What does the parameter XXmm-XXmm of SLR camera lens mean?

The focal length is the focal length range of the zoom lens.

The division of lens focal length: first, the meter head-the so-called standard lens. Its viewing angle is about 43 degrees, and the viewing angle of the photo is closest to the human eye. Different frames have different title focal lengths. The standard lens of 135 film is about 50mm, the focal length of the 6×6 title of 120 is about 80mm, and the title of 300d is about 50 ÷1.5 = 33 mm. Documentary photos can faithfully record what you see by using the title, so most documentary photographers like to use the title, such as French photographers.

When the focal length is smaller than the pan/tilt, the lens can record images with a larger angle of view, so it is called a wide-angle lens. The perspective of a wide-angle lens is exaggerated. Under the wide-angle lens, the closer you are, the smaller you are, especially in landscape photography, you can take more photos with visual impact.

A lens whose focal length is 1.5 ~ 4 times larger than that of the meter head is called medium focus. Generally, the medium focus lens has the smallest deformation, and it is relatively easy to design it into a super-large aperture, so portrait photography is what the medium focus lens is best at. Generally, the focal length of the title is 1.5 ~ 2 times, which is used to take photos of the whole body and chest circumference. Shots larger than 2 times are mostly used for close-ups, and several shots called "portrait king" in the shot all appear in this focal part.

A lens that is more than twice the focal length of the head is called a telescope head. As the name implies, the telescopic lens can achieve "telescopic" shooting, which is mostly used to shoot sports, animals and so on. The application of telescope lens in scenery film can compress the far and near perspective of the scene.