sharper images and better light-gathering ability compared to zoom lenses. Photographers typically use prime lenses for their superior image quality, wider maximum aperture options, and often lighter weight, which makes them ideal for a variety of situations, from portraits to street photography.

In contrast, a lens with a variable focal length allows for zooming, but that does not define a prime lens. Additionally, while some lenses can produce intentional blur for artistic effect, this characteristic does not specifically pertain to prime lenses. Lastly, although many prime lenses may perform well in low-light conditions due to their wider apertures, that feature alone does not define them; they are primarily distinguished by their fixed focal length.