Depth of Field (DOF) Explained
Depth of field relates to the area within the depth of the image that the photograph appears to be sharp and in focus.
The lens is focused on a point and only that point is in focus, in front and behind that point will be out of focus, but as the decrease in focus is gradual it can appear that a larger distance range is sharp. This apparent range is known as the depth of field and often abbreviates to DOF.
The rate at which the focus deteriorates and therefore the depth of field, is affected by a number of factors the main ones being the sensor size, focal length, aperture and distance from the camera.
Some of the variables are linked, for example the reason that the sensor size has an affect is that you can fill the sensor with a wider angle lens, for example a Nikon DX camera with a 40mm lens is the same image as a 35mm with a 60mm lens.
The nearest point that you can focus at, where infinity, the far off mountains and clouds appears to be in focus is known as the hyperfocal distance, the depth of field in this situation is from half way between the camera and hyperfocal distance right through to infinity.
As the loss of focus is gradual it is necessary to have a way of defining when the level of focus is acceptable. This is done using another measurement known as the circle of confusion, this being the smallest sharp image area that the human eye can see in a 10" by 8" print from the image, this is about 1/1250 of the image width. The earliest mention of this term, I have been able to find, was an article published in 1866. For 35mm cameras this is 0.03mm while for the smaller DX sensor this is smaller at 0.02mm as the image has to be blown up more to get to the same size. In effect we are saying that the DX sensor image needs to be sharper but that the smaller sensors allows us to get the same image from a wider angle lens. The net effect is that we have a greater depth of field with the DX lens. Similarly a TV camera with a far smaller sensor has a much greater depth of field and can follow wildlife with extreme zooms far easier than a still photography camera.
Using a depth of field calculator or tables we can work out the depth of field for all combinations of focal length, aperture and focusing distance based upon a camera/sensor. With tables this is usually a page per focal length, with aperture across the top and distance down the side. Tables are produced in small booklets.
A depth of field calculator is a circular device with one piece rotating on another. To use it you rotate one part on the other to make it relevant to the focal length you are using. On one side you look at the distance that you want to include from and to, and count the number or zones, shown in alternate colours to cover this range, you then turn it over and look to see what aperture is required to get coverage over the number of zones required. Going outside the range of focal lengths or other parameters is possible by undertaking some multiplication of the figures shown. The one I have is for a 35mm lens and it has a card with it that allows you to convert the effective focal length to use on the calculator for a DX sensor.
You can see the depth of field in many cameras by pressing a depth of field button, this shuts down the aperture from fully open to the aperture setting you have set. The lens is usually fully open so that you get a bright view and the auto focus system can operate, shutting down the set aperture when the photo is taken. When the depth of field button is pressed the image looks darker and you can see more in focus. You may need to allow your eyes to become accustomed to this lower light level.
Many people think they saw more in focus in the camera viewer then was in focus in the resulting photograph, this is because our brains have a capacity to fill in missing detail with what we expect to see. A little like joining up dots. So as your brain knows birds are covered in feathers you see feathers, often in more detail than are in the resulting image. Carefully looking in the viewfinder at the individual detail will help you to overcome this.
Most cameras also have an ability to show an image taken on the back panel and to zoom in on the detail shown. This can be a useful aid to determining how much was in focus. The use of accessories like the Delkin professional pop-up shade and Hoodman Loupe can make it far easier to see what is happening especially when you are outdoors.
At shorter distances you will find you need to focus a third into the shot, and the image appears sharp twice as far behind the point you focused as in front of it. At greater distances, or where there is much greater depth of field the amount in front and behind the point you focus, that appears to be in focus is about the same. Some people work on a point half way to the Hyperfocal distance (explained above), as being the point where you switch from the third to two thirds to the 50:50.
A good experiment to see this in action is when visiting a place with a long metal fence or similar, look along its length and to focus at various points at different apertures.
To see how this can be manipulated and applied see Application of Depth of Field.
Extra depth of field can be obtained by taking a number of photographs with different DOF and combining them through special software, see Introducing Slicing, and the many other articles that we have, including sets of slices for you to experiment with on this topic.