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Aperture openings also control depth of field. Smaller aperture openings keep a larger part of the image in sharp focus while larger aperture openings will keep the subject or focal point pin-sharp while the rest of the image remains blurred. This effect is even more obvious with tele lenses as they have smaller depths of field than wide- angles.

Aperture Tips

Be mindful when we talk about apertures high numbers (16 or 22) indicate small openings and low numbers (2,8 or 4) mean large openings. The smaller the aperture, results in a larger depth of field. But, be aware that DOF extends 1/3 in front of the point of focus and 2/3 beyond. So, by focusing at infinity, you are wasting 2/3 of the depth of field. Focusing at a point in front of infinity and choosing a small aperture will maximize DOF. This is called hyperfocal focusing.

If your camera has a depth of field scale, here are some tips. Focus your camera at infinity, note the distance on the camera's lens opposite the chosen aperture and set your lens at this hyperfocal distance, noted above. Now everything from foreground to infinity will be sharp. It is best not to choose the widest or smallest aperture setting because lenses do not show their optimum image quality at both extremes of the scale and optical faults may become visible. For optimum image quality select a medium aperture.

In low light situations, fully automatic cameras give preference to fast shutter speeds and select the widest aperture setting for a correct exposure, so depth of field will be minimal. If there is no manual way to select a slower shutter speed keep your subject away from the borders of your image as this is where optical faults are most prominent.

Manual Settings

Not every camera has the possibility to select manual aperture settings. Most will select a fixed combination of shutter speed and aperture at a given Exposure Value. This is adequate for general photography, however there are times when you may want to adjust aperture manually.

When taking landscape shots for example, the object mostly is to get everything from foreground to background in sharp focus. So you will need a large depth of field. To achieve this, set an aperture of 16 or 22. In aperture priority the camera will then automatically select the right shutter speed but in full manual, you will have to decide the right shutter speed for a correct exposure of the scene.

If you would like to isolate your subject from the background, for instance when taking portraits, open up your aperture to throw the background out of focus. Apertures of 2.8 or 4 will focus attention on your subject while keeping the background blurred.

The effect is even stronger when using tele lenses as their depth of field is very shallow at any setting. With wide-angles the effect of a large aperture will be less obvious. When taking macro photos, manually select a small aperture (16 or 22) to get as much depth of field as possible.

Depth of Field

Depth of field (DOF) refers to what is and isn't sharp in a picture. Images with great depth of field have a large zone of sharpness, so that everything from just a few feet in front of the camera to infinity will be sharp. Depth of field at a given focusing distance extends 1/3 in front of the point of focus and 2/3 behind it. The three key factors to depth of field are aperture, lens type or focal length and subject distance:
- Aperture
By closing the aperture, you are not only limiting the amount of light that reaches the CCD but you also create a greater area of sharp focus. The smaller the aperture you choose (say 16 or 22) the larger the  depth of field.
- Lens type
At any given aperture, a wide-angle lens or the wide-angle setting of a zoom lens yields far more depth of field than telephotos or tele settings.  A 28-mm lens set to f8 gives a larger area of sharpness than a 300-mm lens at the same aperture.
- Subject distance
The closer your subject is to the camera the less depth of field you get for any given lens type or aperture. This is the reason there is so little depth of field in macro photography where the subject is often only inches away from your camera.

source: dcviews