How DX Lenses Work
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Defined
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Nikon DX lenses are lenses specifically made for Nikon DX sensors. DX sensors are smaller than a 35-mm film frame. As a result, lenses of a particular focal length have a "crop factor" of about 1.6. For example, a 100-mm DX lens will be similar to a 160-mm lens on a 35-mm frame. Because DX sensors are smaller, DX lenses provide a smaller image circle. Use of a DX lens on a 35-mm camera or a full-frame digital sensor will result in severe vignetting.
Fixed-Focal-Length Lenses
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Fixed-focal-length DX lenses contain a varying number of glass or plastic lens elements. The front lens element gathers the light. It is corrected and refined by the inner lens elements until it passes through the rear element to the sensor. Fixed-focal-length lenses are a specific focal length, which makes them easier to build. They are more reliable and optically superior to zoom lenses. Depending on the lens model, focusing may be via an internal focusing motor or through a focusing motor in the camera body. Manual focusing is always an option with these lenses.
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Zoom Lenses
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DX zoom lenses are more complicated than fixed-focal-length lenses. Zoom lenses are also multi-element lenses but contain far more elements than fixed-focal-length lenses. Light is initially gathered by the front element. It is then refined and corrected by internal elements before being focused on the sensor by the rear element. Zoom lenses, however, also contain internal elements that change the focal length of the lens. This is accomplished by moving an internal element or group of elements forward and backward in the lens to change the effective length of the lens.
Changing the effective focal length of the lens changes the effective magnification of the lens. Longer lenses, or telephoto lenses, act more like a telescope. Shorter lenses, or wide-angle lenses, are capable of taking in a much wider field of view. Although modern zoom lenses are far superior to older zooms, they are sometimes prone to distortion at the two extremes of their range. Depending on the lens model, focusing may be via an internal focusing motor or through a focusing motor in the camera body. Manual focusing is always an option with these lenses.
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