This may look like a silly question but I don't know what the difference between zooming and focusing is. Zooming in will usually let me have fewer subjects in the scene while zooming out will allow for wider view. Focusing is the process adjusting the wanted subject to be in focus and look sharp. When we zoom, lens elements are moved to zoom in or out, what happens in the lens when we change the focus?
The two generally control two different aspects of the image projected by the lens. Focusing sometimes has the effect of changing "zoom" a little as well, however its purpose is different. To keep it simple:
- Focus adjusts the Focal Plane
- The focal plane is the thin plane of reality that is focused clearly on the imaging medium
- Focusing moves this plane nearer or farther from the camera's sensor/film
- Zoom adjusts the Angle of View
- The angle of view is the breadth of the scene projected by the lens
- Wide-angle lenses tend to capture very broad scenes (large angle of view)
- Telephoto lenses tend to capture very narrow scenes (small angle of view)
Focusing in the past used to be achieved by moving the lens itself (i.e. in a large-format view camera) forward or backward (away/towards the imaging medium). This often results in changes to the angle of view as well, since the total focal length of the lens can change via extension...sometimes by a lot. In modern camera lenses, focusing may be achieved similarly...many cheap lenses focus by moving the front or back (or both) lens elements. Higher quality lenses tend to use an internal floating focus group, an internal group of lens elements who's sole purpose is to focus the image. The benefit of internal focusing groups is that the physical length of the lens stays the same, allowing you to minimize the amount of "zoom shift" that might occur when focusing.
Focusing is just changing the plane of focus, so at a very fundamental level, all that is happening is the lens elements are moved as a single group towards or away from the sensor (though in practice there can be minor changes in the separation of the groups to counter unwanted optical effects).
When you zoom, the element groups in a lens will move independently of each other, with the spacing increasing or decreasing between the various groups to change the path the light takes through the lens.
At a basic level, focus adjusts how far from the camera a subject will be sharp (in focus). Zoom adjusts the magnification (angle of view mapped to the same size picture). The two are totally different.
While differnt, on some lenses adjusting one may have a effect on the other. In particular, lenses that focus by sliding in and out actually change their magnification ratios in the process a bit too. This can matter in some cases, particularly for very close subjects. For ordinary non-macro shots from landscapes down to portraits or so, you'd have to look hard to notice this. The zoom adjustment will have a much greater and obvious effect than the small zoom side effect of focusing. Some "internal focus" lenses have less zoom side effect as a result of focus changes.