We will also discuss the AF assist beam method of active autofocus towards the end. A camera's autofocus sensor s are the real engine behind achieving accurate focus, and are laid out in various arrays across your image's field of view.
Each sensor measures relative focus by assessing changes in contrast at its respective point in the image — where maximal contrast is assumed to correspond to maximal sharpness. Please visit the tutorial on image histograms for a background on image contrast. Note: many compact digital cameras use the image sensor itself as a contrast sensor using a method called contrast detection AF , and do not necessarily have multiple discrete autofocus sensors which are more common using the phase detection method of AF.
Furthermore, the above diagram illustrates the contrast detection method of AF; phase detection is another method, but this still relies on contrast for accurate autofocus.
The process of autofocusing generally works as follows: 1 An autofocus processor AFP makes a small change in the focusing distance. This entire process is usually completed within a fraction of a second.
For difficult subjects, the camera may fail to achieve satisfactory focus and will give up on repeating the above sequence, resulting in failed autofocus. This is the dreaded "focus hunting" scenario where the camera focuses back and forth repeatedly without achieving focus lock. This does not, however, mean that focus is not possible for the chosen subject.
Whether and why autofocus may fail is primarily determined by factors in the next section. The photographic subject can have an enormous impact on how well your camera autofocuses—and often even more so than any variation between camera models, lenses or focus settings. The three most important factors influencing autofocus are the light level, subject contrast and camera or subject motion. An example illustrating the quality of different focus points has been shown to the left; move your mouse over this image to see the advantages and disadvantages of each focus location.
Note that each of these factors are not independent; in other words, one may be able to achieve autofocus even for a dimly lit subject if that same subject also has extreme contrast, or vice versa.
This has an important implication for your choice of autofocus point: selecting a focus point which corresponds to a sharp edge or pronounced texture can achieve better autofocus , assuming all other factors remain equal.
In the example to the left we were fortunate that the location where autofocus performs best also corresponds to the subject location. The next example is more problematic because autofocus performs best on the background, not the subject.
Move your mouse over the image below to highlight areas of good and poor performance. This means there is a lower level of overall contrast. In contrast-detection auto-focus, your cameraphone will move the lens back and forth until it finds the point of maximum contrast. This will be when the image is in focus. For this reason, it will always need to go past the focal point before going back to a more focussed image.
This makes it a fairly cheap and easy system to implement. It generally gives fairly good results. Despite being slow, contrast-detection auto-focus generally does a fairly good job in focussing the image. Reasonably slow. Contrast-detection auto-focus is generally quite slow: it normally takes around one second for the image to focus. Phase-detection auto-focus is a newer and more advanced method of focussing an image. The technique is more commonly seen on DSLRs rather than smartphones or compact cameras.
Recently, the technology has appeared on high-end smartphones from Apple and Samsung. With phase-detection auto-focus, your camera analyses the light as it passes through the lens.
It compares light from two apertures on opposite sides of the lens. If the image is properly focussed, the two light rays will give an overlapping intensity profile.
According to Samsung, the Galaxy S5 can focus in an image in only 0. More importantly, when the image is out-of-focus, a comparison of the light intensity profiles will tell you how much the focus will need to be adjusted. It takes only around 0. This is approximately 3 times faster than contrast-detection.
In fast-moving scenes e. You can do continuous auto-focus. With contrast-detection and phase-detection auto-focus, we analyse the light that comes in to the camera.
Collectively, the two methods are known as passive auto-focus. Autofocus is a highly sophisticated and useful tool. Understanding which mode to use and how to choose and move an active AF Point will help you to be more selective and creative with your photography. If you try to take a picture of a blank wall or a large object of uniform color, the camera cannot compare adjacent pixels so it cannot focus.
There is no distance-to-subject limitation with passive autofocus like there is with the infrared beam of an active autofocus system. Passive autofocus also works fine through a window, since the system "sees" the subject through the window just like you do.
Passive autofocus systems usually react to vertical detail. When you hold the camera in the horizontal position, the passive autofocus system will have a hard time with a boat on the horizon but no problem with a flagpole or any other vertical detail. If you are holding the camera in the usual horizontal mode, focus on the vertical edge of the face.
If you are holding the camera in the vertical mode, focus on a horizontal detail. Newer, more expensive camera designs have combinations of vertical and horizontal sensors to solve this problem. But it's still the camera user's job to keep the camera's sensors from being confused on objects of uniform color. You can see how much area your camera's autofocus sensors cover by looking through the viewfinder at a small picture or a light switch on a blank wall.
Move the camera from left to right and see at which point the autofocus system becomes confused. Here's a quick test to tell which autofocus system is in use in your camera some cameras may have both systems :. It is really up to the person using the camera to determine if the subject is in focus. The camera merely assists you in making this decision.
The two main causes of blurred pictures taken via autofocus cameras are:. Your eye has a fast autofocus! Try this simple experiment: Hold your hand up near your face and focus on it, and then quickly look at something past your hand in the distance.
The distant item will be clear, and your hand will not be as clear. Look back at your hand. It will be clear, while out of the corner of your eye the same distant item will not be as clear. Your camera is not nearly this quick or this precise, so you often have to help it. The camera user can often fool the autofocus system. A pose of two people centered in the picture may be unclear if the focus area the area between the two square brackets is in the middle of the two people.
The camera's autofocus system actually focuses on the landscape in the background, which is what it "sees" between the two people. The solution is to move your subjects off-center and use the focus-lock feature of your camera. Typically, focus lock works by depressing the shutter button part-way and holding it while you compose the picture.
The steps are:. You may also use the above procedure in the vertical direction, say when taking a picture with mountains or the shore in the background. Manual focus rings are still available on most SLR cameras. When taking a picture of an animal behind bars in a zoo, the autofocus camera might focus on the cage bars instead of the animal. On most consumer-grade autofocus cameras, use manual focus when:. Autofocus in a video camera is a passive system that also uses the central portion of the image.
Though very convenient for fast shooting, autofocus has some problems:. You can sometimes "see" infrared via this simple experiment , using a camcorder with a TV monitor attached. Point the camera toward a TV remote control.
Push some buttons on the TV remote control and the camera should "see" invisible infrared light from the remote control. Camcorders typically use CCD imaging chips.
0コメント