The difference an aperture makes
One of the biggest considerations when it comes to choosing a telephoto lens, is how much reach you’ll need. Technically, any focal length longer than 50mm on a fullframe camera is considered telephoto. If portraits are your thing, then a flattering look can be achieved with a short or moderate lens in the 70-200mm range. But if small birds are more your bag, then you’ll need all the reach you can get – with 500mm and 600mm lenses being an ideal super-telephoto focal length.