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Aperture

The aperture adjustment on a camera controls the diaphragm, this reduces the hole in the lens from its maximum towards a pinhole. This is done principally as a part of the control of Exposure, controlling the amount of light to reach the sensor or film and to create specific effects, the most common being to increase the range of distances in focus, known as the Depth of Field (DOF).

We have technical definition of aperture at Aperture defined.

The other variables that form the usual controls of Exposure are Shutter Speed  and sensitivity represented as ISO and ASA.  

Aperture is repented by a scale, where the lower numbers i.e. f2 are the larger hole and larger numbers, i.e. f32 the smaller hole. In controlling exposure we work in stops, so that an adjustment in one variable can be offset by a corresponding adjustment in either of the others.

Both ISO and shutter speed are easy to convert to stops and back as they half or double the number, so for example 1/400, 1/200, 1/100 are shutter speeds all 1 stop apart from each other, as are ISO 200, 400 and 800.

Aperture is a little different in that we have to look at a hole allowing light in, like a pipe allowing water to flow, so the number doesn't make as much sense. The result is that doubling or halving the number is two stops instead of 1. 

So f stops go f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32

The sequence while looking complex is simply the doubling of the one before last, so all we need to remember is 1 and 1.4 and we can create the sequence.

Diagram of Aperture holes showing the f-number in relation to size of hole

While doubling or halving a shutter speed or ISO makes a 1 stop difference, doubling or halving the f number makes a 2 stop difference.

To save doing all this in your head you could use an EV table, 1 EV = 1 stop.

What the f means

The f value of a lens is the size of the diaphragm aperture divided into its focal length. So a 50mm lens which has a diaphragm aperture of 50mm is 50/50 = f1 while a 400mm lens with a 50mm diaphragm aperture is 400/50 = f8.

So with longer lenses, as the focal length increases, if they want to stay fast (see below), they have to become extremely large, heavy and expensive. From this you can see that if we were to make a 500mm lens that was f2 it would need to be at least 250mm (10 inches) across, while a 1000mm would have to be 500 (20 inches) across. The largest lens I am aware of that has been made was a 1,800mm f2.8 and this required a special land rover to carry it.

Lenses

Lenses come in different sizes, some often referred to as fast lenses, have a large diameter in relation to their length and therefore a larger aperture going towards f1. The fastest lenses are usually prime lenses, lenses that have a single focal length and do not zoom. Wide angle lenses have a larger aperture than a telephoto lens.

Generally the longer a lens is, the slower, so for example my 1000mm lens is f11.

Telephoto lenses usually, but not always, have a larger aperture at its wide angle end of the zoom and a smaller one at the telephoto end of the zoom.

In the design of a lens the number and shape of the diaphragm blades, as well as other factors, affect the image quality, problems that are overcome at least to a degree is vignetting, the image getting darker towards the outside and aberrations of different types that are caused by distortion and different coloured light focusing at slightly different points.

Depth of Field

The main reason to select a specific aperture is to control the depth of field. With a pinhole camera or camera obscurer you don't have to focus as everything is in focus. When you widen the hole everything becomes more blurred, as the light can now spill over into other areas of the image. We overcome this by adding a lens to focus the light onto a set point in a similar way to a magnifying glass can focus the power of the sun onto a single point. In order for this to work we have to move the lens to focus it, and the larger the hole the less and less that is in focus at one position of the lens.

When the hole is smaller towards f32 more is in focus at a time, while when the aperture is larger towards f2, then the amount that is in focus is reduced.

Depth of field is controlled by a number of factors:-

• Aperture.

• Focal length, wide angle has a greater depth of field, telephoto less.

• Distance from the subject, nearer items less, greater distances more.

• Sensor size, smaller sensors more, larger ones less.

Other articles on Depth of field include:-

Depth of Field (DOF) Explained

Application of Depth of Field 

Sharpness Masterclass

Introducing Slicing

Depth of Field Magic (Project)

What we can do with Helicon Focus

Capturing Slices with a Nikon DSLR

Slices - Coins

Slices - Chips

Slices - Red flower

Aperture Priority

Aperture Priority refers to a shooting mode used in modern cameras. It allows photographers to choose an aperture setting and the camera decides the shutter speed based on what ISO is set to give the correct exposure. It can also be referred to as Auto Exposure, A Mode, or AV Mode.

You can adjust the brightness of the image by the metering method but also by adjusting the exposure variation.

 <<  ^  Exposure Article Route   ^  >>    Shutter speed

for details on Exposure Article Route see the Exposure  page