This post will actually be about technique, more specifically the aperture. Most photographers know that a combination of shutter speed and aperture together ensure a correct exposure whether it’s on a digital sensor or on film. But the aperture has a much more profound role to play in terms of visual language. It determines the depth of field of any photograph. It can make everything from close-up to infinity seem sharp or it can make the focus only a couple of inches wide and knock everything else out of focus. How can we use that creatively? Fundamentally in two ways. By reducing the depth of field we can make the viewer focus on the main subject or we can create an illusion of three-dimensional depth in the photograph.
The fact that a picture in itself is two-dimensional gives rise to special challenges in order to transform the perception of three-dimensional depth onto the flat surface. Depth is simply missing in any picture. It’s not a new challenge and it’s something painters through time have dealt with in various ways. Among other means they have used perspective to bring out a feeling of depth. The ancient Egyptions rendered a man at the far end of a row of marching soldiers as large as the man closest to the observer, and thus really didn’t create much feeling of depth. The old Chinese did the same on their rice paper paintings, but they were still able to create a feeling of depth. They always placed near object down in the left corner and faraway objects in the upper right corner of the frame. So even if a mountain in the foreground and a mountain in the background were rendered at the same size, the painting would still be perceived as being three-dimensional. Eventually painters, particularly in Europe, started to utilized convergence of parallel lines and diminution of object size to create a feeling of depth. And during the Renaissance they even went to extremes, by exaggerating the effects of convergence and diminution.
With the use of limited depth of field it’s possible to create another sensation of depth. The eye can only focus on one plane at a time. Objects in front of or behind this plane appear more blurred the farther away they are from it. As a result, contrasts between sharpness and blur, creates an impression of depth. This is something we can use creatively in our photographic language. A shallow depth of field will at the same time make the eye stay on whatever is focused and this it’s a great way to clean up an otherwise messy or chaotic background.
Most people know that the use of a wide angel lens results in more depth of field than the use of a telephoto lens. But it’s not quite true. What really matters is the scale of the object rendered. If you move in with a wide angel lens so that the object is rendered at the same scale on the image sensor as with a longer lens, the depth of field will be the same with the same aperture, albeit the perspective will be completely different. With this in mind it should also make sense that a camera with a small sensor, give rise to more depth field compared to one with a larger picture frame. As a matter of fact most point-and-shoot cameras have so small sensors that it’s virtually impossible to effectively limit the depth of field. That is why so many photographers chose a so-called full-framed camera, simply to have more options to play with (among other qualities). So to summarize: The only two factors that affect the depth of field are scale and size of the aperture. Use it wisely in your visual expression!