DEPTH OF FIELD
Control how much or how little appears sharp in shots
The ability to control which parts of your pictures are sharp or blurred is one of the main advantages of owning an Canon EOS camera. This allows you to create emphasis where you want it – and to hide distracting elements.
Due to an optical property known as depth of field (aka DOF), a range of distances actually appear sharp. This zone of sharpness can vary depending on three key factors: your aperture, focal length and focusing distance.
Focus closely on a flower with a 100mm macro lens at f/2.8, and depth of field may only stretch for a couple of millimetres behind and in front of your focus point. Switch to shooting a sweeping landscape with a wide focal length of 18mm at f/16, and everything from your feet to the scenery in the far distance on the horizon appears pin sharp. So, to control DOF, first choose your aperture on your lens. The narrower the aperture (the smaller the f-number) the more depth of field you can capture in your shots.
The easiest way is to use Av mode (or Aperture Priority) on your camera’s top dial, then rotate the wheel behind the shutter button to change the aperture – while your camera automatically adjusts the shutter speed – to capture an accurate exposure.
Use a wide aperture (such as f/4) if you want to restrict the zone of focus and capture a shallow DOF. Or if you want to make sure as much of the shot is as sharp as possible, close down the aperture to a narrower setting (such as f/16) to capture a greater DOF.
The second factor is your focal length – in simple terms, as your focal length increases, your DOF decreases, and vice versa. Wide-angle lenses offer much greater depth of field than telephoto zooms. So, for keeping everything in the frame sharp, a wide lens focal length of 18mm, or wider, is a good start. Similarly, if you want to blur the background as much as possible, switch to a long telephoto lens with a focal length of, say, 200mm.
The third factor is your distance from your subject or focal point – aka your focusing distance. The closer your lens is focused, the less depth of field you capture.
Therefore, to maximize the amount of depth of field, use a wide focal length and a narrow aperture, and don’t focus on anything too close to you!
And if you want to keep backgrounds artistically blurred, drawing attention to just a single part of the scene, use a long telephoto, set the widest aperture, and get closer to your subject.