Learn how to control and react to light in our guide to exposure on DSLRs
Developing an understanding of light, and its interpretation, is probably the best foundation on which to build the rest of your camera know-how. We take a closer look at light and how it works to make an exposure, as well as the key features and controls on your camera that will help you in tricky exposure situations.
How cameras measure light
The word exposure refers to a controlled amount of light that is exposed to the film or sensor to record an image. The camera’s job is to balance the amount of light entering the lens with the sensitivity of the sensor, to achieve a fair representation of what we saw with our eyes. This may sound easy but our eyes are far more sensitive to the range of light than the sensor in a camera is, because we can balance out high-contrast subjects and see detail in shadow and highlight areas to a much greater degree than any camera can.
A well-exposed image contains detail from the shadows right through into the highlights. Too much exposure and the image will appear too light, with delicate highlight details disappearing into a pure white mass. But too little and the image will appear dark, with shadow details turning pure black.
Cameras contain three features to control exposure: the shutter speed, the aperture and the ISO setting. The shutter speed determines the length of time that light is passed through to the sensor. The aperture works like the iris in our eyes, opening to let more light in and closing to restrict its flow. The ISO controls the sensitivity of the sensor; in low light, you can increase the ISO to make the sensor more sensitive to light (which in turn, enables faster shutter speeds and minimises the risk of camera shake).
Managing these controls is a light-meter, which measures the light entering the camera and, in auto mode, chooses the right settings to get what it thinks is a ‘correctly’ exposed shot. In manual mode, these settings are suggested by the camera and the user can choose to apply them or override them.
One of the benefits of digital capture is that you can review your exposure instantly and, if necessary, adjust it and take another shot. But don’t be fooled into thinking that good exposure is no longer important as you can always fix problematic images in Photoshop. While it’s true that you can alleviate slight exposure problems on the PC, these can only work successfully on the tones and detail captured in the first place.
This button, used in the auto modes, lets you override the exposure the camera has chosen (perhaps because you think it will have been fooled by the background). Add positive compensation when the subject is darker than the background, and negative compensation when it is brighter. The amount of compensation needed will vary according to the conditions, so you may need to experiment.
A feature found in the menu on many cameras, it’s useful to boost or reduce contrast. At a wedding, for example, you may struggle to get detail in both the bride’s white dress and a groom’s black suit. Turning the tonal compensation down will compress the tonal range to a level the sensor can cope with. You can reverse this process on an overcast day to increase the tonal range and make the images more dynamic.
Highlight WARNING: In this review mode, areas of the image that are judged to be overexposed (with little or no detail) flash on and off. These areas may not be important (eg. a pale sky) but if they are you’ll have to adjust the settings and re-take the photo.
Most cameras have a histogram feature. In this mode, the tones of your image are represented as a graph. The precise spread of the graph will vary according to the tones in your image, but if the bumpof pixels is right over to the left side of the graph the image is probably underexposed, while over to the far right indicates over exposure.
Guide to Exposure on DSLRs – Exposure Functions
Allows you to shoot additional shots either side of your chosen exposure to ensure you get it right
Matrix metering is good all around, centre weighted for portraits, and spot metering for subjects that are off centre
When shooting in a location where the problem with the exposure will be fairly constant, this can avoid changing your exposure constantly
All the menus are available here and also reviewing your histograms and highlight areas to check your exposure
AE lock is useful for metering off-centred subjects. You can take your light reading from a specific part of the image before locking the exposure and reframing your subject. This ensures you are metering for your subject as opposed to the background
You will need to keep an eye on your settings here and through the view to ensure accurate exposure.
Guide to Exposure on DSLRs – Metering System
The majority of DSLRs will give you the choice of three metering systems: evaluative (also known as matrix, or multi-pattern), centreweighted and spot metering.
1. Evaluative/matrix metering divides the frame into sectors and measures light from each.
Evaluative is the most popular for general-purpose photography, as it measures the light in many sectors of the frame to identify the overall exposure. Evaluative metering is still prone to error but, owing to the many areas that it measures from, it is a good, general metering system.
2. Spot metering reads the light from a very small area of the frame, usually in the centre.
Spot metering works in a similar way to centre-weighted, but uses an even smaller percentage of the frame to meter from – usually a 1° circle in the centre of the frame. This gives you a greater degree of accuracy when the main subject contrasts in tone to its surroundings. By taking a precise reading from a specific are a of the frame, such as a face, you can use this as the basis for the overall exposure – and avoid the camera taking the background into consideration, as it would with evaluative and, to a lesser extent, centreweighted metering.
3. Centreweighted metering covers a wide area but is biased towards the central area.
Centreweighted metering still places the greatest emphasis on the central area. This is the preference of manyportrait photographers because it bases the majority of the exposure on the subject.
