A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment

Focus

 
 
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.
 
 
Click to explore focus lock.
 
 
The landscape mode icon.
 
 
Click to explore depth of field.
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 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.
 
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.

Focus Settings

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.

Focus Zones

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.

Servo Focus

Normally focus locks when you press the shutter button halfway down. If the subject moves toward or away from the camera, it goes out of critical focus. However, if your camera has servo focus, focus is adjusted as long as you hold the shutter button halfway down. This mode is designed to keep a moving subject in focus and is great for sports and nature photography, or any other situations where you are photographing moving subjects. If the subject moves after you have focused on it, it remains in focus as long as it's covered by one of the AF points.
 
HAVEN'T I SEEN YOU SOMEWHERE?

Nikon has introduced cameras with a focus mode called Face-priority AF. In this mode the camera automatically detects people's faces in the scene and focuses on them. Only time will tell if this intelligent focus works well enough to catch on.

Predictive Focus

If a subject is moving toward or away from the camera at a constant rate, the camera can predict where it will be when the shutter opens. This predictive focus feature is available on some high-end digital SLR cameras. It's a great addition when photographing sports events and in other situations where subjects are moving rapidly as when a child is running toward you.

Focus Assist

Cameras can have a hard time focusing in dim light. For this reason some cameras will strobe the flash or fire a separate focus assist light. These lights briefly illuminate the scene, but work only at a short range. In some settings these lights are a dead giveaway that you are taking photos because they project a beam of light unto the subject that everyone in the room can see. If you want to be less intrusive or distracting, many cameras let you turn off the assist beam.

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, you can then lock focus independently.
 
 
Pressing the shutter button halfway down locks focus and pressing it all the way down takes the picture.

Focus Bracketing

Although rare, some cameras will bracket focus to help you get sharper images. They take one picture at the calculated focus setting, then two others with focus set behind and in front of the calculated distance.

Landscape Mode

Many cameras have a scene mode designed for photographing landscapes. It sets the focus and aperture so the scene is sharp from foreground to background.

Depth-of-Field Mode

Canon SLRs have an A-DEP mode that checks the distance to the nearest and farthest parts of the scene covered by focus points and selects an aperture so both are sharp.


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