DEPTH OF FIELD
This article is essentially the same as one that I wrote for 35mm film cameras. Most digital SLRs have an imaging sensor that is smaller than 35mm film, creating a field of view (FOV) crop (apparent angle of view) that is different when the same lens is used on a 35mm SLR as opposed to a digital SLR. The depth of field and hyperfocal distance is also different when the same lens is used on a digital SLR as compared to a 35mm SLR. The information below will work for all digital SLRs with a FOV crop of 1.5x or 1.6x. The version of this article for full frame digital SLRs, or 35mm film cameras is here.
Since there are several combinations of apertures and shutter speeds that will give the same exposure on film (see the exposure article), how does a photographer choose which combination to use in a given situation? The answer to this question is what gives photographers creative control over their images. It is the specific combination of aperture and shutter speed that a photographer chooses that determines the depth of field in the image, as well as subject motion, if any.
Depth of Field (DOF) is how much of the image from near to far looks sharp. A landscape photo in which everything looks sharp (from the flowers a few inches from the camera lens to the mountains far in the distance) is said to have a lot of DOF. A photo of a field of flowers in which just one flower looks sharp and every thing else is soft and blurry is said to have very little DOF. The wonderful thing about photography is that the photographer gets to choose how much DOF there is in the photo, subject to certain limitations.
Unfortunately, DOF can't be determined by simply looking through the camera lens. Whenever you look through the viewfinder of a single lens reflex camera, you are looking through the lens, BUT you are looking through the lens at its widest aperture. If you take the picture with the camera set at a smaller aperture, the DOF will be different than what you see looking through the lens.
The exception to this is if you have a DOF Preview button on your camera. A "Depth of Field Preview Button" is one of the most valuable features a camera can have. Some inexpensive cameras have DOF Preview, and some expensive ones do not. The next time you purchase a camera body, you should consider getting one that has DOF Preview.
By pushing the DOF Preview button when looking through your camera lens, THE LENS STOPS DOWN TO THE TAKING APERTURE so you can see the final look of your photograph. Because the lens is stopped down when you check DOF, what you see through the viewfinder will look pretty dark. Some photographers put a dark cloth or jacket over their head and the camera when shooting outside in order to see the dark image though the viewfinder a little better. Yes, this looks just like large format photographers under a dark cloth while looking at the ground glass on the back of their camera.
What creates depth of field? A point of light that is properly focused on the digital sensor (or film plane) becomes a point of light on the sensor. If the point of light is not focused on the sensor, it will create a circle on the sensor/film, called a "circle of confusion". The more out of focus the point of light is on the sensor, the bigger the circle of confusion. Most human eyes can't distinguish between a point and a circle that is smaller than 1/100 inch. As long as the circle of confusion on the sensor/film is not enlarged beyond 1/100 inch on the final print, it appears to be in focus. This is what makes DOF possible. An image captured on film or a digital sensor is made up of lots of points of light on the sensor/film. Even though the camera lens is only focused on one plane at a certain distance from the lens, if the other points of light form small enough circles of confusion on sensor/film, they will appear sharp to the human eye when a print is created.
DOF is a function of how much those points of light are magnified or enlarged when the circles of confusion are enlarged on the final print. That is why the same image may appear to have enough depth of field on an 8x10 enlargement but not on a 16x20 enlargement. The more the circles of confusion are magnified, the less depth of field there appears to be in the final print.
Practically speaking, there are three things photographers can do "in the field" to control DOF.
1) With the same lens at the same focused distance, DOF INCREASES WITH SMALLER LENS APERTURES. F/8 has more depth of field than f/4. F/16 has more DOF than f/8.
2) With the same lens and the same aperture, DOF INCREASES AS THE FOCUSED DISTANCE INCREASES. If you are four feet from the subject your lens is focused on, you will have more DOF than if you are 2 feet from the subject your lens is focused on. If you back up to 8 feet from the focused subject, you will have more DOF than if you are 4 feet from the focused subject.
3) With the same focused distance and the same aperture, SHORTER LENS FOCAL LENGTHS HAVE MORE DOF THAN LONGER LENS FOCAL LENGTHS. If you have three lenses (24mm, 50mm, and 100mm), all set at f/8 and all focused on a subject 10 feet away, the 24 mm lens will have the most DOF and the 100 mm lens will have the least. The 50 mm lens will have more than the 100 mm but less than the 24 mm.
If you want to increase DOF, use a small lens aperture, use a wider angle lens, or get farther away from the focused subject, or a combination of all three. To minimize DOF, use a longer focal length lens, use a wider lens aperture, or get closer to your subject, or a combination of all three.
With the exception of close-up or macro photography, you have more depth of field behind the point of focus than in front of the point of focus.
One very handy way to achieve a lot of depth of field for landscape photos, even if your camera does not have a depth of field preview button, is to use the hyperfocal distance for the lens and aperture in use.
With a given lens and a given aperture, the HYPERFOCAL DISTANCE IS THE FOCUSED DISTANCE THAT WILL KEEP EVERYTHING LOOKING SHARP FROM HALF THE HYPERFOCAL DISTANCE TO INFINITY. It gives you the maximum amount of depth of field you can achieve with that lens at that aperture.
DOF and print size.
DOF also depends on how large a print you make from your original slide or negative. The bigger the print you make, the less DOF you have since you are making the circles of confusion bigger. The DOF scale on many lenses will give you good DOF for enlargements up to 8x12 inches when used on 35mm film cameras of cameras with full frame digital sensors (24 x 36mm). If you want to make bigger enlargements, set your lens for one or two apertures smaller than the lens scale indicates. This will give you some extra depth of field. If the lens scale says you will have adequate DOF at f/8, set the infinity mark opposite f/8 on your depth of field scale just as if you were going to use f/8, BUT turn the aperture ring to f11 if you want a bigger enlargement. The depth of field scales on lenses originally designed for 35mm film cameras are not accurate for use on digital cameras with a 1.5x or 1.6x field of view crop. (As you will read at the exposure article at this site, any time you change the lens aperture, you will need to adjust the shutter speed to have the right exposure).
You can figure out the depth of field mathematically, design your own depth of field chart, or use the one I provide at this site.
The hyperfocal distance is a mathematical calculation based on the film or digital sensor size, lens focal length, f-stop and desired enlargement size.
More information on this formula can be found in Roger Hicks article in SHUTTERBUG, February 1993, pages 42-49. This formula will work with all formats but the values for C change for each format size.
Using the Formula
Example: For an 11x14 enlargement (C = .0137), from a photo taken with a 50 mm lens and the aperture set at f/8:
If you don't want to create your own hyperfocal distance chart, you can use mine. It is based on a value for C that will give 11x14 or 11x16 inch enlargements from an image captured on a digital sensor with a field of view crop of 1.6X.
My new photography book, Digital Photography Exposure for Dummies, has two complete chapters (a total of 40 pages) devoted to an in depth look at depth of field.
Copyright © Jim Doty, Jr. All rights reserved.