CONTROLLING DEPTH OF FIELD
Note to digital camera users: This article was originally written with 35mm film cameras in mind. The principles involved are the same for all film and digital formats, but the values for C in the formula below only apply to 35mm film cameras and digital camera bodies with a "full frame" sensor. If your digital camera has a full frame sensor (approximately 24x36mm in size), stay on this page. If you have a digital camera with a field of view (FOV) crop, a digital version of this article appears here.
For the most detailed information on depth of field, 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.
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 the amount of near-to-far sharpness in an image. 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) has 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 has very little DOF. The wonderful thing about photography is that the photographer gets to choose how much DOF there is, 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 saw 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, what you see through the viewfinder will look much darker than usual when you are checking DOF.
What creates depth of field? A point of light that is properly focused on the film plane becomes a point of light on the film. If the point of light is not focused on the film plane, it will create a small circle on the film plane, called a circle of confusion. Most eyes can't distinguish between a point and a circle smaller than 1/100 inch. As long as the circle of confusion on the film is not enlarged beyond 1/100 inch on the print, it appears in focus. This is what makes DOF possible. An image on film is made up of lots of points of light that show up on the 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 film, they will appear sharp to the human eye.
DOF is a function of how much those points of light are magnified or enlarged when the circles of confusion on film (or the digital sensor) 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 are 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 closeup 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 focal length and a given lens 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.
Using depth of field scales on lenses to set the hyperfocal distance
Some lenses have depth of field scales marked on the lens barrel. This makes finding the hyperfocal distance easier. A lens is focused at its hyperfocal distance when the infinity focus mark is opposite the f stop in use on the depth of field scale on the lens. The distance opposite the same f stop at the other end of the depth of field scale shows the near limit of the depth of field. Everything in between will appear in focus. The distance opposite the focus mark (not the depth of field marks) is the hyperfocal distance for that lens at that aperture.
This is a 24 mm wide angle lens. The aperture is set to f/16 and the lens has been set with the infinity mark opposite the right hand f/16 in the depth of field scale. This sets the lens at its hyperfocal distance for this f-stop. The hyperfocal distance is a little less than 5 feet (the green distance mark above the orange index line). The left hand depth of field scale for f/16 is at a distance of a little less than 2.5 feet. Everything from 2.5 feet to infinity will appear sharp in the final photo.
If you have a lens with a good DOF scale, it will help you quickly set your lens at its hyperfocal distance.
NOTE: If you are using these lenses on a digital camera body with a field of view (FOV) crop, the depth of field scales WILL NOT WORK. You will need to use a hyperfocal distance chart. Read more here and use the digital chart here.
If you have a "full frame" digital SLR, the scales on your lenses designed for 35mm film cameras WILL work on your digital SLR.
DOF and Print Size
DOF also depends on how large a print you make from your original slide or negative. The bigger you the print you make, the less DOF you have since you are making the circles of confusion larger. The DOF scale on many lenses will give you good DOF for enlargements up to 8x12 inches. 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 f/11 if you want a bigger enlargement. (You know from the exposure article at this site that any time you change the lens aperture, you will need to adjust the shutter speed to have the right exposure).
What if your lens doesn't have a depth of field scale? 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 film format 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. Unfortunately, I no longer have access to this article.
If you don't want to create your own chart, you can use mine.
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.