Aperture / f-number written 5 years ago
The aperture of a lens controls how much light can enter the camera. If a scene has lots of light, the aperture can be closed, and conversely if there is little light the aperture can be opened fully to let in as much light as possible.
The image above shows three aperture states: fully closed; partially closed and Fully open.
f-number
The aperture of a camera lens is described in terms of its f-number, or F-stop. The f-number of an lens is the ratio between the diameter of the aperture and the focal length of a lens. Interestingly this means that smaller f-numbers correspond to larger apertures. As an example, consider the following table:
| f-number | Focal Length of lens (mm) | Aperture Diameter (mm) | Aperture Area (mm?) |
|---|---|---|---|
| 1 | 50 | 50 | 7854 |
| 1.4 | 50 | 35.71 | 4007 |
| 2 | 50 | 25 | 1963 |
| 2.8 | 50 | 18.86 | 1002 |
| 4 | 50 | 12.5 | 491 |
| 5.6 | 50 | 8.93 | 250 |
| 8 | 50 | 6.25 | 122 |
There are a number of things to note from this table:
This series of f-numbers is not arbitrary. It roughly corresponds to the series of powers of √2. (√2)0, (√2)1, (√2)2, (√2)3, …
Note that the aperture area roughly halves at each f-number. This means that the amount of light let in between each row is halved also.
In practise f-numbers are written f/##, indicating the division of the focal length (f) by the f-number to give the aperture diameter.
- f-number series
-
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128, …
A note on Stops
The f-number is often referred to as the f-stop. This is due to how “stops” are used in photography terminology to quantify light/exposure ratios. For a proper explanation of stops, please visit the Exposure settings page.
Maximum lens Aperture
Indicated at the end of most lenses will be the max lens aperture.
A couple of things to note:
Shown above the maximum lens aperture is often quoted as a ratio in the form 1:##. This is just a different way of expressing the f number.
Also shown above is “1.8/50”. This is a compact way of saying that it is a 50mm lens, with a maximum aperture of 1.8.
For zoom lenses, there will be two maximum apertures expressed, one for each end of the zoom.
On telephoto lenses making the aperture large becomes more and more difficult. Consider a 300mm lens with an f-number of 1.4 would need a aperture diameter of 214mm! In practise though, lens mechanics will also have an effect on the maximum effective aperture. This means that in order to create a telephoto lens with a large aperture some clever trickery is required, and hence means this type of lenses are very expensive.
References
[tedious] :http://www.uscoles.com/fstop.htm “A Tedious Explanation of the f/stop”
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