Objective: This exercise will demonstrate the how the area of sharpness (also called the depth of field) changes with aperture.
I made three photographs. For each, the camera was set on a tripod and focused at a point about half way between foreground and background. All settings were made manually. Higher resolution A4 size prints are available in my learning log display book, which indicate the area of sharpness more clearly.
1. I chose the largest aperture available for my zoom lens set at 38mm, which was f4.2. I adjusted the shutter to 1/20s. I have indicated the area of sharpness with red lines on the image. It is quite shallow.
What have I learned?
The area of sharpness within the depth of the image can be controlled by the aperture of the lens. A large aperture gives a shallow depth of field (focus) which becomes deeper as the aperture is reduced. This can be used effectively in composing images, i.e. a shallow depth of field can be used to isolate and thereby highlight the key part of the image. It is used in portraiture to blur the background to remove distractions and bring the subject forward. An even textured, out of focus foreground can also be used effectively to add depth to a picture. See the soldier picture below. I know from my experience that the focal length setting of the lens also effects the depth of field at the same aperture. Wide angle lenses have more depth of focus at a given aperture than telephoto lenses.
(These images were taken in room lighting. I reset the white balance on my camera to cool white fluorescent to remove the colour cast)