Depth of Field, Part 2
Normally, modern cameras maintain lenses at their maximum aperture and only stop them down to your selected aperture when you depress the shutter release. Doing so enables you to have the brightest image possible in your viewfinder since more light is allowed through, but it does not give you an accurate representation of the depth of field that will be in the resulting image. Since we know from last week that aperture is one of the main variables that determines depth of field, it is only when the lens is stopped down that the true depth of field will be apparent. Many cameras support a feature called depth of field preview that serves to stop the lens down to the selected shooting aperture.
Making use of the DOF preview can require a bit of getting used to though. When pressing the depth of field preview button, the image will darken (unless of course your selected aperture happens to be the lens's maximum aperture). Indeed, as your selected aperture gets smaller and smaller, the image will grow progressively dark to the point where it can be difficult to discern. The experience can be somewhat akin to walking into a darkened room after having been in a lighted area; it can take your eyes a while to get used to the lower level of illumination. In much the same way, a bit of patience is necessary for your eyes to adjust when using depth of field preview. When you first stop the lens down, the viewfinder can go almost completely dark, but if you wait a bit, your eyes will adjust to where you can see your subject. Some people find this easier than others, in much the same way as some people are more susceptible to night blindness than others. If you find it difficult, one helpful tool is a rubber eye cup for your camera that can help block out stray light coming from the sides. In this way, it serves the same purpose as the black cloth the view camera user puts over his head so he can more clearly see the image he is trying to capture. Also, don't be alarmed that the image darkens. Your shutter speed will still result in the selected exposure since the camera will stop down to this aperture when you fire the shutter anyway. Take some time to practice with the DOF preview control on your camera so you know what to expect.
Next week, I'll talk a little about creative uses for controlling depth of field.