Difference between zoom and focus

[finding83]

What is the difference between zoom and focus?

while we zoom, the lenses are moved to change the size of the image being formed, so what is happening in the camera when we adjust the focus?

[Palladius]

I may very well be wrong on this:

Optically speaking, zooming is better thought of as 'changing the focal length'. You're right that the focal length determines how much the image is magnified, this is because it's a change in how much the lens bends light to a focal point. Focus involves making sure your film or sensor is on that focal point.

It's a bit hard to describe without diagrams but for a simple lens model (i.e. without the clever trickery that lens manufacturers use nowadays), a lens with a focal length of 200mm and focused to infinity will converge light rays to a point 200mm behind the lens, so your sensor will have to be at that point in order to make and image that is in focus. If you then 'zoom out' that lens to 28mm the focal point is 28mm behind the lens, so your sensor has to be moved to that point. If you then wanted to focus on a subject closer than infinity, the light rays from this object will not be parrallel and will be converging on a point much closer than 28mm, so your sensor will have to be moved there.

Of course, that's a much simplified model... few telephoto lenses actually physically require a gap between lens and sensor of hundreds of millimetres, and few cameras physically move the lens or sensor to focus.

Not sure that's entirely answered your question, though

[Palladius]

Oh, I've just seen a rather good discussion of focal length in the 'lens measurement? How so...' thread in this section. That might help clarify some points, too...