Focusing is the moving of the lens in and out until the sharpest possible image of the subject is projected onto the digital image sensor. In order to form a clear image, the lens has to be a certain distance from the image sensor which depends on the distance of the subject from the camera.
The focus options are important considerations when purchasing a new camera. Depending on what features the camera has, there will be several settings that will help you to more easily and more reliably focus on a specific subject or scene.masters-of-photography
There are three ways cameras focus:
Fixed focus in effect means that the camera is with no focusing system of any kind. In such models the lens has a fixed setting to focus subjects from the far horizon through to about 2.5 meters away, and this leads to inability to get sharp images if your subject stands any closer.
When you look through a fixed-focus camera, you typically do not see in the viewfinder the square brackets or circles found in an autofocus camera. However, you may see a "flash ready" indicator.
In fact, on many low-end point and shoot cameras it is the only kind of focus setting present.
There are two types of autofocus systems: active and passive.
Active autofocus system measures the distance to the subject independently of the optical system, and then adjusts the optical system for the correct focus.
There are different ways to measure the subject's distance, including ultrasonic sound waves and infrared light. In today’s cameras the active autofocus system uses an infrared signal instead of sound waves, and is great for subjects within 6 meters or so of the camera.
below The name "active" comes because the camera emits something i.e. infrared signal in order to detect the distance of the subject from the camera.
Passive autofocus system, commonly found on single-lens reflex (SLR) cameras with interchangeable lenses, measures the distance to the subject by computer analysis of the image itself. The camera actually looks at the scene and drives the lens back and forth searching for the best focus.
In certain situations the autofocus can give you trouble. Some cameras will beep or light will go off to let you know that they cannot focus. If this happens, you can switch to manual focus.
You set the camera in focus by turning a ring on the lens.
There are many situations where you will want to override your camera’s auto-focus capabilities:
In scenes with little contrast when the subject in the focus zone is brighter than the rest of the scene, or is poorly illuminated.
The subject is moving quickly.
When a close object and a far away object appear side by side in an image.
In low light level conditions.
Particularly useful in some kinds of photography i.e. macro photography
Most cameras have the ability to switch quickly between autofocus and manual setting by using a convenient switch — either on the lens or on the camera body.
Some cameras have more than one focus zone or area, usually distributed across the viewfinder or the monitor and indicated with rectangles or brackets.
For the camera to focus accurately, the subject must be in this area. The default focus zone is in the centre of the viewfinder or LCD monitor but with the aid of a dial, or a thumb toggle, the photographer can choose which area will be used for primary focus. This enables the focusing of subjects that are off centre in the image frame.
Entry-level cameras offer a single focus area you can move over any point in the scene. This centre focus frame works just fine when you want to focus on a subject that is the centre of the composition. However, if you want to focus on a subject to the left or right, or anywhere else, another approach is required.
Most digital cameras have a two-stage shutter button. When you press it halfway down, it sets and locks focus and exposure. Some cameras indicate that these readings are locked by illuminating the frame or by beeping. What the Focus Lock feature allows you to do is to point the camera at a subject to focus the lens and then hold or lock that focus when you recompose the scene with the subject appearing off centre while 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.
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.
When taking macro (or close-up) images while using autofocus setting, a digital camera may have a hard time getting to focus. It zooms in and out until it can focus on the subject a process called "hunting". Due to the nature of macro photography, this "hunting" can take time because the lens focuses from one extreme to the other. Setting the camera to a macro focus mode reduces the "hunting" time.
In one-shot mode, the button is held halfway down your camera hunts to get focused once—then it locks the focus even if the subject is moving. If the photographer wishes to change the point of focus then they will need to remove their finger and repress the button.
The one-shot shooting mode is thus the best choice for subjects that do not move. Using it with moving subjects is a good way to guarantee getting an out-of-focus picture.
The continuous focusing mode also focuses on the subject when the shutter button is half pressed, but unlike the one-shot mode, when the subject moves the camera will adjust the focusing in order to keep the subject sharp and will continue to focus until the shutter button is fully pressed. 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.
On some high-end digital SLR cameras , the idea of continuous focusing is taken a step further where the autofocus system has "pre-emptive focusing" features that not only track the subject but analyze its movement across the frame and try to predict where it will move to. This in turn helps to keep the main subject fully sharp. This predictive focus feature can be very useful when photographing sports events and in other situations where subjects are moving rapidly.
Although rare, some cameras will bracket focus to help you get sharper images. They take one picture at the calculated focus setting, and then two others with focus set behind and in front of the calculated distance.
Digital Photography Tips › Digital Camera Basics › Focus
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