Focusing modes on a DSLR
As you look through your camera's viewfinder and half-press the shutter release button, you’ll see the multiple autofocus points in the view finder. Depending upon the settings of your camera one or more of these flash along with a beep sound when the focus is achieved. There are different settings available.
One of the several advantages of using a professional camera is that you can control what it focuses on by selecting your AF point manually. For example when shooting portraits, you can focus on the subject’s eyes, or on one of the eyes if it’s off-centre or the face is not parallel to the focal plane. The number of focusing points you have varies from camera to camera. More focusing points means, more chances of your overlapping the focusing point over the area you want to focus and less of the need to recompose after focusing. However, practically you may still recompose a little bit after focusing. Be careful that if all the autofocus points light up at the same time, you’re on Auto Point Selection.
Exposure is also based on the focusing point that is selected.
How to change the Focus points
To manually select individual AF points, press the AF Point Selection button on your camera (check your booklet for the exact location of this dial) and then look through the viewfinder. Use the top dial, crosshair buttons or joystick to cycle through each AF point until the one overlapping your subject is highlighted (generally turns red) Half-press the shutter button to lock the focus, then fully press it to take the picture.
These AF points are interestingly located along the imaginary lines which may help you compose the photograph keeping in mind the Rule of thirds.
Single Point AF
This is the best mode to use as you have maximum control over the exact point you wish to focus. In case of Portraits, you may focus on the eyes and is especially useful when shooting with a very shallow depth of field. So you want to be very sure that the camera focuses exactly where you want it to. By default, the centre focus point (which is the most accurate and fastest) is used for focusing in single point autofocus. This can be changed to overlap the subject which may be on the sides and not in the centre.
Dynamic Area AF
This mode is good for shooting fast or erratically moving subjects. Some cameras may have a lot of autofocus points spread over the viewfinder – you can choose one, but if the subject moves away from that point, the camera will continue to focus using one of the adjacent AF points. However, personally, I prefer the single point AF as the exact point is more in control.
If all the autofocus points light up at the same time, you’re on Auto Point Selection. In this case generally the camera focuses on the nearest subject.Not recommended if you have something in the foreground and the subject is not the nearest element in the frame.
Yes, in situations where two people are in the frame on the sides but not in the centre, you would not need to shift the focusing points over the face of one of them but the camera will automatically focus on them as they are closest to the camera.