The depth-of-field (DOF) preview function, available on many single-lens reflex (SLR) cameras, offers the ability to visualize the extent of sharpness in an image at the selected aperture before capturing the photograph. It operates by stopping down the lens aperture to the chosen f-number, simulating the effect on the image. For example, if a photographer intends to shoot at f/2.8 for a shallow DOF effect, activating the DOF preview will darken the viewfinder image but reveal precisely which parts of the scene will appear in focus and which will be blurred.
The implementation of this function provides a significant advantage in critical focusing and composition, particularly when utilizing wide apertures for portraiture or macro photography where the margin for error in focus is minimal. Historically, this feature was especially vital in film photography, as the photographer had no immediate visual feedback on the captured image. While digital cameras offer image review capabilities, the preview remains beneficial for pre-shot assessment of focus and DOF, saving time and potentially preventing wasted shots. It aids in determining whether a smaller aperture is necessary to bring more of the scene into focus or if the current aperture adequately isolates the subject.
