In photography the word aperture refers to the opening of the lens iris. It seems like a simple concept, and on older interchangeable lens cameras it’s pretty easy to see the lens iris (or diaphragm) in action. As the iris opening gets smaller less light is transmitted to the film or sensor. If you increase the size of the iris opening you get more light passing through the lens. That’s the easy part. When you get beyond the basics of exposure, aperture is the most subtle adjustment photographers will make. One of the major distinctions between beginning and advance photographers is their knowledge and use of aperture to control how their photographs render a scene.The aperture controls the depth of field (DOF) of any lens. A lens can only focus on a single plane, but DOF extends the perception of focus in front of and behind the plane of focus. Roughly speaking that perception of focus is distributed 1/3 in front of the plane of focus and 2/3 behind the plane of focus. A lower f-stop number (larger opening) will have shallow DOF and create separation between the subject and both the foreground and background. A higher f-stop number (smaller opening) will have deeper focus and the photograph will show more of the foreground to background in focus. This is a good starter simplification, but there are subtleties to using depth of field that take years to master.
Depth of field is not an absolute. A short focal length lens will have more DOF than a longer lens at the same aperture number. Very long lenses in the 400mm and above range will have fairly shallow DOF even at smaller openings. Very short lenses, less than 10mm have deep focus even wide open.
Depth of field does not change based on the size of the camera’s sensor, but will change based on subject to camera distance. When the camera is very close to a subject DOF will be shorter. As the main point of focus moves away from the camera we will perceive greater DOF.
The next four images are shot on a camera with a sensor the size of traditional 35mm film, often called a full-frame or FX sensor. On this camera the 28mm lens a moderate wide angle. In the first two images the camera is about three feet from the words “NO TRESPASSING.” In the second two images the camera is at near minimum focus distance.
Depth of field gets even shallower when the camera is moved closer to the subject. The red words are less than 18” away in the next two photographs. Even with a small aperture the entire scene is not in focus.
Depth of field is a curve. There is no right amount of depth of field. Shallow focus is not always the best choice nor is deep focus. DOF is the tool of the photographer. By understanding the three factors influencing DOF, aperture, lens focal length, and camera to subject distance, a photographer can get the correct balance to create the photograph he or she desires. It has become common Internet folly that portraits must have very shallow DOF and the landscape images should have nearly everything in focus. Nothing could be further from the truth. For more than 50 years Playboy has published countless beauty portraits with no shortage of depth of field. The single bloom in shallow focus is and will remain a nature classic.
The next two exposures are made with a 50mm lens on an FX camera at about 25 feet. It is a scene rendered very nearly as our eyes would see it. The lens is focused on the gate.
The added depth of field of the second image brings the foreground grass into focus, which shows it to be unruly weeds rather than a green frame at the base of the image. Given the number of textures, grids and patterns in this image the additional DOF puts the gate in competition with the background without telling us more about the scene. For this landscape photo I prefer the first exposure at f2.
Get Closer for Shallower Depth of Field
Subject to camera distance is an often-overlooked factor in determining Depth of Field. Moving the camera will change more than the perspective of a photograph even with the same lens and the same aperture. When the primary subject is 30 feet or more from the camera even a lens with a near normal angle of view will have fairly deep focus at 5.6 or larger f-numbers. Only the near foreground will appear completely out of focus. When the camera is very close to the subject DOF will be very shallow to a point where distant background elements will not appear in focus even at f22.
In the following three photos the chain is photographed at 4 feet a with a 50mm lens on an FX camera. The camera is focused at “2002” on the lock and the camera is tipped down so the plane of focus is not parallel with the chain.
In the next five photos I moved the cameras to about 20 inches from the chain. Using the same 50mm lens on an FX camera the lens is focused at “2002” on the lock. The camera is even more tipped down so the plane of focus is not parallel with the chain. If the camera was plumb and in the same plane as the chain the focus planes would follow each other. Because the camera is tipped the plane of focus and the plane of the chain intersect at only the point of critical focus.
There isn’t one clearly better photograph. We need to closely consider the details in an image and how aperture subtly effects them. The choices come down to the photographer’s vision. Of the previous five photos the exposure at f4 is my favorite. To my eyes the f8 shot is the worst of the five. The light patches in the background foliage are just getting clear enough at f8 to be distracting but not clear enough to identify–they could be litter or flowers. Interestingly f8, my least favorite of all the choices, is probably what the Auto mode would have chosen for an image on this bright day. The f16 image is an improvement over the f8 image because the background confusion is resolved.
It’s Okay to Be Shallow
In a more traditional landscape with both near and far detail it is often considered ideal to have deep focus, but that is not always the case, and may not be the choice of the photographer. The following photographs are made with a 28mm lens, a moderate wide angle of view, on a camera with an FX sensor. The camera is level and plumb at about five feet from the two orange blossoms that are together in center, where the lens is focused. Each of these images has its own merits. It’s not a question of right or wrong, or even better or worse, but a question of which image the photographer wants.
More Than One Right Answer
The next photographs are made with a 50mm lens, a nearly naked eye angle of view, on a camera with an FX sensor. The camera is level and plumb with the lens focused on the sculpture inset in the door. The different apertures yield four different images; to my eyes the shallowest and deepest focus are the two most successful. There is only one choice I would reject outright.
THE HOMEWORK ASSIGNMENT: Choose one or more scenes and make the same sort of test images I have here using your camera and your lenses. You will notice that, although my work is mostly with people, all of my examples are of static subjects. When learning to master aperture and depth of field it is important to start with subjects that stay put. As I did in all of these examples, use a tripod to eliminate camera movement as a factor in your results. When learning aperture basics it is best to keep the variables to a minimum. Even a photographer who has no interest in landscape photography will benefit from practice with fences and flowers and other things that don’t move.
Aperture must be used to completely control how a camera and lens will record an image. At larger openings, smaller f-numbers, a lens will have shallower depth of field. At smaller openings, larger f-numbers, more will appear in focus. But there is no correct amount of depth of field. There is no ideal aperture for any specific photograph. Rather photographers must learn to use the range of apertures to create images that match what they see in their minds. The examples above are just a few practical applications of aperture choice. In future posts I’ll explore more elements of photography where aperture choice is critical, and how to use aperture to make photographs that match your vision.