What does the number of lenses in a SLR camera mean? For example, what do 1855 and 18 135 mean respectively?

Focal length is a measure of light convergence or divergence in an optical system, which refers to the distance from the optical center of a lens to the focal point of light convergence. It is also the distance from the center of the lens to the imaging plane such as the backplane or CCD in the camera. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the vertex of the mirror.

The zoom range is 18mm (wide-angle end) -55mm (telephoto end), 18mm (wide-angle end)-135mm (telephoto end). Buy 18- 135mm! However, according to your question, taking Canon as an example, you can also buy 18-200mm (the original factory is more expensive and the auxiliary factory is cheaper), 18-250mm (there is no original factory, only the auxiliary factory) and 18-270mm (there is no original factory, only the auxiliary factory). 5, around 600 yuan) or 85mm F 1.8, 50mm F 1.4 (you have more money, choose one of these two). Nikon can also choose these, but the fixed focus 85 aperture has 1.4. When your camera is updated, you will know what camera and lens to buy (unless you buy a full-size second camera, you can continue to use the lens on it)!