Online Tutorials: Exposure

Exposure is the product of how long light is allowed to be exposed to the media and the intensity of light. This is always controlled by the shutter speed and/or aperture.

The settings on your camera allow the intensity or time to be fixed. These are commonly known as aperture priority (A) where the aperture is fixed and shutter priority (S) where the shutter speed is fixed. For each setting, the other factor varies depending upon the available light.

Most modern cameras have built in exposure meters that allow us to make a reasonably accurate shot and some have other features to help with the exposure of the whole image. This is not always accurate and experience will need to be developed to produce consistent results every time.

The following show how a photographer can gauge the required exposure for their image.

 

Spot metering

With this technique, a very small area of the image will be metered correctly. With experience and time, several meter readings can be taken in areas of both high light and shadow to improve the balance of the shot. This method is often used when needing to focus on a very specific point in a scene, such as a lit castle on a hill during the night.

This would allow the photographer to get the correct amount of light for the castle whilst leaving the surrounding areas dark.

spot metering

*your camera may have a different display

Centre-weighted metering

The exposure is calculated by using a main central area of your viewfinder. This is based upon the idea that a photographer will place the subject near to the centre of the composition.

If there is a lot of contrast or the main subject is not central then this will produce unusually underexposed
or overexposed images.

centre weight metering

*your camera may have a different display

Evaluative metering

Also known as matrix or honeycomb metering. Light is evaluated over several areas of your composition and an average reading is taken, which is fairly accurate for images with low contrast levels. It can however cause your image to look a little flat if you are wanting to photograph something very specific.

A lit castle photographed on a hill during the night would look completely washed out as the light levels around would over compensate for it being dark.

evaluative metering

*your camera may have a different display

 

Bracketing

There are some situations where accurate metering across the whole composition is difficult. This can be a major problem if you do not have much room for error or you do not have sufficient filters or exposure latitude.

Bracketing is simply taking a standard meter reading from your camera's meter or your light meter and shooting at that value and at the same time shooting, for example, one light stop below and one stop above the metered reading.

sky
Exposure focused on the sky - building is under-exposed.
sky
Exposure focused on the building - sky is washed out and over-exposed.
sky
Combined image, taking the correct exposure from both sections.

This should take account of any difficult highlights or shadows as the three images can be combined to produce more accurate results. Bracketing can also be used for a range of stop increments from 1/3 of a stop to 3 stops. The example above is of the extremes, +/-3 stops and later combined using the computer.

If your camera has the capacity to record your picture as a "RAW" image, bracketing is less of a necessity as the amount of information stored will allow for recovery of burnt-out, over-exposed highlights or under-exposed shadows.