What does the f-number printed on a lens mean?
Asked 9/13/2010
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I understand what an f-stop is in general, but what does the f-number printed on the lens itself indicate? Is it the lens’s maximum aperture, the best aperture to use, or something else? Also, why do some zoom lenses show a range like f/4–5.6?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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It refers to the maximum f-stop (which is defined as the ratio of focal length to effective aperture diameter). Smaller number means larger opening and more light:
Source: Wikipedia
Some (possibly overgeneralized) examples:
- f0.9 to f1.2 is exceptional
- f1.4 is very fast, usually the pro primes with less than 100mm focal length have this aperture
- f1.8 to f2.4 - still quite fast
- f2.8 - good pro zooms and long pro teles usually have f2.8 constant maximum aperture (constant only applies to zooms, see below)
- f4 - good consumer-prosumer zooms and very long pro teles usually have f4 constant maximum aperture
- f4-5.6 - aperture range denotes variable aperture zooms, meaning you lose light when zooming in (70-300 f4-5.6 is f4 lens at 70mm and f5.6 lens at 300mm)
Read more about aperture from Wikipedia.
Originally by user75. Source · Licensed CC BY-SA 4.0
user75
15y ago
0
Generated from our catalog & community — verify before relying on it.
The f-number printed on a lens usually indicates its maximum aperture.
Aperture is expressed as an f-number: focal length divided by the effective diameter of the opening. A smaller f-number means a larger opening and more light, so f/1.8 lets in more light than f/4.
If a zoom lens is marked with a range like f/4–5.6, that means it has a variable maximum aperture: at the wide end it may open to f/4, and at the long end only to f/5.6. A lens marked f/2.8 without a range is typically able to maintain that maximum aperture throughout the zoom range.
This printed number is not the “ideal” aperture for image quality. Maximum aperture is useful in low light and for faster shutter speeds, but many lenses are often sharpest a few stops down, commonly around f/8 or f/11 depending on the lens and subject.
On some older or manual lenses, extra aperture markings around a center line are depth-of-field markings, not additional maximum aperture values.
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