Depth of Field
Creating a captivating photograph has a lot to do with how you lead the viewer through your image. In previous tips on composition we have discussed a few ways to do this such as leading lines, anchor points and how the direction of your subject can lead the eye. Used in all types of photography, but most noticeable in portraiture is the ability to blur the background to lead the eye where you want in the image.
In the in depth look at exposure that we have recently taken, one of the exposure variables that we were talking about was the aperture. As you open the aperture in the lens to let more light in (a lower f-number), the area that stays in focus becomes more narrow. The area that is reasonably sharp in an image is known as depth of field. If the background is blurry like the example above on the right, we refer to that as a shallow depth of field. Inversely, when the background and foreground are in focus, it is known as a deep depth of field (higher f-number, less light entering lens).
Like Taking A Dip In The Pool
If you think about this like the depth of a pool, it makes sense. If I am on the 2ft side of a pool (like my drawing?), I am in the shallow end. If you take that water’s shallow depth and imagine it as your in focus area, you only have a small amount sharp and in focus. When you only have a small amount in focus and the rest is blurry, you have a shallow depth of field. When you are on the 16ft side of a pool, you are in the deep end. If you are at f16 on your lens you have a deep depth of field or more in focus throughout the image like the example above on the left. There not only can you see the subject, but the distracting background behind him.
Zoom vs Wide Angle
In a landscape image, people will often strive to have every flower in the foreground just as sharp and in focus as the mountains on the horizon. For this you would need a deep depth of field. It also would help to have a very wide angle lens to achieve this. The wider your lens, the harder it is to blur the background; the more telephoto your lens, the easier it is. In the example image to the right, the photograph on the top was shot at 35mm and then cropped to show the same field of view as the 70mm. Both lenses were shot at f2.8. You can see how much more pronounced the blur is to the background with the telephoto versus the wide angle.