The Nikon DX image sensor format is about 24×16 mm.
These dimensions are about 2/3 those of the
35mm film format (29mm vs 43mm diagonal, approx.). This format was created by Nikon for its digital SLR cameras, many of which are equipped with DX-sized sensors.
The 1/3 smaller diagonal size of the DX format amounts to a 1/3 narrower angle of view than would be achieved with the 135 film format, using a lens of the same focal length. Strictly in angle-of-view terms, the effect is equivalent to increasing focal length by 50% on a 135 film camera, and so is often described as a 1.5× focal length multiplier.
This effect can be advantageous for telephoto and
macro photography as it produces a tighter crop without the need to increase actual focal length and sacrifice
depth of field. However it becomes disadvantageous for wide angle photography as a
wide angle lens for 135 film (e.g., 28mm) effectively becomes a
normal lens for the DX format (28mm × 1.5 = 42mm 135 film equiv.). This has led to the increased development of the DX format-specific lenses for the
Nikon F-mount. Since these lenses do not need to cover the 135 film area, they are smaller and lighter than their 135 format counterparts of equal angle-of-view. The production of DX-specific lenses has also enabled the production of affordable wide angle lenses for the format (e.g., 12mm), whereas costly ultra-wide angle lenses from the 135 format were formerly required.
When DX format lenses are used on 35mm film cameras, vignetting often occurs, as the image circle doesn't cover the entire area of the 35mm film.