A portrait can turn on a small detail: whether a hand resting on a chair remains distinct or melts into the room. A camera’s preview of depth of field offers a way to inspect that transition before the shutter is released.
Depth of field is the zone in front of and behind the focus point that appears acceptably sharp. The aperture is the opening in the lens; its f-number describes its size. At the same focus distance and focal length, a smaller opening such as f/8 generally gives a deeper zone than f/2.8. The change can bring a second face into clearer view or make a background compete with the subject.
Set up the preview
To preview the depth of field, choose aperture-priority or manual exposure, set an aperture, and focus on the subject. Find the depth of field preview button, often near the lens mount, and hold it while looking through the finder or at the screen. The lens closes to the selected working aperture, revealing the effect of that setting. Release the control to return to the normal viewing state. On the Nikon D6, for example, the Pv button performs this action while held.
Controls vary. Some cameras require a function to be assigned to a customizable button; some displays already show the effect of the selected aperture in certain modes. The Canon EOS R50 manual describes settings in which holding an assigned button makes the display reflect both aperture and exposure. The camera’s manual is the best guide to its particular control and display behavior.
Read the image
Begin at the focus point, then look toward the nearest and farthest details that matter. In a portrait, these might be an eye, a hand, and a patterned wall. With a preview of the depth of field active, compare the hand and wall at f/2.8 and f/8 without moving the camera or changing focus. The wider setting may separate the face from the room; the smaller opening may make the hand easier to read while giving the wall more presence.
Sharpness changes gradually, not at a visible boundary. A face slightly outside the focus plane may still look clear at a small viewing size but softer when enlarged. When previewing depth of field, remember that moving closer generally makes the zone shallower, while moving farther away makes it deeper. A different focal length or focus point will also change the comparison.
Account for the finder and viewing size
A depth-of-field preview looks different in an optical viewfinder, where light travels through the lens to the eye. Stopping down lets less light through, so the image can become noticeably darker, especially at high f-numbers. The Canon EOS 90D manual describes this darkening and the wider apparent zone at larger f-numbers. Give the eye a moment to adjust, or use the rear screen if the details are hard to see.
An electronic viewfinder displays a sensor-based image and may brighten it electronically. Its brightness and aperture simulation depend on the camera’s settings, so a bright display does not mean the lens has stayed wide open. Noise, display resolution, and limited refresh in dim light can make fine detail difficult to judge. For a closer look, use screen magnification where available, then take a test photograph and enlarge it in playback.
The depth of field preview is a guide to the arrangement of sharp and soft areas, not a guarantee that a final print will look sharp. Print size, viewing distance, focus accuracy, camera movement, and lens performance all affect the result. To see the difference now, photograph the same scene at f/2.8 and f/8, focus on the same point, and compare the important details at both full-frame and enlarged views.