A lens system refers to the complete range of interchangeable lenses and associated optical accessories designed, manufactured, and made available by a camera manufacturer or third party optical company for use with a specific camera body or camera mount standard. It encompasses the full breadth and depth of the optical options available to photographers working within a particular camera system, from ultra wide angle and fish-eye lenses at one extreme through to the longest super telephoto lenses at the other, including all the specialist, macro, tilt-shift, and zoom lenses available in between.
The scope and quality of a manufacturer's lens system is one of the most important factors influencing a professional or serious amateur photographer's choice of camera system, as the lenses available for a camera are in many respects more critical to the ultimate image quality and creative flexibility of the system than the camera body itself. A camera body can be upgraded or replaced as technology advances, but the investment in lenses - which represent a far greater combined financial outlay for most photographers - is essentially tied to the mount standard and lens system chosen at the outset. For this reason, photographers entering a new system or considering switching between systems typically evaluate the available lens system very carefully, assessing not only the current range of available lenses but also the manufacturer's historical commitment to developing and expanding the system over time.
A comprehensive lens system typically includes lenses covering all principal focal length ranges and categories of use, including ultra wide angle lenses for architecture and landscape, standard lenses for general photography, portrait and short telephoto lenses, macro lenses for close up work, long telephoto and super telephoto lenses for sports and wildlife, zoom lenses spanning various focal length ranges, and specialist lenses such as tilt-shift and fish-eye designs for specific creative and technical applications. The availability of fast, large aperture prime lenses across a range of focal lengths is particularly important to professional photographers working in low light or requiring shallow depth of field, and the presence or absence of such lenses in a system can be a decisive factor in the choice of platform.
Third party lens manufacturers such as Sigma, Tamron, Tokina, and Samyang complement and extend the lens systems of major camera manufacturers by producing lenses in compatible mounts that offer alternative combinations of focal length, maximum aperture, optical quality, and price point, giving photographers working within a given system additional choices beyond those available from the camera manufacturer directly. The availability and quality of third party lens support is therefore also a relevant consideration when evaluating the overall breadth and practical utility of a lens system.