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--------------------------------- Optics Primer - Zoom Lenses

--------------------------------------------------------- By Bill Claff

We have already seen how focal length can be changed to perform focusing. This same effect can be used to construct a zoom lens.

Here we see you the lens groups will move as we zoom from wide (W) to telephoto (T) in this 70-200mm lens.
We also see how group G3A moves as we focus closer than infinity.
Also, there are lenses in G4 that move up/down; this is the Vibration Reduction (VR) feature.
On each side of the VR elements are field stops to maintain image quality when these elements are moved off-axis.

Here's what happens at infinity as we zoom from 70mm to 105mm to 200mm:

As focal length goes up the entrance pupil gets larger and moves further to the right; this is a constant f/4.2 lens.
Also, as we would expect from the initial drawing, this lens does not change length during zoom (or focusing).