About the data on the lens: 50 mm, 17-40 mm, 24-70 mm, 70-200 mm, what do you mean by ..........?

These data are all the focal length data of the lens. Any small value is the nearest focal length, while the large value is the farthest focal length. Example 17 is the closest focal length,-means that 40 is the farthest focal length. Generally, the nearest focal length can reach about 10, and the farthest focal length can reach more than 800. If the focal length is close, the picture is large or the object is large, and if the focal length is large, the distant scenery can be photographed. If you use a lens with a focal length of 800, you can clearly shoot ants dozens of meters away. Photography belongs to an art and science. If you are interested, you can go to the bookstore more often, which will increase a lot of knowledge and fun.