The Mid-Grey Rule
You can start to predict situations that will fool the camera’s lightmeter and adjust any necessary settings to compensate for these before they happen. Lightmeters are designed to find the average exposure, and while this works most of the time to produce an accurately exposed image, certain situations can fool a camera into recording a scene inaccurately. When you point your lens at a scene, the meter averages the various colours, tones and level of brightness to a mid-grey. For the most part, the shutter speed and aperture recommended by the camera is based on reproducing this mid-grey tone.
Guide to Exposure on DSLRs – Exposure Mode Dial
Correct exposure is one of the essential ingredients of a successful photograph. If too much light reaches the sensor all the brightest parts of the scene may burn out into a stark white, with no visible details. This is called over-exposure. Conversely, an inadequate amount of light (under-exposure) will turn dark areas of the scene, such as shadows, into a sea of blackness.
Determining the correct exposure is also about recording a scene or subject in the way you want it to appear in the final image (which may or may not be the way it appeared to the eye at the time). You may decide that you wnat a dark, moody effect, or a clean, bright one. Deciding what the important element in a scene is and setting an exposure that will record that focal point correctly will put you in creative control of your images.
Exposure is controlled by the camera in three ways: by the sensitivity of the sensor (its ISO); by the size of the hole in the lens through which light passes (the aperture); and by the duration for which the sensor is being exposed to light (the shutter speed).
There are numerous combinations of these that will provide a technically correct exposure, but perhaps only one that will record the scene in the way you want it to be seen in your photograph. This is why a knowledge and understanding of the relationships between all these factors is so important.
Here the camera sets a small aperture, and other parameters that lend themselves to good landscapes.
The camera chooses parameters suitable for good portraits, incl. wide apertures.
Beginners will want to head here. The camera does everything and trusts the user with nothing.
The next step up from Auto, you can override some of the camera’s choices.
You choose the aperture you want, and the camera picks the right shutter speed to go with it.
You choose the shutter speed, the camera matches it with the appropriate aperture.
In this mode you control both shutter speed and aperture yourself, so you can set your own exposure.
Yes, Landscape, Portrait, etc are scene modes too, and on some cameras they’d be in here too. On this Olympus E500 there are modes for lots more special subjects.
An ISO boost, and an exposure that balances the flash with the ambient light.
Fast shutter speeds, continuous drive and servo focusing will help you catch the action.
For close-ups head here. A small aperture is among the camera’s priorities in this mode
Generally, the ISO is set as low as possible for the conditions, since unlike the other controls, increasing it has a detrimental effect on image quality. This is because raising the ISO is, in effect, boosting the signal to the sensor, and just as raising the recording level on a tape recorder will increase the background hiss, so raising the ISO will increase the appearance of ‘noise’, which most commonly appears as specks or grains (often multi-coloured) over the image. In bright conditions, the ISO is generally left at 100 (usually its lowest setting) and is raised to 400 when there isn’t enough light to get a sharp, hand-held exposure. There will be more on this later.
Guide to Exposure on DSLRs – Apertures & Shutter Speeds
Once your ISO is set for the conditions, exposure on a shot-by-shot basis is controlled by the aperture and shutter speed. Each has its own numerical scale (see diagram) whereby each number either doubles or halves the amount of light reaching the sensor (depending whether you’re going up or down the scale).
With shutter speeds, this mathematical relationship is easy to see, as the numbers themselves more or less double as you go up the scale. With apertures it’s less obvious because they’re measured in ‘f-numbers’ or ‘f-stops’ – a figure derived by dividing focal length by the diameter of the aperture. But the relationship is the same: f/8, for example, passes twice the amount of light as f/11, but half as much as f/5.6. Each doubling or halving of exposure is known as a ‘stop’. So widening the aperture from f/11 to f/8, or changing the shutter speed from 1/500th to 1/250th sec, will increase the exposure to the sensor by one stop, while doing both (which will let 4x more light reach the sensor) increases it by two stops. On the other hand, if you widen the aperture from f/11 to f/8 (doubling the light reaching the sensor) but change the shutter speed from 1/500th sec to 1/1000th sec (halving the light reaching the sensor) the overall amount of light reaching the sensor (Exposure Value) will remain the same.
Digital cameras make it more difficult to learn this mathematical relationship because they can usually be adjusted in half or one-third stop increments, so you’ll have to move the setting by three positions (for third stops) or two (for half stops) to affect exposure by a whole stop.
How Apertures and Shutter Speeds Affect Your Pictures
Why does it matter what aperture or shutter speed is used to take a picture, as long as the exposure is correct? Because they have a direct influence upon how the picture looks. As well as controlling how much light enters the camera, the aperture also has a strange effect on the zone of focus in the picture. The smaller the aperture, the greater the area of sharpness in front of and behind the point you focus on. The wider it is, the blurrier the areas surrounding your focus point will be. With shutter speeds, a slow shutter speed records movement as a blur (the slower the speed, the blurrier the subject) while a fast shutter speed freezes fast action so it looks stationary. We’ll cover these effects in more depth in forthcoming issues.
