Hyperfocal Distance: h = f2 / ac
f = focal length of lens
a = aperture diameter (f/stop number)
c = circle of confusion
Depth of Field, Near limit: hs / h + (s - f)
Depth of Field, Far limit: hs / h - (s - f)
h = hyperfocal distance
s = distance from camera to object
f = focal length of lens
What is Depth of Field? Depth of Field (DoF) is the distance range between between the nearest and farthest objects that appear to be in acceptably sharp focus within the image plane. DoF involves one image plane, and the area between two target planes (in front of the lens). What is Hyperfocal Distance? Hyperfocal Distance is the distance of the nearest object in sharp focus, when the lens is focused on infinity. It varies with each f/stop number. When the lens is focused on that distance, everything from one-half the distance from the camera to infinity will be sharply focused. Depth of Field is dependent upon the following variations: A. The focal length of the lens.
B. The diaphragm opening (effective aperture).
C. The distance from the lens to the object that is focused on.
D. The distance from which the image is viewed.
E. The viewer's personal standard of the permissible degree of sharpness (or unsharpness).
Other variables in the formula remaining constant, it follows that: A. The shorter the focal length of the lens, the greater the DoF.
B. The smaller the diaphragm opening (larger f/stop number), the greater the DoF.
C. The greater the distance to the object being focused on, the greater the DoF.
D. The greater the distance from which the image is viewed, the greater the "apparent" DoF.
E. An often-used standard of acceptable sharpness is the reproduction in the image of a small point in the object plane by means of a "Circle of Confusion," or disc not greater than 1/100 of an inch. This is often expressed as 1/1000 of the focal length. Sometimes a figure of 1/300 of an inch or 1/3000 of the focal length is used.
At this point in the discussion, the image size will not remain constant. If you change the focal length in the above example, the subject (Target Size) will get larger or smaller depending upon the change in focal length. How do I apply this in the real world? Much of what is written above is common practice for many of us. If I want more DoF, then I can increase my f/stop number to f/11 or f/16 or even higher. If I want less Dof, I can lower the f/stop number, add Neutral Density filters or a polarizer to reduce the light, thereby forcing the f/stop number lower. Move the camera closer for less depth of field, further away for more. But if you zoom the lens at the same time (to maintain a constant subject size), then the depth of field will stay the same. If the Target Size remains the same, by moving the camera all you have done is change perspective. In the real world (such as a news anchor person behind a desk), the Target Size needs to consistently remain the same size. As you move closer to decrease DoF, the image size will increase, so you must decrease the focal length to maintain the same Target Size. The two variables (lens-to-subject distance and focal length) cancel themselves out due to the Law of Reciprocity. What about 1/3 inch CCD's having more DoF than 35mm film? Just as the Target Size can vary as in the above paragraph, so can the size of the film stock or the size of the CCD image sensor. If the Target Size in front of the lens is to remain the same when we change CCD sizes, then DoF will indeed change. What happens when we change chip (format) sizes? If the focal length of the lens stays the same (such as a 100mm lens on 1/3 inch CCD, and a 100mm lens on 35mm film), then the Target Size will increase 7.2 times, because a 1/3 inch CCD is 7.2 times smaller than the 35mm image plane. To maintain the same Target Size, either the lens-to-subject distance must be 7.2 times greater, or you must reduce the focal length 7.2 times. By either increasing the lens-to-subject distance, or by reducing the focal length, the DoF is increased. If all formula variables stay the same and the Target Size (CCD) behind the lens changes, then DoF will not change. If the Target Size changes in front of the lens (by changing focal length or len-to-subject distance), then DoF will change. Bring me back to the real world. What does all this mean to me? The real world experience is that under most conditions (a TV set, some product shots, a speaker at a podium), the subject must consistently remain the same size due to the basic rules of image composition, head room for subject, or because the Art Director says the box will must appear so big. The Art Director also wants less DoF. So you move the camera closer. Now the AD says the box is too large. So you zoom the lens wider to make the box appear smaller. Then the AD says hey, you've got too much DoF again. By moving closer and zooming wider, you've cancelled out the change in DoF. This is the Law of Reciprocity. The only effective way to reduce DoF while maintaining the Target Size is to lower the f/stop number (f/2.8, f/2.0 etc.). In a well-lit scene, you'll need to use Neutral Density filters to reduce DoF.
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