Fact: Depth of field will remain constant across all formats and focal lengths as long as the aperture and the image size remain the same. There are several factors that determine how sharp, or not sharp, an image may be. One of those is depth of field, and another is the movement of the subject matter that occurs while the shutter is open. Here, I will discuss depth of field.
One of the most frustrating things that has happened to me in the creation of photographs is making a photograph I think will be stunning, sending the film out for processing (color film only for me), or processing a black-and-white negative, looking at the film on the light table where it has great tonality and a very pleasing and dramatic composition—everything I wanted—and then, on closer inspection, seeing that I didn't have enough acceptable depth of field to render a sharp photograph throughout the scene. To gain a clear understanding of acceptable depth of field, let's first define it.
Acceptable depth of field is the amount of space in front of and behind the plane of critical focus that appears to be acceptably sharp. The plane of critical focus is the place where the lens is focused, and nothing in that image can be sharper than the object(s) at the plane of critical focus. As the distance of material from the plane of critical focus increases—either toward the camera or toward infinity—objects will be less and less sharp until they are so unsharp that their lack of sharpness can be seen by the viewer of the photograph. What is acceptably sharp in one photograph may not be acceptable for another.
Depth of Field
If the camera and lens are focused at exactly 8 ft, as in the drawing, an object included at 7 ft or 9 ft will be more out of focus than what is at 8 ft. These objects may appear to be sharp and may be acceptably sharp, but they aren't critically sharp. That is a critical thing to understand.
We were staying in Northern Colorado, almost in Wyoming. I lost myself in the wonder of this aspen forest. An aspen grove, by the way, is one of the largest living organisms in the world since all the trees in a grove can originate from a sucker put out by one tree. They may all be the same tree... that is a marvel. In my wandering, I came to this tree and was struck by how old it must be. There was a slight mist forming, as often happens in colder weather at altitude, which helped to separate the main subject of this image from the background trees. I call that "atmospheric depth of field." Then, using a large aperture, I employed "photographic depth of field" to throw the trees in the background slightly out of focus. The point of this discussion is that you can and should use depth of field as a compositional element.
Managing Depth of Field
Depth of field is managed using two factors: image size on the viewing screen and aperture size. As the aperture number increases, from f/5.6 to f/32, depth of field also increases. Conversely, as the aperture number gets smaller, depth of field also gets smaller. Said another way, as the size of the aperture gets larger—going from f/32 to f/5.6—depth of field decreases, and vice versa. We can control what is acceptably sharp to a great degree by the size of the opening in the lens that allows light to pass through the lens, through the shutter, and to the media, whether the media is electronic or film. The laws of physics governing this phenomenon do not change between film and electronic means. (This should be obvious, but I have had people argue vociferously that the optics governing film and electronic media are different. I want to lay that one to rest: the laws of optics remain the same across media types and media sizes.)
The size of the image at the focal plane is another factor in controlling depth of field. Some people will say that three things control depth of field: the f-stop being used, the camera-to-subject distance, and the focal length of the lens being used. That statement is true, but only partly so. First, the f-stop being used is always a factor in determining your working depth of field—always. The second factor is the image size at the focal plane. Because of perspective, objects become smaller as they recede from the focal plane and larger as they approach the camera. Next, the focal length of the lens being used causes the object to become larger or smaller, depending on the focal length. A wide angle lens, like a 17 mm or 15 mm lens, will reproduce a certain image size at a given distance, while a 300 mm lens from the same spot will produce a larger image at the focal plane. The reason this is important is that the depth of field with a certain lens, at a certain distance, and at a common f-stop will remain the same whether you are using a crop frame or full frame digital camera, a medium format camera, or, as I do, a large format camera. The commonality is that depth of field will remain constant across all formats and focal lengths as long as the aperture and the image size remain the same.
If you photograph an object at a distance of exactly 11 ft using a full frame digital or film camera or an 11x14 large format camera with a 300 mm lens set at f/22, the image size and depth of field for both cameras will be exactly the same. While the focal length of the lens and the distance from the camera to the subject influence depth of field, acceptable depth of field only changes as the two are used in combination. The primary method of controlling depth of field is by changing the size of the aperture in the lens.
There is actually another factor in determining depth of field that is not often discussed but is a primary benefit of using large format cameras or a tilt-and-shift lens on a 35 mm-based camera. That is the "Scheimpflug Principle," and I will address that in a future article. Briefly, the Scheimpflug Principle allows the photographer to shift the plane of critical focus from running horizontally to vertically. That makes depth of field run in a vertical direction, making it possible to have everything in front of the camera in critical focus, from just in front of the camera to infinity.
