Depth of field in photography is usually described as the amount of the frame that is in sharp focus. When we look at a scene with our naked eye almost everything appears sharp. But that is not the case when we compose the same scene with cameras. Some part of it is blurred out. The more the frame is in sharp focus, the more is the depth of field and vice versa.
Depth of field (DOF) depends on two factors: aperture and distance of the subject from camera (magnification). Wider aperture and closer subject give the shallower DOF.
There is an inverse relationship between the aperture of your lens and the depth of field. Narrower is the aperture more is the depth of field. Usually when you set your lens to f/1.2 or f/1.4 (in a prime lens) or f/2.8 (in a zoom lens) a soft blur is created around the back and front of the subject. This is also known as bokeh. When this happens only a small part of the frame (depending on where you’re focusing) is n sharp focus and the rest is blurred out.
When you reduce the aperture the whole frame starts to get into sharp focus. The narrower the aperture the sharper is the image. As you move from of f/4 or f/5.6 all the way to f/11 and beyond the images get sharper and sharper. The reason is the light rays reflected against the subject and passing through the aperture gets refracted by the front element of the lens and meets the sensor at a precise point creating a sharp image. Narrower the aperture more precise is this meeting of light creating a sharper image and thus bigger depth of field. However, after certain point increasing aperture number will give greater DOF, but you’ll start to loose sharpness. This phenomenon is called Diffraction.
Depth of field also depends upon the kind of lens that you use. A wide angle prime lens offers more soft blur because it has a wider aperture. But if you stop down to f/8 and even narrower, a large depth of field can be created. A wide angle lens reduces the perspective of the subject as it fits in more inside the frame; also know as field of view. As such a hut on the beach will appear smaller than the actual real life size when the picture is finally taken. On the other tele lenses have an effect of increasing the size of the subject on the final image. The higher magnification allows you to capture more detail of the subject but also reduce the field of view and the depth of field.
Great photography is about depth of feeling, not depth of field -Peter Adams
Basic tips to play around with depth of field
Set your aperture to a minimum of f/8 to make sure you have a good platform to start with. Always ensure that you zoom out and exploit the widest limits of your lens, even if you’re using a tele lens. Tele lenses have a narrow maximum aperture so once you have zoomed out completely you may still have an aperture of f/4 to start off. Wide angle lenses can pack in a lot provided you can start at f/8 or even narrower. When shooting landscape or any other subject, never focus on the infinity. Always focus on something closer so that you have a sharp depth of field behind the point where you’re focusing.