Click to explore how focusing shifts the plane of critical focus.
Shutter buttons have two stages. When pressed halfway down, the camera sets and locks focus (and often exposure).
It seems the more expensive the camera, the more focusing areas you get to choose from. These are from a Nikon SLR.
Click to explore the way focus zones work.
Click to explore the effects of servo focus.
Pressing the shutter button halfway down locks focus and pressing it all the way down takes the picture.
Click to explore focus lock.
One of the most important things to look for in a new camera is how well it focuses. This is important because a lens can only bring one part of the scene into the sharpest possible focus. This part of the scene falls on what is called the plane of critical focus. Subjects falling on this plane will be the sharpest part of the picture. You move this plane toward and way from the camera as you
Imagine the part of the scene on which you focus (A) as a flat plane, much like a pane of glass, parallel to the back of the camera or the image sensor. Objects falling exactly on this imaginary plane will be in critical focus and be the sharpest part of your picture. This plane of critical focus is a very shallow band and includes only those parts of the scene located at identical distances from the camera. As you point an autofocus camera at objects nearer or farther away in the scene, the camera refocuses and the plane of critical focus moves closer to or farther from the camera.
As the plane moves, objects at different distances from the camera come into or go out of critical focus.
PLANE OF CRITICAL FOCUS
The plane of critical focus in your image will be the area that falls on the active focus
area. As you point the camera at various subjects and press the shutter button halfway down, you'll see the subjects pop into focus in the viewfinder.
There are three ways cameras focus—fixed focus, autofocus, and manual focus.
- Fixed focus is found on the least expensive cameras, almost all camera phones, and one-time-use cameras. It is sometimes called focus-free for marketing purposes, a euphemism one reviewer suggests they change to unfocusable.
- Manual focus found on SLRs and some expensive fixed lens cameras lets you focus by turning a ring on the lens-in many situations this is the best way by far. On point and shoot cameras you often have to use buttons or dials to manually focus—a slow and unsatisfying process at best.
- Autofocus is available on all but the very cheapest cameras. In fact, on many
low-end cameras it's the only kind of focus. When you press the shutter button halfway down, the camera automatically focuses on the center of the scene or some other designated focus area. It's important that the camera do this quickly and accurately. Autofocus often has trouble focusing on off-center subjects or on scenes with little contrast, when the object in the focus zone is
brighter than the rest of the scene, when the subject is poorly illuminated, when both near and distant objects fall within the focus zone, or when the subject is moving quickly. If the camera can't focus, some cameras beep or blink a lamp. If this happens, it's best if your camera lets you use focus lock to focus on a subject at the same distance or switch to manual focus.
Some cameras have more than one focus zone or area, usually indicated on the screen or monitor with rectangles or brackets. Others offer a single focus area you can move over any point in the scene. Both approaches make it easy to focus on off-center subjects. If the camera displays multiple focus zones, it will usually focus on the center one or on the part of the scene closest to the camera
covered by one of the zones. Multiple zones are especially useful if the camera lets you manually select the one to use.
Here three areas are
indicated with the active one used to set focus shown in green. The camera normally chooses the focus point that covers the closest part of the scene but you can also select the point manually.
Some cameras let you
move the focus area around the screen. You may also be able to link spot metering to the focus area.
When focusing you should also be checking composition. One thing we often forget is to check how the main subject relates to the background.
Using Focus Lock
To change the position of the plane of critical focus, you can use a procedure called focus lock. Most digital cameras have a two-stage shutter button. When you press it halfway down, it sets and locks focus and exposure. Some cameras beep and illuminate a lamp or frame in the viewfinder when these readings are locked in. If you don't release the shutter button you can then recompose the image and the settings remain unchanged. This procedure normally locks exposure too, but if you first use AE Lock to lock exposure (page 71), you can then lock focus independently.
1. When photographing an off-center subject, you position the focus area, in this case the center of the viewfinder, over the subject and press the shutter button halfway down to lock focus.
2. Without releasing the shutter button, compose the image the way you want it and press the shutter button the rest of the way down to capture it. If this hadn't been done here the camera would have focused on the wall.
Some cameras have a focus assist light that makes it easier to focus in dim light. These lights illuminate the scene, but work only at a short range.