This is the scale of aperture values. These are whole 'f-stops', but there are apertures in between ('half stops' or 'one-third stops'):
- f1 (very rare)
- f1.4 (some fixed focal length lenses)
- f2 (some zooms)
- f2.8
- f4 (most standard zooms start at f3.5, which is mid-way between f2.8 and f4
- f5.6 (the maximum aperture of most standard zooms at their maximum focal length*)
- f8
- f11
- f16
- f22
* Zoom lenses rarely offer the same maximum aperture throughout their zoom range, which is why the lens specs will quote something like 'Nikkor 18-55mm f3.5-5.6'. That means the maximum aperture at 18mm is f3.5, but at 55mm it drops to f5.6.
Each aperture value in the list above is one stop (or 1EV) 'faster' than the one below it. F1.4 is two stops 'faster' than f2.8, which means you can use an ISO setting two stops lower or a shutter speed two stops faster.
There are zoom lenses with constant maximum apertures, but these are much bigger, heavier and more expensive.
Fixed focal length lenses like this Nikon 50mm f1.4 have wider maximum apertures because their optical design is simpler and the designers can push the envelope in different directions.
