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Aperture, Speed & Sensitivity (Part 1)

2013-03-01 21:07:00 GMT

By ACDSee Guest Blogger & Professional Photographer Alexandra Pottier

In photography, three things matter: Aperture, speed and sensitivity. These three facts, when combined, make a good exposure.

About aperture in photography:

The diaphragm opening in a camera is composed of thin metal curtains that open or close in increasing sizes.

The aperture of the diaphragm corresponds at the size of the surface that will allow the light to come in and hit the sensor during the exposure. You can compare the diaphragm to a window, the bigger the size, the more light comes in and the room is more bright.

The size is expressed in f/stops where large numbers (f/22) are letting very little light in, which is useful when there is a lot of light, like a bright sunny day.

Whereas smaller numbers (f/2.8) allow a lot of light to come in when there isn’t much light around.


f/2.8                                                 f/8                                               f/22

The numbers are proportional, if you increase one f/stop (from f/11 to f/8 for example) you double the size of the hole from which the light comes through. It is the same proportion if you decrease from one f/stop, (from f/8 to f/11) you let half of the light to come through.

The most common f/stops are f/2.8, f/5.6, f/8, f/11, f/16 , f/22.

Usually, the biggest or the smallest aperture given by a lens depends on its focal length. The average lens with a focal length of 50mm, has a maximum aperture around f/2.8. A bigger zoom lens usually offers a smaller aperture, around f/5.6.

The more a lens opens wide, the more complex it is to build. The zoom lenses with a big focal length (200mm) and a wide aperture are usually very expensive.

The aperture also affects the depth-of-field (DOF), the sharpness of the different levels of the picture. In other words, the depth of the depth of field.

With a large aperture (f/2.8) the depth-of-field is small and, on the contrary, a small aperture (f/22) gives a large depth-of-field.

In Portrait photography, it is recommended to use a small depth-of-field, so everything that is behind the subject will have a nice artistic blur. Where in landscape photography, it is better to use a large DOF, and so a small aperture to have every level in the picture sharp.


The image on the left has a big aperture (f/2,8) and has a small depth of field
The image on the right has a small aperture (f/22) and has a large depth of field.

You can change the aperture by using the manual mode of your camera, or you can use the A or Av mode. When you do so, your priority is to set the aperture first, the other settings will follow.

Some cameras have a depth of field measurement where it shows you what is invisible. When you change your aperture while using this tool, you can see the depth of field changing through the lens. Very handy to compose a nice picture.

Now get your camera and practice!