1. Zoom lens, telephoto lens and retrofocus lens A zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length (and thus angle of view), as opposed to a fixed focal length (FFL) lens.lens elementsfocal lengthangle of view A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes. A lens that loses focus during zooming is more properly called a varifocal lens (see also below).parfocal lensvarifocal lens Parfocal lens and varifocal lens:

2. A parfocal lens design Applications: Video, motion picture cameras

3. A varifocal lens design Application: still camera

4. Telephoto lens system

5. Single-lens reflex camera

6. Maksutov-Cassegrain-Telescope Schmidt-Cassegrain-Telescope Shrink system length

7. Retrofocus lens system Wide field lens system retrofocus