On aperture and focusing of SLR camera.

The lens has a parameter called the nearest focal length. Generally, SLR lenses are of great value, which means you have to stay away from objects to shoot. Otherwise, you can't focus;

Second, the lens is marked with the maximum aperture. What you mean by this is that you can use 3.5 at the wide-angle end of 18mm, but it has shrunk to 5.6 at the end of 135;

As for the lowest, it is generally around F22-F32; The best aperture of this lens is F8; At this time, the picture quality is the best;

Generally, the aperture of some high-end lenses is called constant aperture, such as 70-200F2.8, which means that whether it is 70 or 200, the telephoto end can use the maximum F2.8 without shrinking.