In many cases, especially in landscape photography, we are interested to get an image which is as sharp as possible from somewhere near the camera all the way to the horizon. Common wisdom is to adjust the focusing distance to the smallest possible value such that the depth of field, or more specifically the far limit gfar, still reaches infinity
This focusing distance is known as the hyperfocal distance (literally super focusing distance, which maximizes the scene in focus).
As derived in equation (D8b), the far limit depends on the object distance g, the focal length f of the lens, the f-stop A and the chosen circle of confusion c as follows:
|gfar||=||g f2 / (f2 - A c (g - f))||(H2)|
Since the focal length is finite, the expression on the right can only become infinite if either the object distance is infinite or the denominator is zero. The latter is used to calculate the hyperfocal distance ghyper:
|f2 - A c (ghyper - f)||=||0||(H3)|
Solving for ghyper results in
|ghyper||=||f2 / (A c) + f||(H4)|
Note that ghyper is an object distance, measured from the lens. With equation (M6), the corresponding focusing distance dhyper can be derived as
|dhyper||=||f2 / (A c) + 2 f + A c||(H5)|
This is the formula used for the hyperfocal distance calculator.
For all practical values, everything but the first term in equation (H5) can be neglected, thus we get the following approximation
|dhyper||≈||f2 / (A c)||(H6)|
Since we have established that we can focus at the hyperfocal distance or at infinity (or in fact anywhere in between) with the far limit still reaching infinity, it is interesting to compare the near limits of these approaches. These can be calculated using equation (D8a).
1. For a lens focused at the hyperfocal distance, the near limit is approximately given by
|gnear||≈||f2 / (2 A c)||(H7)|
In other words, the depth of field already begins at half the hyperfocal distance.
2. For a lens focused at infinity, we get the approximation
|gnear||≈||f2 / (A c)||(H8)|
In this case, the depth of field starts at the hyperfocal distance (which is quite a coincidence).
For example, for a focal length of 50 mm, f-stop 8 and a circle of confusion of 0.033 mm, we get a hyperfocal distance of 9.6 m (31 ft) and a near limit of 4.8 m (16 ft). For the lens focused at infinity, the near limit is at 9.5 m (31 ft), somewhat further away from the camera.
With today's autofocus lenses, manually focusing at some numerical distance seems not only a bit outdated, but also rather impractical. Even more so, as manual focusing is often operated by wire, many of these lenses don't provide a distance scale any more. As a workaround, if you use the autofocus on some object a few meters beyond the calculated hyperfocal distance, you should be on the safe side.
The situation is a lot easier with dedicated manual focus lenses which usually provide a depth of field scale, such as the classic Leica M lenses (fig. 6). Left and right of the marker for the focusing distance, additional markers for the various f-stops indicate the respective extension of the depth of field. To focus at the hyperfocal distance, simply turn the focusing ring such that the infinity symbol of the distance scale is aligned with the right DOF marker for the f-stop that you are using. This ease of use has certainly done a lot for the popularity of the hyperfocal distance.
Distance and depth of field scales on a Leica Summarit-M 35 mm f/2.4 ASPH. To focus at the hyperfocal distance, simply turn the the focusing ring until the infinity symbol is aligned with the right marker for the f-stop that you are using (in this case 16). The hyperfocal distance is at about 2.4 m (8 ft), with the depth of field extending from 1.2 m (4 ft) to infinity.
So problem solved ... or is it? Read why a lens focused at infinity may actually give better results.