This sequence of images, taken on a Nikon D200 fitted with an 85mm f/1.8 Nikkor lens, shows how the particular exposure combination you choose has a direct effect on how your picture looks. Every picture in the sequence has received the same overall amount of light. Notice how, as the aperture size is halved with each successive image, the exposure duration (shutter speed) doubles to compensate. They are all ‘correctly’ exposed, yet all look different.
Guide to Exposure on DSLRs – Exposure Modes
Don’t panic – you don’t need to learn all those apertures and shutter speeds off by heart (though doing so would be useful). Modern DSLRs can control both of these elements for you automatically. The trouble is, although you’ll get a fair record of what you saw, if you leave everything to the camera then you’re not in creative control of your pictures, your camera is. While the Program and Auto modes are useful for quick snaps, the sooner you can graduate to the more creative modes, the sooner you’ll be able to impose your own vision on your pictures. Your camera will then become an artist’s tool rather than simply a recording instrument. Here is a look at the modes found on a typical digital SLR.
In this ‘mother knows best’ mode, the camera makes all the decisions about what exposure to set. You can’t override it in any way, and your only input is to press the shutter at the right time.
Superficially the same as Auto, except that if you don’t agree with the settings the camera has chosen there is some scope to override them. You can, for example, choose to add flash or change the particular shutter speed/aperture combination to obtain more depth of field than the camera had set. This makes it an ideal mode for the slightly more knowledgeable user.
This mode asks you to choose which aperture you want and the camera then matches it with the shutter speed that will provide, in its opinion, a correct exposure. This is probably the most popular mode with experienced photographers since by taking control over the aperture it enables them to determine the most fundamental aspect of the composition – how much of it is rendered in focus.
In this mode you get to choose which shutter speed you want to use and the camera then selects the aperture required for the correct exposure. This is ideal for action photography where you want to control how sharp or blurred your moving subject will be recorded. It can also be useful in lower light conditions where you don’t want to go below a hand-holdable shutter speed.
The camera tells you what exposure it thinks you should give, and it’s up to you how you use that information. Unlike the Priority modes, where the exposure adjusts for changing light conditions, in Manual it won’t, so you’ll need to be aware of light levels. Manual is ideal for situations where exposure needs to be constant, such as shooting panoramas that you’ll want to stitch together later.
Most digital cameras offer a selection of scene modes. By telling the camera what you’re photographing, it can choose the parameters to match. Shooting a portrait? Then a wide aperture will be useful. Scene modes can also adjust the white balance, sharpness levels etc, to suit the subject too. They’re a good stepping stone from Auto, but you’ll get more fine control from the Priority modes.
Guide to Exposure on DSLRs – Tricky Metering
A portrait taken in front of a window with a great view in the background sounds like a guaranteed scenario for a great photograph but, as with many scenes where the light is coming from behind the subject, the contrast range between your subject and the background is likely to be too great for the sensor to handle.
If the camera’s meter is influenced by the background, the subject will be too dark and, in some cases, even a silhouette.
To get a correct exposure on the face you’ll need to take a selective light reading, though of course all the background detail will then be overexposed.
The only way to balance the two is to increase the light level on the face (using either reflectors, or flash) to more closely match the background level.
Bracketing allows you to shoot an additional exposure (or two) either side of your first reading, giving you a greater chance of obtaining an optimum exposure. The camera calculates the extra exposures based on your initial reading and by what increment you wish to bracket your shot by and takes a sequence of exposures at the different settings, in quick succession.
Using a Grey Card
A grey card is the absolute middle value in a tonal range and is often referred to as 18% grey. Shooting a grey card as a test image allows you to find the centre-point for your exposure. This will also help in post-production when looking to correct exposure or to spot colour-casts in your monitor or prints. Here, we have used a fold-out grey card in the image to help act as a basis for the exposure.
We used a tripod-mounted camera and shot against a window looking out across London, using only available light.
Lightmeters are programmed to pick an exposure that will record the scene as a mid-grey. Most scenes contain a mix of light and dark areas so this ‘average’ usually works, but there are some circumstances where it won’t. With subjects that are mostly dark the camera will try to lighten it to a mid-grey. Knowing this, you’ll have to override the camera to give the sensor less light than the camera wants to provide. The amount will vary depending on the tones in the scene.
Lighter-than-average scenes are a problem too. Examples include shots typically taken on holidays, such as beach shots or snow scenes, which contain large areas of brightness. Leaving the camera to its own devices will result in muddy, underexposed pictures as the camera tries to turn the beach or snow mid-grey. You’ll need to add more exposure than what the camera suggests. Again, the amount will vary according to the circumstances.