If you have a DX camera (Also called "Cropped" or APS-C-lenses), such as a consumer-level Nikon, or Pentax, the lenses have an implicit magnification factor when compared to the old-style 35mm film format.
If you grew up with film, a 28mm lens was a moderate-wide-angle, a 50mm was a "normal" lens, and a 135mm was a short-telephoto. Think of these as magnification or field-of-view. But these same focal lengths on a DX camera act as 1.5-times those magnifications.
For years, I called these focal-lengths "old-school" vs "new-school" -- this may not be true for you. If you are blessed with a full-framed digital camera, such as a Nikon FX (full frame), focal lengths are the same as the old days. But most of us have less expensive equipment, cameras with the smaller APS-C sensor size, and because of this we have to do the mental gyrations when comparing focal lengths.
For example, on a newer DX camera, a 50mm lens behaves as if it were a 75mm short-telephoto.
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The math takes the new focal length time 1.5, which equals the approximate focal-length on a full-frame camera:
Or, going the other way, you can convert a 200mm DX lens to the old-style focal length by dividing 200 / 1.5 = 135mm.
Thus
Although a DX 18mm lens sounds impressively-wide, it is really a boring 28mm moderate-wide-angle. On the other side, a 200mm DX is a respectable long telephoto, clocking in at 300mm -- almost good enough for some bird-watching.
Why the 1.5x Magnification on "DX" lenses?
My Nikon manual is paraphrased the issue like this: APS-C cameras have a smaller (and cheaper sensor, and the lens's circle of view is also smaller (making for less-expensive glass):
Some camera brands have different cropping factors. Entry-level Canons are 1.6. Some Olympus, Panasonic, and Sony have crop factors of 1.3 or 2.0. Check your manual.
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