Free book: Drawing Portraits #004

1.2.3 Depth of field

This refers to how much is in focus. When you take a picture, the right amount of light is required on the film. Too much light will cause the picture to be washed out, and too little light causes it to be too dark. Traditional photographic film has a speed of light-response. You can get fast film which is very sensitive to light, and slow films which require more light. Depending on the amount of light available when you take the picture, you need to control how much reaches the film. There are two ways to control how much light lands on the film. The first is to control how long the shutter remains open. The second is to control the size of the shutter. The size of the shutter is called the aperture. A big aperture lets more light in for a given shutter speed. A small aperture obviously does the opposite. What's important for us to understand is that a small aperture causes the light beams to hit the film at an angle which is near 90 degrees. Where these light beams cross will give good focus, and for a very small aperture, there is considerable slackness in how we focus the lens onto the film. Alternatively, you can consider that a range from near objects to far objects will be in good focus for a small aperture. But as the aperture is opened up, light can strike the film at a more obtuse angle and the range at which all these light beams cross is narrowed. This means less depth in your subject is in good focus. If you have more light, or a faster film, you can get away with a smaller aperture, and get more of the picture in focus. But this effect is mostly controlled by physics, and the photographer must work with the limitations. To put this into a practical situation, let's imagine that we are taking a picture of someones face. Imagine that you focus the camera on the person's nose, and there is very little light, and you are using a slow speed film. You will need a wide aperture, and only the tip of the person's nose will be in focus. This represents a shallow depth of field. With computers, and a subject which does not move, we could take several pictures at different focal points, and then stitch them together in the computer to create a single low-light image with a good depth of field. If we are drawing a person from life, we can do this on the paper without technical limitations. Although we focus only on one part of the face at a time, we can draw each part in focus as required. But sometimes, we might want to emulate a limited depth of field because the general public is used to looking at photographs, and you may wish to emulate this limited depth of field to create impact. When a portion of your drawing is in focus compared to another part, it will tend to stand out. We can use this as part of our composition. Alternatively, by de-focusing confusing details, like images in the background, we can create more interest in the subject matter. The de-focused images can be thought of as a kind of impressionism.