A great number of photographers use Aperture-Priority (Av) mode as their preferred creative mode. In this mode the photographer selects the aperture and the camera automatically selects the shutter speed to achieve the correct exposure. But what is so magical about this “aperture thing”? Why do photographers want control over the aperture used when taking photographs?
Aperture refers to the opening in the lens that controls the amount of light that passes through to the sensor or film. That opening changes based on what the photographer or camera feels is the suitable “aperture setting” or f-stop. The letter “f” is assigned to represent aperture. Depending on which lens you are using you may see apertures ranging from f/1.4 to f/22.
When a photograph is taken with a large aperture (like f/1.4 or f/2.8) there is a small area in front and behind the point of focus that is relatively sharp or in focus. When a photograph is taken with a small aperture (like f/11 or f/16) there is a large area in front and behind the point of focus that is relatively sharp or in focus.
The area of acceptable focus that stretches from the foreground of the point of focus to the background is called DEPTH OF FIELD.
When photographing a flower, a fruit or the torso of a model you do not want any distraction in the foreground or the background. One way to eliminate or reduce distraction is to use a large aperture that tends to give a small depth of field and a blurry foreground/background. The following 3 images show what happens to the background when the aperture is changed:
The lime was photographed with a Canon EOS 7D camera and a Canon EF 24-70mm lens set on a tripod. The first image was shot at shutter speed 1/640 and aperture f/2.8. The second image was shot at shutter speed 1/125 and aperture f/5.6. The third image was shot at shutter speed 1/40 and aperture f/11. The ISO was set at 200.
The leaves and branch to the left of the lime in the first photograph are blurry, less so in the second photograph and clearly visible in the third photograph. The aperture was decreased from f/2.8 to f/5.6 to f/11 and as a result more details in the background appeared.
WHEN TO USE LARGE APERTURE
Use a large aperture (like f/1.4, f/2.8 or f/4.0) when you want to make your subject(s) stand out against the background. This works best when photographing a flower, a fruit, a pet and sometimes the face of a person. (Remember: large aperture for small depth of field.)
WHEN TO USE SMALL APERTURE
Use a small aperture (like f/8, f/11 or f/16) when you want as much of the foreground and background to appear relatively sharp. This works best when photographing a beach, a farm, a crowd or any subject that covers a large area. The beach scene below was captured with a small aperture of f/16 to reveal as much detail as possible in the foreground and the background:
This is Dania Beach in south Florida. The image was taken at 8:12 AM on a very cool and sunny day. The Canon EF 24-70mm f/2.8L lens was set at 28mm. The Canon 7D was set at ISO 200, aperture f/16 and shutter speed of 1/200 second.
Another very important thing to consider in the discussion of aperture and depth of field is the focal length of the lens you are using. You normally shoot a landscape or a crowd with a short focal length such as 15mm, 20mm, 24mm or 28mm. However when shooting a flower, a fruit or sometimes a model, you are more likely to use a longer focal length such as 70mm, 85mm, 105mm or 200mm.
Lenses with short focal lengths give much better depth of field than lenses with long focal length. Hence short focal lengths for landscape and focal length like 105mm, 135mm or 200mm for taking headshots and shooting models.