 Aperture
If you have an eye (even two) you'll understand easily the concept of aperture: an eye has an iris that regulates the amount of light that the brain will finally interpret. If it is dark, like in a romantic dinner, the pupil will become bigger to gather more light. No wonder the eyes of your beloved looked stunning. But at a very sunny day, those pupils will need to get reduced to avoid the entrance of extra light. The same goes with a camera, there is a mechanism (diaphragm) that controls the amount of light reaching the sensor.
The aperture combined with the ISO sensitivity and the shutter speed, the degree of exposure to light is regulated. Now, the thing is that your eyes don't work exactly like your still photographs camera. In your camera there will be a combination of parameters to expose correctly the image: for how long the shutter will be opened, how sensitive the sensor of the camera will be and how big the hole (aperture) that allows light into the sensor will be.
The lens aperture is usually specified as an f-number. Although it may sound tricky, if the "f" number is small (like f/1.2, f/1.4 or f/1.8) the aperture is very big. If the "f" numbers are higher (e.g. f/11, f/16, f/22) the aperture size is smaller.
A common aperture range may be f/2.8 at the widest aperture and f/22 at the minimum aperture. These values differ from lens to lens. There are lenses that allow a really big maximum aperture (like the Canon EF 50mm f/1.2 L), thus they are ideal for low-light photography.
 That's a really fast lens
There was a time when Canon manufactured a 50mm f/1.0 lens. Stanley Kubrick decided to shoot some scenes for his film Barry Lyndon using the lens with the widest aperture in history (f/0.7). This lens was developed by Zeiss for NASA for use in the Apollo moon landings. Kubrick wanted to shoot a scene with just some candles.
 Professor Otto A. Fishonado
Doctor in Physics
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Getting technical
Professor Otto A. Fishonado will expose some technicalities of aperture:
 "Well, dear students, the diaphragm that controls the aperture in a camera carries a scale of so-called f-numbers, or stop numbers, in a series: such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. In the good ol' days, that scale was right in the lens, the lens at the right has the following scale: 2.8 4 5.6 8 11 16 22.
Now hold your breath and read this slooooowly:
The squares of the f-numbers are inversely proportional to the amount of light admitted. Each setting admits twice as much light as the next higher f-number, or stop (giving twice as much exposure.)
Read it again and the continue... |
Let's take another look to our aperture diagram:
When the lens is at its minimum aperture (in this case f/22) less light enters the sensor. At f/16 the sensor receives twice the amount of light compared to f/22. Consequently, at f/11 receives twice the light as with f/16."
That's it for Professor Otto A. Fishonado. Thank you, Professor!
Aperture values
Although the aperture values will differ depending on the lens, here it is an example of a typical f/stops scales for aperture:
1.0 1.1 1.2 1.4 1.6 1.8 2.0 2.2 2.5 2.8 3.2 3.5 4.0 4.5 5.0 5.6 6.3 7.1 8.0 9.0 10 11 13 14 16 18 20 22 25 29 32 36 40 45 51 57 64 72 81 91
Prime Lens
 A fixed focal length lens (called a "prime") has a bigger maximum aperture than a zoom lens. Even a cheap prime can have a maximum aperture of f/1.8 and a luxury zoom lens may have a maximum aperture of f/2.8. A lens with a big maximum aperture allows to shoot at faster speeds (that's why people call this kind of lenses "fast" lenses).
Zoom lenses
 The zoom lenses have a smaller maximum aperture that gets even smaller towards the tele end. E.g. the Canon EF 70-300MM f/4-5.6 has a maximum aperture at the wide end of f/4 and f/5.6 at the tele end. A good (and expensive) zoom sometimes has a fixed maximum aperture at both wide and tele ends.
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Aperture and Depth of Field
The aperture has a second important characteristic: it modifies how sharp or blurred a scene will be at different distances. That means that the aperture affects the depth of field.
At f/2.8 only the focused subject will be sharp; foreground and background won't.
This effect may be very dramatic with portraits and is known with the japanese word "Bokeh". |
Many photographers prefer very sharp images. With a really wide aperture images may look soft. The Bokeh will be greater, but using a smaller aperture will improve the sharpness of the photograph dramatically. |
With smaller apertures (f11) everything in the scene will be very sharp. Many lenses have a so-called sweet spot where the lens reaches the maximum sharpness (in many cases at f/8 or f/11). |
The following diagram shows that at f/2.8 more light is entering the sensor, but the depth of field is narrower and only some stickers on the car are clearly focused . At f/11 the aperture is smaller, the sensor receives less light but the depth of field is wider and the whole car is sharply focused:
| Aperture |
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| Depth of field |
It all depends in what you want to achieve. Sometimes you'll want a shallow depth of field to emphasize a little detail, or you may want the complete scene to be crystal clear.
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Narrow depth of field |
Wide depth of field |
Choose the depth of field through aperture values to fit your needs |
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The "Av" (aperture priority) mode
There is a mode that allows the photographer to set the aperture and then expose the photograph automatically. This mode is called "Av" (aperture priority) and it is useful when you want to control the depth of field or the amount of light entering the sensor. Using the Av mode in a macro photograph of a flower, you may decide to set the aperture at f/11 to have a wider depth of field; you would choose a value of f/11 and the camera will set the ISO sensitivity and shutter speed to expose the photo. Even if you don't like the setting of the camera, you can adjust the exposure to over-expose or under-expose the photograph to your taste. In the same case, if you want only a small part of the flower focused and everything else blurred you should use the maximum aperture (e.g. f/2.8). That will have two benefits: enhanced Bokeh and faster shutter speed that would be really useful if you are hand-holding the camera.
Conclusion
Understanding depth of field may seem tricky at first, but it is crucial if you want to shoot a portrait with a beautiful background blur (Bokeh), or in very low light, or if you want to know how to get sharper images.
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