What does the telephoto lens do?
On a 35mm system, a telephoto lens is any lens with a focal length greater than 50mm or so. A telephoto lens can be used for any of the following reasons: 1. A flattering perspective for portraits. 2. Taking photos of far away subjects like athletes and animals 3. Compressed landscapes 4. Increased background blur However, the “telephoto” lens that you show is just a screw on magnifier for another lens, and will result in significantly decreased image quality. It doesn’t represent a good investment, although a real telephoto lens from a major manufacturer like Canon or Nikon does hold its value extremely well. Its difficult to advise you on lenses without knowing your specific goals for your photography, but I would encourage you to wait and purchase a real telephoto lens.
