The most obvious effect of altering the focal length of a zoom lens, or of fitting a lens with a different focal length to an SLR, is the change in magnification. Anyone with a zoom lens camera will be familiar with this effect. If you want to take a photo of something a long way away, you zoom in and the subject appears closer. This series of photos shows the effect of a wide-angle zoom of 27mm, a medium zoom of 75mm and a telephoto of 450mm, all taken from the same position.
What is happening here is not the subject being brought closer, but the angle of view being reduced, so that a smaller proportion of the scene fills the frame of the sensor. If we look at a cropped-down section of the centre of the wide-angle view, we can see that it looks very similar to the 450mm view, although rather less sharp, because this tiny section of the frame obviously has far fewer pixels than the full-frame zoomed-in image.
Incidentally, this is the way that digital zoom works. A small centre section of your CCDâ€™s view is enlarged, resulting in a much lower resolution image. This is why digital zoom should never be used as a substitute for optical zoom. In fact, itâ€™s better never to use it at all.