All lenses attach to the camera using some sort of locking lens mount. These come in bayonet, screw-thread, and friction-lock varieties, and they act to attach the lens to the camera body and join any electrical connections.Each camera manufacturer has its own design, and while they often look very similar, they are not interchangeable. When shopping for a camera lens be sure it has the proper fit so that it will mount on your camera. Most third-party lens manufacturers sell the same lens with different mounts to cater for this.
Focal length is the most important factor in any camera. This determines which type of lens it is, and what subjects it will be able to photograph. Focal lengths range from a few millimeters up to over a meter, and can be grouped as follows:
|Focal length||Lens Type||Purpose|
|8-24mm||Ultra wide angle / fisheye||Wide panoramas, interiors, landscapes|
|24-35mm||Wide angle||Interiors, landscapes, architecture|
|135-300mm||Medium telephoto||Close sport, wildlife|
|300mm+||Super telephoto||Far sports, astronomy, wildlife|
Lens focal lengths are specified for a full-frame 35mm sensor camera. Most amateur DSLRs and all compacts use a smaller sensor, and this has the effect of cropping off the edges of the photograph, resulting in an image which is more “zoomed in” than it would be on a full-frame sensor. Because of that it makes it seem as the lens has a bigger focal length than it really does. We can calculate a lens’s “effective focal length” by multiplying the real focal length by the camera’s “crop factor”. A typical crop factor is around 1.5x, meaning that a 50mm lens actually has an effective focal length of 75mm when fitted to this camera. You should get familiar with your camera’s crop factor and take it into account when buying new lenses.