20th Oct 2005

Digital Cameras. Exposure


The aperture of a camera is made out of retractable blades which can open and shut to make the gap in the middle bigger or smaller.

The way in which an aperture works can be likened to an iris in the eye, it opens and closes to let more or less light in. The aperture value is calculated using the following formula: f / # = f / A where f = the lens focal length and A = the diameter of the aperture. Aperture values can be written in three different ways: F8, f/8 or 1:8 – it makes no difference.

The larger the aperture value, the less light it lets in and every setting lets exactly half the amount of light as the previous setting.

Lenses are usually marked with the maximum aperture setting that will work with that particular lens. This is because aperture, focal length and shutter speeds are all interrelated.

Auto Bracketing

Bracketing involves taking several photos in a burst but using different exposure setting for each photo taken. This is typically used if you are not sure how a picture will turn out. The exposure settings are assigned to each photo that is set to be taken before the shutter release button is pressed.

Exposure - EV

The term ‘exposure’ refers to the length of time that light is allowed to hit the sensor. Most modern digital cameras use a mechanical shutter, similar to those in film cameras but some older ones use electronic shutter systems which automatically start and stop light being recorded by the sensor.

The Exposure Value (EV) is calculated taking the sensitivity of the sensor, the aperture size and the shutter speed into account. On fully automatic cameras this is all done automatically but manual cameras or cameras with a manual mode will allow you to set the aperture, shutter speed and sometimes the CCD sensitivity yourself.

Manual Mode

Digital cameras that can be put into manual mode are becoming more and more popular as more people switch from film to digital. Manual mode simply means that the settings such as shutter speed, size of the aperture, and focus are adjusted manually rather than automatically.

Metering System

The metering system inside a digital camera measures the amount of light in the composition of your photo and calculates what part of the image should determine the exposure value. All digital cameras come with an automatic exposure and the metering mode is the only that needs to be set manually.

There are three main types of metering, which are explained below:

Centre-Weighted Average Metering

This is the most common form of metering and cameras without adjustable metering systems usually have this system as standard. It measures the amount of light in the whole composition and calculates an average, but gives more emphasis on the centre of the image.

Spot (Partial) Metering

This allows you to meter the subject in a small area in the middle of the frame, ignoring the rest of the frame. This system is particularly useful for close up, macro shots and subjects, which are heavily backlit.

Matrix or Evaluative Metering

The secret as to how the Matrix metering system actually works is closely guarded by the different manufacturers but the basic principle lies in the frame being split up into a number of sections (sometimes up to 300). Each section is then assessed individually and the overall exposure value is calculated from these.

Remote Capture

Remote capture involves connecting the camera to a PC via a USB cable so that the computer controls the camera rather than it controlling itself. Remote capture is used for two reasons:

  • Images can be instantly viewed on the larger computer screen.
  • Photos can be stored directly to the hard drive.

Shutter Speed

The shutter speed describes how long the shutter allows light onto the charge-coupled device (CCD). For example, a shutter speed of 1/250s means that the shutter allows light onto the sensor for precisely 1/250 of a second.
Most people can take a picture with a shutter speed of as low as 1/60s with the picture suffering from camera shake but any lower than that usually requires the camera to be supported buy a tripod or something similar. Most modern digital cameras come with a large selection of shutter speed settings ranging from anything between 1/4000 of a second and 16 seconds.

In order to take an action shot where you need to catch the subject in motion, the shutter speed should be the same speed as the focal length in mm (the distance between the lens and the subject). For example, if the focal length is 500mm, the shutter speed should be 1/500.

Earlier digital cameras used electronic shutters, which electronically allowed light to be recorded by the CCD for the period of time the shutter speed was set to. However, newer cameras use the old fashioned mechanical style shutters which physically block out the light.

Time Lapse Feature

The time lapse feature allows the camera to take a series of photos over a period of time. For example, if the camera was mounted on a tripod, it could be set up to take a photo every 30 minutes or hour to monitor the progress of its subject. This feature varies from camera to camera, with some it is a built in feature, whereas with others it requires the camera to be connected to a computer (remote capture).

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