Depth of Field, or DoF for short is the area in front of and behind of the focus point that is in focus. For any lens, there can only be one true focus point. The DoF in essence extends the infocus area 1/3rd in front of, and 2/3rds behind of the focus point. In the example below, the single tree is what the camera is focusing on, and the range of what is in focus near the tree  both in front of and behind  is called the Depth of Field. Depth of Field then is the area of the photo in which everything is in focus. A shallow Depth of Field means that the distance of items in focus is short, perhaps only a few inches or a few feet, while a deep Depth of Field means a long range of distances are in focus  even up to infinity in some cases. For the photographer adjusting the camera settings, the actual depth of the DoF is dependant on the aperture, focal length of the lens, and the distance the subject is to the camera. There are other factors involved, including the sensor size, Hyperfocal Distance, and "Circle of Confusion"; mostly things you don't really need to know on a daytoday basis. In simple terms, the more open or larger the aperture is , the shallower the DoF will be, and the more closed or smaller the aperture, the wider the Dof will be. Large aperture openings will give limited DoF, which is desireable for portraiture, as it hides the background. Small aperture openings are essential for landscapes so everything is in focus. So how do you know what the DoF is for a given situation. Experience mostly. An indepth discussion of DoF is beyond the scope of this page. If you have an iPhone, iPad, or Touch, there is a nice free DoF calculator available on iTunes  called "Simple DoF". I have it on my iPod Touch, and it is a standard part of my equipment bag. The following charts, created by Simple DoF, will provide some information for various DoF you should be able to expect.
How to read the chart: To determine DoF for any particular setting, first find the chart corresponding to the focal length you are using. Then locate the aperture you are using in the leftmost column, and then find the distance the subject is from the camera in the second row. Intersecting those two points will give you the TOTAL DoF. You have to multiply the DoF by 0.33 to find the point 1/3rd in front of, and multiply the DoF by 0.66 to find the point 2/3rds behind the focus point. For instance, assume your lens is 200mm (or zoomed to 200mm), and your subject is 80ft from you. And you are using f/5.6; from the chart, your total DoF would be 11ft. The DoF (in focus) range would be 3.6ft in front of, and 7.26ft behind the subject, which is located 80ft away. Or, the total infocus range would be from 76.4ft to 87.26ft. Anything within that boundary will be in focus. From examining the two charts, you can glean the following information:
Suppose a camera is setup with a 100mm lens, and an aperture of f/8. If the subject is 30ft from the camera, the DoF will be 10ft. But if you move back to 60ft, without changing anything, the depth of field is now 40ft. How is this so? Remember that optics behave according to the Inverse Square Law, so that doubling the focal length results in a fourfold change in DoF. As you can see from the above discussion, increasing focal length will decrease the depth of the DoF (or decreasing focal length will increase DoF). So just how important is focal length? To answer that, you have to also consider the sensor size. In a compact camera (point & shoot) the sensor size is typically 5.6x smaller than the typical DSLR. That small size affects the focal length of the camera's lens. For instance, a 200mm lens, divided by 5.6 results in a 35mm lens. What that means is that a 200mm lens used on a DSLR would be equivalent to a 35mm lens used on a compact camera. This focal length "compression" greatly extends the Depth of Field for the compact camera. By consulting the DoF calculator, if we compare that 200mm vs 35mm focal length, we find that:
So while the composition of the photo will be the same, the DoF in the compact camera will be 10 times the DoF of the DSLR. This results in both advantage and disadvantage. The advantage is that the focus system in the compact camera does not have to be as precise, but the disadvantage is that it limits the ability of a compact camera to have any kind of background blur. Especially for portraiture, it is just about possible to take a photo of a person in a parking lot and by using a short DoF, to "blow out the background" as it is often called. That way you will not see cars, other people, telephone poles or anything else in the background to distract from the main subject. Understanding this technique will allow you to take much better and pleasing photos. However, it needs to be mentioned that this technique is all but impossible to achieve with a compact camera, as the smaller sensor size and lens results in a 10 fold increase in the DoF for those cameras. The DoF can be determined for any lens, subject distance, and aperture by using the "Simple DOF" Calculator mentioned above.
If you have a sharp eye, you might have noticed that in the above photo, the flag photo on the right, that even though the background is more in focus, it is still not in focus. A small f/22 aperture was used, so it should be in focus, so why isn't it? The answer; the distance from the lens to the subject is 8ft, so the depthoffield is not to infinity. Had the photo been taken from a distance of 16ft, then the depthoffield would have been 4 times longer, so the background would have been in focus. Give yourself an A if you figured it out. This is the kind of reasoning that will make you a better photographer  the ability to discern why something is not right.
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