Which one is better, Canon 17-40mm or 24-105mm? How is the focal length defined?

Focal length: Parallel light passes through a convex lens and converges into a point, which is called the focus. The distance from the convex lens to the focus is the focal length. Photographic lenses are very large. We can only say that the distance from the equivalent optical center of the lens to the focal plane (that is, the photosensitive plate) is the focal length.

For a full-frame camera, the nominal focal length of the lens is the actual focal length.

For APS format cameras, the equivalent focal length is

APS format equivalent focal length = nominal focal length 1.33 times longer than the nominal focal length on the lens.

Long focal length, small viewing angle, and large object image; short focal length, large viewing angle, and wide scene. Therefore, the focal length has different uses, and you cannot use the focal length to judge whether it is good or bad.

17-40mm is a wide-angle segment, suitable for landscape and large-scale shooting. That is to say, if you need to see the vastness, choose it.

The 24-105mm standard zoom range has wide-angle and medium focal lengths, making it easy to compose pictures. Suitable for traveling. But if it's on the camera for a long time, it's the so-called hang-up. The disadvantages are that the wide angle is not wide and the long end is not long, so it is not good for shooting birds.