MORE ON DEPTH OF FIELD
Another article on depth of field
(DOF) at this site explains how to use hyperfocal distance as a tool to control depth of field.
DEPTH OF FIELD WITH INFINITY FOCUS
One approach to depth of field is to focus on infinity and set a small enough lens aperture so everything important in your photograph will be
resolved. Look at the smallest subject in your photograph that you want to be resolved. If you can set a lens aperture whose diameter in millimeters is as small as the smallest subject to be resolved in your
photo, you can set that lens aperture, focus the lens on infinity, and everything bigger than the diameter of your lens aperture will be resolved.
For example lets say you are taking a photograph with a 50
mm lens. There is nothing in the photograph smaller than 1/4 of an inch (about 6 mm) that needs to be resolved on film. If you set the lens at f/8, the diameter of the lens aperture will be 6.25 mm (50 mm
focal length divided by f/8) and focus on infinity, everything 6.25 mm and larger will be resolved on film. Go out and try it!
DOF AND NON-INFINITY FOCUS
What if the smallest subject to be resolved in your photograph is smaller than the diameter of the lens aperture? A
different approach is required. You will need to focus at a different point than infinity.
Whatever you focus on will be sharply resolved on film (if you
do everything else right!!), even if the subject is much smaller than the diameter of your lens aperture. The father away from the point of focus, the less sharply everything will be resolved. The questions are
"How far away from the point of focus?" and "What size subjects will be resolved?"
First the basic principle, and then some examples. The ratio of the subject size to lens opening is a fraction,
F that you can use to determine depth of field. Multiply F
by the focused distance, and it will tell you how far in front and behind the focused distance that your subject will appear sharp.
Put as two formulas, we have the following:
F = S/O
Near depth is D - (D)(F)
Far depth is D + (D)(F)
F is our desired Fraction. S is the smallest Subject size we want to show up in our photo. O is the diameter of the lens Opening. D is the focused Distance.
This should make more sense with a few examples.
Let us say you want to focus on some narrows reeds that are 12 feet away. The reeds are about 3 mm thick. You want to use a 100 mm lens at f/11. The diameter of the lens opening will be 9 mm (100 mm divided by
f/11). The ratio of subject size to lens aperture is 3 mm to 9 mm or 1/3. The ratio of subject size to lens opening is the key fraction we will be working with. This fraction will tell us how far on
either side of our focused distance our chosen subject size will be resolved. Multiply the focused distance by this fraction to determine how far on either side of the focused point our subject will be resolved.
Our focused distance is 12 feet. Our 3 mm subject will be resolved at a distance of 12 feet plus or minus 1/3 of 12 feet. Our depth of field for subjects 3 mm or larger will be from 8 feet to 16 feet
(from 12 - 4 to 12 + 4).
Suppose we take the same situation (100 mm lens, 12 foot focus distance, and 3 mm wide subject). We choose an aperture of f/16
instead. The diameter of our lens opening will now be about 6 mm (100/16 and rounding off a little). Our subject to lens opening fraction will be 1/2 (3 mm / 6 mm).
Now the distance at which our subject will be resolved is 12 feet plus or minus 1/2 of 12 feet. Our practical depth of field for 3 mm subjects at f/16 will be from 6 feet to 18
feet (from 12 - 6 to 12 + 6).
One more example. The smallest subject you need resolved is 4 mm in diameter. You are using an 80 mm lens at f/4. The
lens opening will be 20 mm (80/4). The fraction of subject size to lens opening is 1/5 (4/20). You want to focus at 10 feet. The depth of field for 5 mm subjects will be 10 feet plus or minus 1/5 of 10 feet.
In this example, anything 5 mm or larger will be resolved on film from 8 feet to 12 feet (from 10 - 2 to 10 + 2).
You will need to adjust aperture and
focusing distance to make sure everything you want to appear in focus actually ends up in focus.
You can find more information in
Harold Merklinger's series of articles: Adjusting Depth of Field.