Depth of Field (DOF)

Hi there, today I start tackling one of the more technical subjects in photography – the topic is Depth of Field, also referred to as DOF.

Firstly to explain aperture:

Aperture on your camera defines Depth of Field. The Aperture is the opening of the lens – how large or small the lens opening is. The smaller the number indicated on the camera – the larger the aperture. The higher the number in your lens, the smaller is the aperture. For example, some lenses could provide a lens opening at its largest of f/1.8, other lenses may only be capable of opening the aperture to f/4. This is quite critical in a lens, because the larger the opening, the more light one has to work with. So indoors for example, and in the evening, at their largest aperture settings, an f/1.8 lens would retrieve more light from the surroundings than an f/4 lens would.

So large aperture = more light and smaller aperture = less light.

Do not get aperture confused with shutter speed. Shutter speed is measured in fractions of a second, 1/10 sec, 1/50 sec, 1/100 sec and so on. This represents the length in time that the film or the sensor is exposed to light – called exposure time. Photography is about balancing Aperture (lens opening) and Shutter speed (exposure time) to get the correct picture. Sometimes better effects are achieved by deliberately under exposing, (shorter exposure time = less light & less motion blur) or even by deliberately over exposing (longer exposure time = more light and more motion blur)

Now on to DOF:

DOF is very important as this blurs or sharpens details in your picture. Say you have to take a portrait photo of someone in the garden. Behind the person is a hedge with a lot of leaves and flowers. How does one take a picture that does not have the leaves and flowers detracting from the subject and leaving the picture too busy…?… well DOF is the answer. Using a larger aperture will cause the person (the subject) to be in focus and the background to be blurred.

Most appropriately small apertures would be used when doing landscapes, as here you would like to capture as much detail as possible. Portraits may demand larger apertures so that the background of the image can be blurred to make the subject stand out nicely in focus.

Aperture settings directly affect DOF. The rule of thumb is the smaller the aperture the more detailed your picture will be. The following diagram tries to explain it…

The top section shows a lens on the left opening at a large aperture of f/2.8 the green box to the right indicates how much of the image is in focus. You will see that just a small area is in focus in front of the subject and behind it.

The bottom section of the diagram shows a lens opening at a very small aperture of f/36. Notice how the area in focus deepens so that a lot more detail behind the subject is in focus.

The next picture illustrates the top section of the diagram. It was taken at f5.6 (a fairly large aperture). Notice how short the depth of field is – the numbers on the ruler blur quite soon.

Compare that to the last picture, taken at f36 (a very small aperture) and notice how the numbers on the ruler remain in focus for quite a distance…


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