A note on exposure
Exposure refers to the amount of light passing through the lens and reaching the sensor. Too much light and the image will be overexposed, too little and it will be underexposed. It is controlled by two devices on the camera, the aperture and the shutter speed.
The aperture is the hole in the lens through which light passes, and the shutter speed determines for how long the light is allowed to pass through. It is important to understand how both of these work together. Although in ‘auto’ mode the camera will calculate this for you, there are many occasions when you will want to set the exposure manually to achieve a desired affect.
Exposure refers to the amount of light passing through the lens and reaching the sensor. Too much light and the image will be overexposed, too little and it will be underexposed. It is controlled by two devices on the camera, the aperture and the shutter speed.
The aperture is the hole in the lens through which light passes, and the shutter speed determines for how long the light is allowed to pass through. It is important to understand how both of these work together. Although in ‘auto’ mode the camera will calculate this for you, there are many occasions when you will want to set the exposure manually to achieve a desired affect.
Aperture
The aperture has an ‘iris diaphragm’ that controls its diameter thus letting in more or less light.
The aperture does 2 things:
Firstly, because it controls the amount of light that enters the camera it follows that it controls how light or dark the image is. Secondly, and more interestingly, it controls the amount of the image that is in focus. This is known as ‘depth of field’.
The size of the aperture is referred to in ‘f-stops’ or ‘f-numbers’. The smaller the f-number the larger the aperture and the larger the f-number the smaller the aperture. Thus at f2 the aperture is opened wider then at f9. The typical f-stops on a camera are: f1, f1.4, f2.8, f4, f5.6, f8, f11, f16, f22 etc. Each f-number on the scale lets in twice as much or twice as less light then the next or previous number on the scale. So f1.4 lets in twice as much light as f2.8 and f16 lets in twice as less light then f11.
The affect of the f-stops on the brightness of the image is shown in the following example:
Notice how the images are brighter at lower f-stops (aperture wide open) then at larger f-stops (aperture smaller).
Depth of field
The aperture also affects something called ‘depth of field’ – Depth of field is the amount of the image that is in focus. When you focus your lens on a subject, anything at that same distance will similarly be in focus. Things that are closer to or further from the camera lens will gradually - or drastically - be less sharp. Your camera's aperture controls how large of a zone is acceptably in focus.
At large apertures depth of field will be ‘shallow’ so less of the background will be in focus and at smaller apertures depth of field will be ‘deep’ so more of the background will be in focus.
In this image the tip of the red pencil is sharply in focus and the acceptable focus zone fades quickly so that the other pencils are more and more blurry. This was achieved by using a large camera aperture.
(left to right : f8, f5.6, f2.8)
As you increase the size of the aperture from f8 to f2.8 you'll notice how the heads in the background become more and more out of focus.
What else apart from aperture can affect the depth of field?
Assuming we keep the aperture constant there are 2 other factors that affect the depth of field.
1) Distance of the camera from the subject. Moving the camera closer to the subject will give you a shallow depth of field thus giving you a more blurry background. Moving the camera further away from the subject will give a less out of focus and blurry background.
2) Zoom setting. Assuming aperture and distance of camera from the subject are constant then zooming into a subject will give shallow depth of field and an out of focus background and vice versa.
Thus to summarise, for shallow depth of field:
a) set aperture to maximum
b) move the camera closer to the subject
c) zoom in
When is depth of field useful?
Depth of field is a great creative tool. All images communicate something and what they communicate will depend on where you place the emphasis in the photograph. For example are you trying to give the subject a sense of place by ensuring as much of the background is in focus (deep depth of field – small aperture) or are you trying to isolate the subject from the background so that the eyes are drawn to it? Whatever it is, you must be able to control the depth of field to get the affect you want and controlling the aperture will help you control depth of field.
In portraiture it is considered good stylistic technique to isolate the face from the background. This is achieved by focusing on the eyes and then adjusting the aperture to get the desired depth of field. In the image above, I focused on Alisha and selected a large aperture of f1.8 to blur out the background.
(top left). Notice how shallow the depth of field is here. The focus point is the eyes. Anything behind that focal point and forward of it is out of focus. This has been achieved by using a lens with an aperture of f1.2!
So we can see that shallow depth of field is quite handy when we want to isolate the subject from the background so that the image literally jumps out at you!
What about large depth of field? When is it useful? Answer: landscape photography. When shooting landscapes we want as much of the scenery to be in focus so we select smaller apertures. This will ensure that the whole image from the camera to the background is in focus.
Notice in the image on the left how everything from the wooden post to the mountains is in focus. This was achieved by selecting a smaller aperture (> f9)
Controlling aperture on the camera
When you put your camera in ‘auto’ mode the camera automatically selects the aperture for you. Thus you have no means of controlling depth of field. All SLR cameras and many ‘point and shoots’ have an ‘aperture priority’ mode. In this mode YOU select the aperture and the camera automatically selects the shutter speed to ensure correct exposure. The aperture priority mode is normally designated as ‘Av’ on most cameras.
Another point to note is that the maximum aperture that a camera is capable of is a function of the lens not the camera. The lens of the camera will have on it an f-number. For example the Canon G7 (top right) has f2.8-4.8 written on the front of the lens. This means that the MAXIMUM aperture capability of this lens is between f2.8 - 4.8. When on the mimimum zoom setting the maximum aperture capability of the lens is f2.8. When at maximum zoom the maximum aperture decreases to f4.8 – hence the range.
The great advantage of SLR’s to point and shoots is that you have dedicated lenses with much wider maximum apertures thus allowing you to achieve more pronounced and nicer depth of field affects. In addition, for the more expensive SLR lenses, the maximum aperture stays fixed even when you zoom in. These lenses maintain a fixed maximum aperture throughout the zoom range. But such lenses tend to be more expensive!
In Summary
To achieve nice depth of field:
- select aperture priority mode and ensure the aperture is large (small f-number)
- zoom into the subject to get more shallow depth of field
- move the camera closer or further from the subject for an added layer of control
