Today most camera's have the capacity to focus, that is previously there were cameras that had an infinite preset focus but today any camera selling above 50- 60$ have an auto focus system. Auto focus is a system where the focusing lens arrangement is repositioned to attain a focus image, by the camera itself to get a sharp image.
Auto Focus Sytem
The above image shows the standard auto focusing system in a very simplified diagram. This is the system that most DSLRs implement. Such a system is expensive dues to the cost of sensor and the dedicated modules etc. To make auto focus system for cheaper cameras the sensor and the dedicated modules are removed from the system and the CPU take their responsibility.
Let us first look at a property of a image when in focus consider the three images below,photo-034-300x109 What is and How to set contrast based auto focus? photo-036-300x109 What is and How to set contrast based auto focus? photo-039-300x109 What is and How to set contrast based auto focus? the first one is completely out of focus , second one more in focus and the last one completely in focus. Now the first thing that we can see from the three images is the difference in amounts of color in each, that is the first image has the lest overall colour difference or entropy. The second one has higher entropy than the first and the third has the highest entropy of all the three images. This is not an isolated case with this image. Blur causes reduction of entropy, and out of focus image is blurred and thus has lower entropy.
The contrast based auto focus system uses this very principle to achieve focus. When the area of interest is in focus the total entropy of the region of interest will be highest. That is as the image becomes more in focus the entropy increases, and as the area goes out of focus entropy decreases. The cameras utilising the contrast based auto focusing method usually have an electronic shutter, so the image is constantly being received into the CPU. The CPU then scans the area of interest for contrast levels and when trying to focus it does a try.fail,retry method to get the area in focus. That is the CPU measures the current contrast then makes a small adjustment in one direction then measures again if the contrast decreased it means that the adjustment was in the wrong direction then it corrects and scans until the contrasts starts decreasing or the maximum point is found.
Depending on the capacities of the camera in question the auto focus system can have more advanced features including focus area selection different size area of interests and so on. The focus area selection is done by using an square area of interest, that is usually 10% of screen area , this area of interest can be moved around the viewing area and the focusing is done according to that area.
Let us see some simple ways of using auto focus. We are considering features available on a higher end point and shoot camera. The auto focusing features include
* Center area based focusing
* Centered small area based focusing
* Centered large area based focusing
* Movable small area based focusing
* Assisted focusing
* Macro Focusing
Small centered area
Center area based focusing
This is the default option on most cameras. In this option the camera looks at a central area that usually makes up less than 25% of the total image area and depending on this area the focus is obtained. This is the option that is used when the camera is in auto mode. The exact percentage of screen area analysed is dependent on the camera manufacturer and model. This mode usually give good results and is recommended for the beginner. To use this the easiest way is to set the camera to auto, which is usually marked by the green colored symbol on the mode Dial.
Centered small area based focusing
The illustration gives a basic idea of how such a setting would look the grey area actually will be the area in interest when trying to focus. This area will usually be marked by a white box in the cameras view finder image or the LCD image. When using this mode what ever you bring into this box will be focused on. That is if you are shooting the picture of a cat, the cat should be in the white box that is it should be in the grey area shown on the image. Then only it will be focused on that is then only you will get a sharp image of the cat. Let us analyze this a bit if the cat is towards the left and there is actually a bottle in front of the cat and in the image the bottle is in the grey area, then the bottle will be sharp and the cat will be out of focus or blurred. This mode is usually used for shooting small object like a dog or a cat or a bottle that is when we need to focus on an object that fits in the grey area shown in the illustration.
Centered large area based focusing
The illustration again gives a basic idea of the area. This area makes up a large part of the image. The exact size of the area is manufacturer and model dependent. This setting is usually used for taking picture of a larger object. That is some thing that is nearer. The important thing to keep in mind is that as the area of interest expands the processor will take more time to process, but it will be possible to attain focus in lower light conditions. This large area can also be used to get focus on an object when the light is dim. This can also be used when you are taking a group photo and so on. One thing to note is that with cameras having features like face detection it is always better to use the automatic mode when using the camera in such ideal condition that is when taking portraits or group photos or landscape and so on.
Movable small area based focusing
The illustration image shows this as a small Gery box. In the camera this box can be moved around to the desired location. The main requirement for this option is that there should be ample light for this to work as a small area is under consideration. This is very useful when trying to capture only a particular part of the image in focus. That is when you want to focus towards the sides of the image etc. This is most useful in the macro mode because you will need to focus ons something in particular in a enlarged scene. This mode can create wonders with a little imagination. Do understand that what ever falls in this area gains focus so this mode can also be useful in other conditions like if you set it towards the center when photographing a scene where ther is a person in the center. The person will be in focus because the area has the person in it. This can be faster to obtain focus than the other modes when conditions are ideal. Note that this is very sensitive to light and if there is little or too much light this mode might not get focus at all.
When there is too little light to acquire focus in any of the above modes the camera either activates a dedicated assist lamp or strobes the flash to acquire focus. This setting can be changed by looking at the focus assist options in your camera. This can be battery consuming thus you can disable this mode to save battery but it is a very helpful tool when in low light.
This mode enable the camera to focus on a object that is very close to the camera. One can be beautiful pictures of flowers, insects etc using this mode. The above modes all work with this mode. The spot area option will be most beneficial in this mode. Some camera have a dedicated macro button that can be enable when in other modes too. The image of the flower with leaves can be used to identify this button.
The first three options can be set on you camera either by looking at the auto focus settings on your camera or by manipulating the dedicated auto focus button on your camera if it has one. This guides how to section is more towards making you understand why to choose the setting than how to. To get information for your camera specifically refer the manual. The aim of the how to section is to make u understand why you would choose the specific setting.
In all the above setting when focus is obtained the white box becomes green. To get best results make sure that you have obtained focus by looking for the white box before depressing the button completely. (referring to models that focus on depressing the shutter button half way, most models use this method only) Some cameras also give a audible signal when focus is obtained. To enable this(if your camera has this option) you can go to the sounds option in the menu or it can be found in you auto focus options .
Note:- This article is written with only Point and Shoot cameras in mind.