First and foremost, any system of lenses that we create must be able to produce a clear image. If we are unable to do this, our lens is never in focus and serves no purpose. Secondly, once focused, we must be able to magnify the image. If there is no magnification, there is no "zoom" and we would have failed miserably in our vernture to construct a zoom lens. (Something to think about: A normal window produces a clear image but doesn't magnify. Would that make a good zoom lens? Nope!) Finally, once we've focused on an object, the image must remain in focus as we zoom in on it. We do not want to create a lens where we focus, zoom in, and then have to re-focus. The image must always be in focus.
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