Controlling your Depth of Field (DOF)
When you are shooting an image as well as composition and lighting you should also think about how much of the image to be selectively in focus. It may just be a small part of the image such as a portrait where the eyes are going to be in focus and to a lesser extent the rest of the face but the background would be best out of focus as much as possible. Or say a landscape where far more of image will be in focus as in my examples here.
I an order to be selective with focus clearly the lens focal point is key but the amount of focused area in front and behind this focus point will depend on the depth of field. So how can we control this ...?
If we assume we going to take a landscape shot where we want as much in focus in the foreground of the image as we do in the a good deal middle ground right and right of into the background then we are looking for as much DOF as possible. Two things will help us achieve this. Firstly the lens we us a wide angle lens this has a far greater DOF than a telephoto so we should choose a wide angle lens to get a good DOF. Secondly the aperture of the lens will allow us to increase the DOF of a lens for instance the lens I have used for the illustration here is a 50 mm lens which goes from F3.5 - F32 at F32 we will get the maximum DOF from the lens, however as I am sure you are aware this produces a very small aperture for light to pass through the lens meaning that the the shutter will be required to open for a much longer time than say F3.5 which will mean the camera cannot be hand held and will need to be placed on a tripod.
Here is the first shot to illustrate this (click on the image to enlarge so you can see this more clearly). This shot is at the the widest aperture of the lens F3.5 and as this allows a lot of light into the lens the shutter speed is 1/320 so this shot could easily be hand held. As you can see here the focus is roughly in the center of the image and there is little in focus to either side of the focal point bringing the viewers eyes to a particular clump of daffodils.
Now if we keep the focus in the same place but reduce the lens aperture to F11 you will see much more of the image is focus.The result of increasing the aperture means the shutter speed is reduced so that at F11 it is now 1/50 of a second and at such speed a tripod is needed to reduce camera shake. As you can see now far more foreground and the house in the backgrounds detail has increased changing the image so the eye is drawn into far more of the view.
If we now increase the aperture further to F32 the shutter speed is going to mean the tripod has to be very firm and on a very still day a long shutter delay would be fine to use. However in this example the shutter speed was 1/20 rather than say 1 or 2 seconds because the daffodils where being blown in the wind and would appear very blurred by this movement. To achieve this the ISO (light sensitivity) of the digital back was increased.
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