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AF Zoom-Nikkor 70-210mm f/4 lens.

This lens uses the optics of the earlier Nikon Series E 70-210mm f/4. In 1986 it appeared in an AF version. The Series E version had been a slide zoom, but when it was introduced as an AF lens, it became a two-ring zoom. The Nikkor 70-210mm f/4 AF was Nikon's first AF telephoto zoom of 1986. It was made for only 18 months, to be replaced by the smaller, slower and cheaper AF 70-210mm f/4-5.6. The AF 70-210mm f/4 is largely distortion-free, whereas the successor AF 70-210mm f/4-5.6 is handicapped by high levels of distortion. Optically both the Nikon Series E 70-210mm f/4 and the Nikkor 70-210mm f/4 AF are similar to the Nikkor 80-200mm f/4 AI-s.
The short-lived AF 70-210mm f/4 was rushed out as a stop-gap when Nikon realised customers were more enthusiastic over automatic focus technology than the company had supposed. The lens was never regarded as a commercial success, although part of the reason might have been its fairly high price. From the point-of-view of the amateur photographer, however, the lens is useful on several counts. If your digital SLR has the AF screw drive, you can use all the features of this lens without hindrance. It is a good deal lighter to carry than an f/2.8 alternative, and only one stop different throughout the focal range, being a lens of constant aperture.
A constant aperture is a useful quality in a zoom lens. Constant aperture zooms tend to be higher quality lenses. These days only professional zoom lenses are of this type. Most consumer zoom lenses have a variable maximum aperture. If you look at zoom lenses that are not constant aperture, you will notice that as you zoom out, the maximum aperture changes to a bigger f/ number. In practical terms what this means is that as you zoom out, the viewfinder gets dimmer because the maximum aperture is getting smaller. A constant aperture lens is designed so that the size of the aperture opening will change in relation to the zoom position; thus if you select f/4, you will have f/4 at any focal length.
There are other advantages of a constant aperture lens: there is a lot less refocussing as you zoom; in manual mode, you don't have to change your shutter speed whenever you zoom in or out; your exposure won't change as you zoom; and the lens barrel doesn’t extend or retract when the focal length changes.
There is an informative article about the Nikkor 70-210mm f/4 AF at www.dantestella.com/technical/70210.html