What focal length can a SLR lens be used to take long shots?

Generally speaking, 135mm~180mm is enough to take pictures of ordinary scenery.

Focal length, simply put, is the range of change in focal length of a zoom lens.

Division of lens focal lengths: The first is the header - the so-called standard lens. Its viewing angle is about 43 degrees, and the perspective of the photo at this time is closest to the human eye. Different film formats have different focal lengths. The standard lens of 135 format is about 50mm, the focal length of 120 6×6 lens is about 80mm, and the focal length of 300d lens is about 50÷1.5=33mm. Documentary photos use headers to faithfully record what you see, so most documentary photography masters like to use headers, such as French photography master Bresson.

When the focal length is smaller than the header, the lens can record images from a larger angle of view, so it is called a wide-angle lens. The perspective of a wide angle lens is exaggerated. Under a wide-angle lens, the close objects are larger and the far objects are smaller. Especially in landscape photography, photos with more visual impact can be obtained.

A lens with a focal length 1.5 to 4 times longer than the focal length of the mast head is called a medium focal length. Generally, mid-focus lenses have the smallest distortion, and it is relatively easy to design a very large aperture, so portrait photography is what mid-focus lenses are best at. Generally, the focal length of 1.5 to 2 times of the header is used to take full-body and half-body photos. Lenses with a magnification of more than 2x are mostly used for shooting close-ups. Several lenses known as “portrait kings” all appear in this focal length.

A lens that is more than 2 times larger than the focal length of the masthead is called a telephoto lens. As the name suggests, a telephoto lens can achieve "telephoto" shooting, and is mostly used for shooting sports, animals, etc. The application of a telephoto lens in landscape films can compress the far and near perspective of the landscape.