30 Jul 2011


Submitted by Roger

Put your camera on manual and, if you have one, use a hand-held light meter. The purpose of this article is to give you an understanding of exposure.

Exposure of a photograph depends upon four factors:

  1. Level of illumination
  2. Sensitivity of the film or digital sensor
  3. Shutter speed (exposure time), and
  4. Lens speed (aperture)

Understanding how these factors contribute to the correct exposure of a photograph, and how these factors interact to give the photographer a degree of creative licence, is the basis of anything else you will ever do with a camera.


It is a common misconception that the more light you have, the easier it is to take a photograph. Yes we need light, but we also need shadow, and we have to preserve detail in both. At this point it is sufficient to say that light is necessary. I will be revisiting the question of lighting later.


The sensitivity of the film or sensor is expressed as an ISO number. This number is simple to understand. If you are taking a photograph of a sunlit scene at midday under a cloudless sky, with the aperture of your lense set to f 16 (f 11 for digital), your shutter speed will be (the reciprocal of) the ISO. So if your ISO is 125, your shutter speed will be 1/125 of a second. If your ISO is 500, your shutter speed will ne 1/500 of a second for a correct exposure. So as the ISO number increases, so does the sensitivity.

In the days of film, increased ISO meant larger photosensitive crystals in the film, so each crystal would have more opportunity to capture light. Increased film sensitivity therefore meant increased graininess in the image. Digital sensors accomplish some increased sensitivity by electronic amplification of the stored image, but really large increases are accomplished by using adjacent pixels in the CCD sensor and adding the signals. The result, as with film, is increased graininess at higher ISO's. Digital sensors also exhibit electronic noise, particularily at higher ISO and/or with longer exposures. Noise reduction software has helped to compensate for this effect, but it adds to the apparent graininess of the image.


The camera shutter determines the length of time the film or CCD sensor will be exposed to the light. The more light reaching the film plane, the shorter the required exposure. If something is moving in your scene, the shutter determines how much of that motion is recorded. Slow shutter speeeds will allow slight motion to create a blur in your photograph, and fast shutter speeds will 'freeze' all but the most rapid motion.


Aperture diagram

The aperture of a lens determines its light-gathering capabilities. Two factors effect the aperture of the lens, namely the effective optical diameter of the front element, and its focal length. If a 100mm lens has a front element with a 50mm diameter, then the lens is said to be an f2 lens, meaning the diameter of the front element is 1/2 the focal length. The effective diameter may be modified using an internal adjustable iris diaphragm (stop) which blocks some of the light allowing a smaller portion to get through the lens. This iris acts very much like the iris in your eye.

The interesting thing about aperture is its effect on out of focus elements in the image.  To focus something in a photograph, the lens is moved back and forth until all light coming from the required plane of focus comes to a point on the film or sensor. Everything that is the same distance from the lense will be focused on the film plane. Anything in front of or behind the plane of focus, will also come to a focus point, but that point will be in front of or behind the film plane. The light from these out of focus elements, at the film plane, will be in the form of a circle, not a point, as shown in the diagram. As the aperture is made smaller, these circles also become smaller. The result is an increase in the apparent depth of field.


Putting it all together

As the intensity of illumination decreases, you must either increase the sensitivity or increase the exposure. Sacrificing resolution by increasing ISO is fine if graininess adds to the artistic effect of the photo, or if you will not be enlarging the photo enough to show the grain. To increase the exposure, you use a slower shutter speed and/or a wider aperture. Slower shutter speeds increase blurring due to motion, either in the subject or the camera, and wider apertures decrease your depth of field. If you need foreground and background both in sharp focus, you need a narrow aperture (f11 - f32) and should take measures to minimize the effects of motion. If you need to stop action, you need a high shutter speed, which means a wide aperture. You should practice rapid focusing and framing with the camera in front of your eye.

We speak of exposure in stops: one stop will halve or double the amount of light reaching the film plane. Increasing shutter speed from 1/125 to 1/250 of a second reduces the exposure by one stop. Likewise increasing aperture from f8 to f5.6 will increase the exposure by one stop. Aperture numbers begin to make more sense if you understand that doubling the aperture number is two stops. So f4 is four times the exposure of f8, which in turn is four times the exposure of f16. The standard aperture numbers, at one-stop intervals are: 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32. Most cameras will allow you to adjust aperture by 1/2 or 1/3 stop intervals for finer control.

f 2.6, 1/500 secf 32, 1/4 secThese two photos of a blade of grass illustrate the different effects of shutter speed and aperture. The exposures are equivalent, but shutter speed and aperture were varied by 7 stops. Both pictures were taken using a tripod to eliminated camera motion.

On the left, a shutter speed of 1/500 of a second froze all motion. The aperture of f 2.8 created a very narrow depth of field. The background is an even green colour, and parts of the grass are out of focus.

On the right, a shutter speed of 1/4 second allowed the light breeze to move the grass during exposure. The aperture of f 32 kept all parts of the grass in focus, and details of the background are becoming apparent.

Exposure decisions are not simply about the amount of light captured, but must take into account any motion which may occur in the scene, and the desired depth of field. These decisions should be made by you the photographer, and not left to chance by trusting the camera's automatic settings.

As an exercise, put your camera through its paces and learn how shutter speed and aperture effect the final image. The better you know your camera, the better the images you will create.

depth of field