One writer Mr. Brian Peterson wrote a book illustrating the three main elements (ISO, Shutter Speed, and Aperture) that need to be considered when playing around with exposure. He called it ‘the exposure triangle’. Each of the three aspects of the triangle relate to light and how it enters and interacts with the camera.
The three elements are:
- ISO – the measure of a digital camera sensor’s sensitivity to light.
- Aperture – the size of the opening in the lens when a picture is taken.
Shutter Speed – the amount of time that the shutter speed is open.
It is at the intersection of these elements that an image’s exposure is worked out.
Note: A change in one of the elements will impact the others. This means that you can never really isolate just one of the elements alone but always need to have the other two at the back of your mind.
A Metaphor for understanding the digital photography exposure triangle:
Many have described the relationship between ISO, Shutter Speed and Aperture using different metaphors to help us get our heads around it. But like most any metaphors, this is far from perfect but only for illustrative purposes.
The Window
Imagine your camera is like a window with shutters that open and close.
Aperture is the size of the window. If it’s bigger, more light gets through and the room is brighter.
Shutter Speed is the amount of time that the shutters of the window are open. The longer you leave them open, the more that comes in.
Now imagine that you’re inside the room and are wearing sunglasses (hopefully this isn’t too much of a stretch). Your eyes become desensitized to the light that comes in (it’s like a low ISO).
There are a number of ways of increasing the amount of light in the room (or at least how much it seems that there is. You could increase the time that the shutters are open (decrease shutter speed), you could increase thesize of the window (increase aperture) or you could take off your sunglasses (make the ISO larger).
Ok – it’s not the perfect illustration – but you get the idea.
Summary
Mastering the art of exposure is something that takes a lot of practice. In many ways it’s a juggling act and even the most experienced photographers experiment and tweak their settings as they go. Keep in mind that changing each element not only impacts the exposure of the image but each one also has an impact upon other aspects of it (ie changing aperture changes depth of field, changing ISO changes the graininess of a shot and changing shutter speed impacts how motion is captured).
The great thing about digital cameras is that they are the ideal testing bed for learning about exposure. You can take as many shots as you like at no cost and they not only allow you to shoot in Auto mode and Manual mode – but also generally have semi-automatic modes like aperture priority and shutter priority modes which allow you to make decisions about one or two elements of the triangle and let the camera handle the other elements.
On the next lesson we will talk about the ISO.
Photography The Blog 
