Aperture

You will know that the pupil of the human eye shrinks in bright daylight and expands in dark rooms. The pupil is opening or closing its "aperture" to let in more or less light.

The aperture diaphragm in a lens serves exactly the same purpose. To let more light in it needs to be wide open (a lower f-stop like f/2) or shut-down (a higher f-stop like f/22).

Inside the diaphragm is a series of spiralling blades that open in or out.

There is some technical detail behind f-stops and why they seem to be such odd numbers. Each f-stop is a factor of the square root of 2 - about 1.4. So each f-stop number is 1.4 times the previous f-stop. Each f-stop is double the number of the f-stop TWO before it. Each f-stop lets in half the amount of light as the previous number.

Early lenses had very fixed f-stops, but modern units have much finer control usually down to thirds of an f-stop, which is a very small change of exposure.

Depth of Field

Apart from controlling exposure, aperture also controls the depth of field. This is a crucial artistic control in your photography.

As we've discussed in focussing not everything in your picture can be in sharp focus. If you focus on a close object, then distant objects must be out of focus and blurred.

The girls face is lovely and sharp, but the brick wall 3 metres behind is completely soft and out of focus.

When focussed at a particular distance, the focus becomes increasingly blurred the further you move from that focussed distance. Depth of field describes the zone that is most in focus - or more accurately is close enough to being in focus to still look sharp.

So depth of field is range of acceptable sharpness. There is a gradual loss of sharpness the further you move from the core depth of field.

The wider the aperture (smaller the f-stop) the shorter the depth of field.

The narrow the aperture (higher f-stop) the greater the depth of field.

Interactive Depth of Field

Move the slider left and right to see the effect of different apertures on depth of field. In each image the aperture changes. The distance from camera to subject remains the same.

The focus is constant on the face, but look at the writing on the wall behind. Notice how as the aperture becomes smaller (larger f-stop number) it becomes crisper.

Now repeat this and look at the bollard - because it is closer to the focus on the girl it comes into focus much sooner than the writing on the wall.

To shoot a portrait with a blurred background use a wider aperture.

To shoot a landscape with everything in focus use a narrow aperture.

But depth of field is also affected by other variables. The distance from the camera to the subject has a major effect. The focal length of the lens also has an effect.

The very busy background is reduced to a blurred sense of environment - giving a context for the image without distracting from the portrait.

85mm lens, f/1.8, 1 metre from subject, 3 metres from brick wall. Brick wall is soft and blurred.

Focal Distance

Greater distance between the camera and the subject will give a greater depth of field. Photographing a distant tree in front of mountains will have everything in focus. Photographing a person close to you in front of mountains is going to leave the background out of focus.

Same setup, but 4 metres from subject, 3 metres from brick wall. Brick wall is now apparently sharp.

Interactive Example

Use the slider to see the effect of variable distance.

In this case the focal length is maintained at a constant and the distance from subject to camera is changed.

Ignore the change of the width of the view, concentrate on the background elements and when they come into sharp focus. You can see that the further away the camera gets, the more sharpness there is from subject to background.

Focal Length

If you use a wide angle lens (say 24mm), then there is greater depth of field and so more will be in focus. If you use a long lens (say 100mm) then there is less depth of field - little will be in focus.

Of course all these variables work together, so if you have to use a long lens (100mm) you can compensate by using a small aperture (f/22).

Depth of field is a crucial artistic element and a technical element.

Interactive Example

Use the slider to see the effect of varying the focal length but maintaining the same crop. In this example the focal length changes, so the camera position has to change to maintain the same framing in the image.

Firstly, look at the change in depth of field. At 24mm everything appears sharp and in focus.

As the focal length increases notice that the background objects become increasingly out focus.

Secondly, you will of course see that the width of the view is also changing behind the subject. Although Leah remains the same size in the frame the background becomes more and more restricted as the focal length gets longer.

Look at how all the details on the man's face and turban are lovely and sharp, but the background is completely out of focus with no detail visible at all.

Depth of Field for a Portrait

Portrait. For a close up portrait you do not want a distracting background. You can use a shallow depth of field to throw the background out of focus. This will reduce distractions from the background. So you might shoot at 1 metre distance with an f2 aperture to give you a short depth of field.

Depth of Field for a Group Shot

This is an F-22A Raptor

Group shot: For a larger group photograph you want all the faces to be in focus, so you want a greater depth of field. You might shoot at 10 metres distance with an aperture of f/22.

Depth of Field Preview

Higher end DSLRs will have a "Depth of Field Preview" button. Under normal circumstances when you look through the viewfinder the aperture is kept wide open so the viewfinder is as clear as possible. However this means you are not seeing the effect of depth of field.

Pressing the depth of field preview sets the aperture to how it will be for the final photograph. This may result in a darker image in the viewfinder but will give you a chance to see how depth of field affects your image.

This use has now largely been superseded by using the live preview option on the rear LCD screen.

Hyperfocal Distance

You will have noticed in the diagram above that depth of field only comes a little way in front of the focussed distance. But the depth of field extends a long way behind the focussed distance.

Technical definition: The depth of field will stretch from half the hyperfocal distance to infinity.

In other words, for a given aperture and focal length there is a focussing distance which will put most of the picture into focus.

This measurement is normally used in landscape photography, where you want to get both foreground objects and the far distance in focus.

Selective Focus

Selective focus is the art of using depth of field to concentrate on your subject. Your subject will be sharp and in focus, but objects in the foreground and background will be soft and blurred.

This technique concentrates the viewers' eye on the important part of the picture. It uses the background and foreground to provide context and framing.

Selective focus really highlights the bottle top.

Video

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Exercise

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Assignment: A Depth of Field Library

The best way of really understanding depth of field is to take photographs to explore all the variables.

The easiest way of doing this is to cajole a friend into standing for you as photo subject. Put them in front of a brick wall or hedge or trees. Put them about 3 metres (10 feet) away from the background.

Get record shots of what happens when

1) You move different distances from your subject

2) You keep a close distance but change the aperture from small to large

3) You zoom in and out keeping the same distance

4) You zoom in and out keeping the same crop, but changing your distance

Go over the photographs and compare the backgrounds in each shot.