Getting your head around exposure can be daunting, so let’s start with the basics. When taking photographs, an image is recorded by light reaching your camera’s sensor. You need a certain amount of light to expose the scene correctly; too little and the image will be too dark, or under-exposed; too much and it will be too bright, or over-exposed. The amount of light reaching your sensor is controlled by three key components: aperture, shutter speed and ISO.

The aperture controls how much light is allowed through the lens – a wide aperture lets in more light, a narrow aperture less. The shutter speed determines the length of time the shutter remains open. Aperture and shutter speed work in unison to expose the image correctly, and if you adjust one you have to adjust the other: if, for example, you increase the shutter speed (therefore decreasing the length of time that light hits the sensor), then you have to use a wider aperture to let in the same amount of light to expose the shot correctly.

The other variable that affects exposure is ISO. The ISO setting affects the sensor’s sensitivity to light. The lower the ISO, the more light is required to expose the image.

Understanding exposure

To determine the aperture and shutter speed required to expose a shot correctly at a given ISO, your camera measures the amount of light reflected back from the scene using a built-in light meter. The key word here is ‘reflected’. Camera meters assume that the scene you want to photograph contains a full range of tones, and tries to expose it accordingly. This means that certain lighting conditions can confuse them – dark scenes can be over-exposed and bright scenes under-exposed – because the meter will try to expose the light or dark areas as midtones.

Dark scenes

A built-in light meter will try to produce an exposure made up of average midtones. A dark scene that’s been exposed so it’s dominated by midtones will therefore look over-exposed.

Mid-tone scenes

Scenes containing a relatively even mix of shadows, midtones and highlights, such as in the photo above, won’t give your camera’s metering system too many problems.

Bright scenes

As with dark scenes, because the built-in light meter will try to render any scene as an average midtone, very bright subjects like snow will end up looking a bit grey, rather than pure white.

How to read a histogram

You can call up the histogram on your camera’s LCD to judge the tonal distribution in a photo. The horizontal axis shows pixel brightness, ranging from pure black on the left to pure white on the right. The vertical axis shows the number of pixels at a particular brightness level.

Under-exposed

Not enough light has reached the sensor, caused by either too fast a shutter speed, too narrow an aperture, or both. This shot’s histogram is pushed all the way to the left, a situation known as ‘clipping’. If printed, the clipped areas will appear completely black, and all shadow detail will be lost.

Correctly exposed

The histogram should correspond with the tones of your scene; a dark scene should have a histogram with a bell shape on the left; a light scene should have a histogram with a bell shape on the right. For daylight scenes, expose the scene as far to the right as possible without clipping the highlights.

Over-exposed

Too much light has reached the sensor. Notice how the histogram reveals that the tonal distribution is pushed all the way to the right, and that the highlights are clipped. In the printed photo, areas of the scene that should reveal detail will appear as pure white, with no detail.

Metering modes

Multi-zone metering

This is also known as matrix, evaluative, multi-segment and pattern metering, depending on the camera you have, but all serve the same purpose. Multi-zone metering is generally the default setting on your camera when you first switch it on. In this mode, the camera divides the scene into sections and takes a reading from each section to determine an overall reading for the whole scene.

Centre-weighted metering

Centre-weighted metering also takes a reading from the whole scene, but concentrates mainly on the central 60% of the frame. It’s handy for portraits, especially when the model is in the centre of the frame. Centre-weighted metering can easily be fooled by very bright or very dark areas, but it can be easier to predict when you need to adjust the exposure than it is with the multi-zone metering mode.

Spot metering

This is the most accurate metering mode because it enables you to take a reading from a small, precise area of a scene. However, it can take a bit of practice to be able to judge precisely what constitutes a suitable midtone. To simplify matters, some photographers meter off a so-called ‘grey card’ placed in the same light source as the subject. You can pick one of these up from Jessops for around £6.

Back to: Explore your SLR

Forward to: Understanding aperture