TODD VORENKAMP, EXPLORA:
Leave it to the world of photography to confuse size and speed. In lens speak, the term “fast glass” refers to lenses with large apertures. The aperture is the opening of a lens. Its size is expressed as a number that shows the ratio of the opening to the size of the lens. This number is referred to as an f/number, f/stop, focal ratio, f/ratio, or relative aperture.
How fast is “fast?” Or, how big of an aperture opening gives me a truly fast glass?
In “professional” zoom lenses, the aperture of f/2.8 is generally regarded as fast. When it comes to prime lenses, depending on your level of lens snobbery, what is truly fast starts between f/2.0 and f/1.4 with many “professional” lenses featuring f/1.4 maximum apertures. Faster-than-f/1.4 lenses are the exotics of the optical world.
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