Aperture and Depth of field

Aperture explained firstly,

The aperture of a camera is effectively an opening that allows light through to the recording media (be it film or a digital sensor). It follows, that for a given shutter speed a larger opening will allow more light through and a smaller opening will allow less light through.

Aperture will also be found referred to as f/number and f/stop, but they are all the same thing. The actual value is the ratio of focal length to aperture size.

Looking at the numbers in the above table it can be seen that the aperture a camera is operating at can be quite confusing. This is because the larger the aperture actually is, the smaller the f/number it will have. A large aperture which will let a lot of light through and require a fast shutter speed will have a small f/number. Conversely, a small aperture which will only let a small amount of light through will require a slow shutter speed and have a large f/number.

This links into depth of field, like so

Depth of field (D.O.F.) is another concept that is not too easy to grasp. Basically, it is the amount of the picture that remains sharp in front and behind the position your camera has focussed on. Depth of field can be controlled by two factors, the aperture value used and your distance to the subject.

Using a smaller aperture (larger f/number) increases the depth of field. From the above table, the f/number of 12 will give you a greater depth of field than the f/number of 2. This increase in depth of field is greater, the shorter the focal length of the lens (shorter focal length means less 'zoomed in')

f 16	f 8	f 2.8
Smaller aperture	Medium aperture	Larger aperture
Larger D.O.F.	Medium D.O.F.	Smaller D.O.F.
1/60 sec	1/400 sec	1/1000 sec

The above photos were taken with a 60 mm lens at f16, f8 and f2.8. As you can see the larger the aperture the smaller the depth of field. I have also included shutter speed details to illustrate the effect of aperture choice on the shutter speed.

As mentioned above, depth of field is also affected by proximity of the subject. In this case, the further the subject is from the camera, the larger the depth of field. In the extreme case of macro photography where objects are very close to the lens, this results in extremely small depths of fields - millimetres or less.

It may initially be thought that the larger the depth of field, the better. While this may be true for some instances (like landscape photography), varying the depth of field can have great creative benefits to your pictures.

A large depth of field can be created using a small lens aperture, a wide angle lens (with a short focal length), focussing on a distant subject or a combination of these things. The most obvious use of a large depth of field is landscape photography, when the photographer would generally like all elements of the picture to be kept in focus.

A small depth of field can be created by reversing the above factors. I.e. use a large lens aperture, a telephoto lens (with a long focal length), focussing on a close subject or a combination of these things.

The main use of a small depth of field is to separate the subject from any background distractions, clutter etc. This can be used for portrait photography and taking photographs of flowers where you want to separate a particular flower from it's surroundings. This image was taken with a 60mm macro lens at f7.1 to acheive a shallow depth of field but deep enough to ensure that all of the hoverfly is sharp.

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