Focus and Focal Length: the True Benefits of using a DSLR

Using a DSLR offers a number of unique advantages over compact cameras and smartphones – chief of which is their ability to change lenses. In this quick beginner’s guide we’ll explain some of the key benefits a DSLR offers in terms of focus and focal lengths.
Focus and focal length: the true benefits of using a DSLR
When you look through the viewfinder and half-press the shutter button, you’ll see the multiple AF (autofocus) points that flash when they’ve achieved focus on whatever’s in your frame. These are designed to help you take sharp shots.
However, if you shoot using basic modes (Portrait, Landscape, Sports and so on), most DSLRs will automatically select the AF point, so your camera uses the AF point to focus on what it thinks you want to focus on – usually whatever’s closest to you, which might not be what you want to be in focus at all!
However, one of the many benefits of using a DSLR is that you can control what your camera focuses on to ensure sharp results every time. You have the option of manually selecting your AF points.
This means, when shooting portrait photography, for instance, you can focus on somebody’s face that’s off-centre in your frame for an improved composition. The number of focusing points varies enormously depending on your camera model.
For example, an old Nikon D40 has three AF points while the Canon EOS 7D has 19. The Olympus OM-D E-M5 offers 35.

Focal Lengths, Wide-Angle and Telephoto

Focal lengths, wide-angle and telephoto
A major advantage of using a DSLR over a compact camera is that you can change lenses, opening up a whole world of focal lengths.
Lenses with fixed focal lengths are known as prime lenses, while zoom lenses have a range of focal lengths. Focal length is measured in millimetres, the longer the measurement the narrower the angle of view.
A 35mm lens roughly equates to what you see in the real world, those with larger focal lengths (telephoto) have a magnifying effect, bringing far-away objects closer, while shorter focal lengths
But as well as a cropping effect, focal length has a dramatic effect on the shape of your subject: the wider the angle, the more distorted the image appears – great for landscapes, not so good for portraits.
By changing both focal length and distance you can alter perspective and control how much background appears in shot.

How to Control What You’re Focusing On

How to control what you're focusing on
To manually select individual AF points, press the AF Point Selection button on your camera and look through your viewfinder.
Use the top dial, crosshair buttons or joystick to cycle through each AF point until the one you want falls over your chosen subject in your scene.
Half-press the shutter button to achieve focus on your subject, fully press the button to take an exposure.
Helpfully, the AF points are placed on (invisible) vertical and horizontal lines at points one-third into your frame, so they can also help improve your compositions; use them as guides as to where to place the subject in the frame.
Note that if all the AF points light up you’re still on AF Auto Point Selection.