What is the SLR camera lens built-in zoom
Built-in zoom refers to the zoom lens when the length of the same, from the outside does not seem to change. Some lens zoom when the lens will be stretched out, this is not. 18-135 and 17-70 are zoom range, such as 18-135, the lens is pulled to the recent 18-focal length, pull the farthest is 135 focal length, you can always zoom from 18 to 135.17-70 Similarly.
For example, you take a picture in the grasslands, there are horses in the distance, the lens pulled 17 to see the whole scene you see down, that 18 paragraph than 17 to see a narrow point, and so on to 70 Even more narrow to 135 can only be photographed on both sides of the film can not shoot. 18-135, is a tour of the mirror to go the world of the lens, go out with a lens on it, and want to shoot wide-angle also, want to shoot the details of the distance there.
But why photographers take pictures with a lot of lens it, because this kind of mirror to go the world what can meet the lens image quality is not so good. From the use of up to say, to take a clear film, go out with a lens to use 18-135, but from the pursuit of the quality of the photo, the smaller the scope of the focus of the lens imaging effect is better. 17-70 pull can also shoot big scenes, far from 135, but the image quality will be better. See what you are looking for. If the pursuit of quality or sub-lens better, wide-angle wide-angle lens, shooting telephoto detail with telephoto lens. But the price is more expensive