Digital Photography Basics
A Guide To ISO, Aperture & Shutter-Speed

Throughout this tutorial I will be using the analogy of the human eye. This is because the mechanics of a camera are very closely related to the mechanics of the human eye.

a.) What is an SLR? What does it mean?
     It stands for Single Lens Reflex!

  Before we start to talk about the principles of photography and the mechanics that accompany those principles, we first need to understand the term Single Lens Reflex (SLR).
Single Lens Reflex (SLR) is in reference to a series of mirrors located inside the body of the camera. The purpose of these mirrors is to divert the light entering through the lens up to the viewfinder. The advantage of an SLR system is that it allows the photographer to see exactly what the camera will see when the shutter-release-button is pressed. For us to truly understand how the SLR system works, we will have to take a look at some of the other internal components of an SLR.

The other internal components:-

1.The Sensor
  In essence, the sensor is the ‘eye’ of the camera and can be found near the back of the SLR body. The sensors primary function is to capture light, convert it into an electrical signal and then pass that electrical signal to the ‘brain’ of the camera (the processing chip).

2.ISO (International Organisation for Standardisation)
  Like the human eye, the sensor can have different sensitivities to light, however, unlike the human eye, this sensitivity to light can be changed at will; this is achieved by changing the ISO.

On all digital SLR’s there is a setting for changing the ISO. The normal range of ISO’s is ISO-100, 200, 400, 800, 1600, 3200, 6400 and 12,800.

As you may have already noticed, ISO starts at ISO-100 and keeps on doubling until it reaches ISO-12,800. Every time the ISO-Number doubles, the sensor is twice as sensitive to light and therefor only half the light is required to obtain the correct exposure.

The downside to raising ISO is that we start to loose image quality; this appears in the form of ‘noise’ (‘noise’ is random and unwanted pixel colouration). It is important to strike a balance between obtaining the right exposure and maintaining an acceptable level of image quality.

3.Shutters & Shutter-Speed (Part 1)  If we extend the analogy of the human eye (the human eye being the camera sensor) the camera shutters would be the eye-lids. The main purpose of the shutters is to prevent light from reaching the sensor, and only when the shutter-release-button is pressed (the final button pressed to take the photograph) do the shutters open and consequently expose the sensor to light. The amount of time the shutters stay open controls two important aspects of photography:
(a) The duration of time the sensor is exposed to light; therefor the amount of exposure the image
      recieves.
(b) The sense of motion in an image.

Shutter-speeds are measured in seconds and micro seconds, and will normally range from 30 second exposures through to 1/8000 exposures (eight thousandths of a second). If I were shooting on a bright day in the middle of summer, then the shutter-speed would likely be very fast, i.e. 1/200; conversely, if I were shooting in low light, the shutter-speed would be slower (the greater the duration of time the shutter is open, the more light the sensor receives).

4.Shutters & Shutter-Speed (Part 2)  I mentioned in ‘Shutters & Shutter-Speed (Part 1)’ that the speed of the shutters also control the sense of motion in an image; to explain this concept I will use the example of trying to photograph a speeding bullet.

If I wanted to capture a speeding bullet, I would need a VERY fast shutter-speed in order to freeze the bullet in motion. Let’s say for example that 1/8000 was an adequate shutter-speed to create the illusion that the bullet is frozen in mid-air, if I were then to lower the shutter-speed, the speeding bullet would start to create a motion blur (a blur extending out in the opposite direction to the direction of the bullet). If I continued to lower the shutter-speed, the bullet would become more and more blurry and eventually I wouldn’t have a shutter-speed fast enough to capture the bullet at all.

Because shutter-speed affects both the exposure and the sense of motion in an image, we have to strike a balance between the two affects. In trying to show the motion of an object, I might inadvertently set my shutter-speed so slow that my image becomes over-exposed.

5.Aperture
  If the camera sensor is the human eye, then the aperture would be the iris of the eye.
The word ‘aperture’ is in reference to an adjustable opening normally located within the lens of the camera. Just like ISO and shutter-speed, aperture controls exposure; but also like ISO and shutter-speed, aperture has a secondary effect on the photograph, namely, ‘Depth Of Field (DOF)’.
The simplest way of describing DOF, is to say that it is the range of acceptable focus within an image. If for example we were photographing a flower but wanted the background to fall out of focus, we would open the aperture wider. When opening the aperture, we allow a greater amount of light to reach the sensor, so we have to be careful not to overexpose the image.

Aperture is measured in something known as F-Stops, for example, F/2.8 or F/22. The ‘F’ is in reference to the focal length of the lens, and the ‘number’ is a mathematical constant that relates to the circumference of the aperture opening (F/2.8 literally means ‘focal length divided by 2.8’).

If I were shooting at a focal length of 100mm at F/2.8, the circumference of my aperture opening would be 35.71 in circumference; if I were shooting at a focal length of 100mm at F/10, my aperture opening would be 10 in circumference.

A simple way of thinking of this is as follows:-
I.)An increasing ‘F-Number’ equals a decreasing ‘aperture opening’, which also equals a reduced  exposure, which further equals a deeper DOF.

II.)A decreasing ‘F-Number’ equals an increasing ‘aperture opening’, which also equals an increased exposure, which further equals a shallower DOF.

Summary & Conclusion
Photography is about striking a balance. Whilst ISO, aperture and shutter-speed affect the exposure of an image, they all have a counterpart effect.

Just remember these 3 simple rules:-

1.) ISO affects exposure and noise (image quality)
2.) Aperture affects exposure and DOF (acceptable range of focus)
3.) Shutter-speed affects exposure and the sense of motion

Master these 3 fundamental corner-stones of photography and you will be well on your way to producing better photographs!

Subin Sebastian

(Creative Photography)