The standard DOF scales were determined before World War II and were decreed as being the best resolution available at the time on a print from the newly developed 35 mm camera. This was 4 lp/mm on a print (see below) or 33 lp/mm on the 35 mm film and to date the standard has not been changed. So, if you use the DOF scales on your camera lens you are limiting yourself to DOF available with 1930’s equipment and you will be disappointed. If you don’t believe me read Camera Lens News #2 (1997) on the Ziess (Germany) web site.
Required DOF is determined by the print and the viewing distance and I will assume that your 16 x 20 prints will be viewed at the standard distance of approximately one major dimension (20”). I am not sure how familiar you are with optics but this is an optical problem and optical resolution is measured in line pairs per mm (lp/mm) and this is half the number of pixels per mm if you refer to digital systems.
It has been well established that the human eye with 20-20 vision can resolve objects with 1 minute of arc which is 7 lp/mm (actually 6.88) or 14 pixels/mm at 250 mm (10”). Typically an adult can resolve around 5 to 6 lp/mm on a print at 250 mm (this equates to 254 to 305 dpi on the printer) and 2.5 to 3 lp/mm at a viewing distance of 20”. The magnification from say your 67 camera to the 20” print is 20x2.54/7=7.3 and so if we say we want 3 lp/mm on the print this equates to 3 x 7.3=22 lp/mm on the film (assuming no enlarging or scanning losses in resolution) and 40 lp/mm should be achievable on the film, so the resolution requirement is acceptable.
Lp/mm is related to the circle of confusion or the size of an out of focus dot and the size of this dot is 1/lp/mm and for 22 lp/mm this is 0.05 mm and so the smallest dot that we can have on the film to look like it is in focus on the print is 0.05 mm (0.002”).
The limiting factor setting DOF is not the distance (as shown on the lens scales) but the f-stop and this is why the lens manufacturers show the f-stop as well as the distance on the DOF scales, so the main thing we need to be concerned with is the aperture. The diffraction limit for any lens using white light is set by the approximate relationship 1500/f-stop in lp/mm and so f10 has a diffraction limit of 150 lp/mm, but you do not have to calculate this but you need to realise that for current film and lens technology and for optimum resolution, you should not stop down a 35 mm camera more than f8, or f11 to f16 for medium format or f16 to f22 for a 4x5 otherwise the sharpness of the photograph will be diffraction limited.
For those comfortable with math and who have the time, you can calculate everything out, however for those without the time (or can’t be bothered with the math), there are some simple rules of thumb based on the Merklinger approach. When you take a photograph, decide what is the closest thing that you want to resolve in the picture and estimate it’s size. Then stop down the lens to the aperture you are thinking of using and if the object you want to resolve is larger than the aperture then focus on infinity and take the shot or you change the aperture to suit provided you stay in the diffraction limits. If the object you want to resolve is smaller than the stopped down aperture size you will need to focus between infinity and the object.
In this case use the focusing ring on the camera (or a range finder if you have one) to estimate the distance of the nearest and farthest objects of equal size that you want to resolve (say a 5 mm (0.2”) twig in the foreground and a 5 mm (0.2”) crack in a door in the background). Now determine the distance between the objects from these estimates and the focus distance will be half way between the two objects. For example say the distance to the twig is 50 m and to the crack in the door is 150 m so the distance between them is 100 m and the focus distance is 50 + 100/2 = 100 m.
To determine the f-stop required you will need to ratio the sizes and the distances. Some people believe that long focal lenses have narrow depth of field and therefore DOF has something to do with focal length This is not true, DOF relates only to the aperture and the magnification ratio for otherwise optically identical lenses of different focal lengths. Consequently, with identical f-stops and with all other things being equal a 50 mm lens focused at 5 m has the same DOF as a 500 mm lens focused at 50 m, because the magnification ratios and the f-stops are the same.
Since the sizes of the objects in the example are the same, whatever is worked out for the front object will also be true for the far object. Hence the ratio of the twig to its distance is 5/50 = 0.1 and consequently the ratio of the focus distance to the aperture size must be the same and so the lens aperture must be: focus distance x 0.1 = 100 x 0.1 = 10 mm (in this case it does not matter if we mix m and mm). All that is necessary now is set the lens to an aperture of 10 mm, take a light reading, adjust the shutter speed, compose and take the shot.
In this example, if you are using a 50 mm lens then the f-stop will be 50/5 = f5 (say 4.5), which may not be at least two stops down from fully open. If you used a 100 mm lens the f-stop would be 100/10 = f10 and setting the lens at f8 for 35 mm would be perfect. For medium format if you used an 80 mm lens the f-stop would be 80/10 = f8, which is okay but for a 160 mm lens the f-stop would be 160/10 = f16 which is probably a bit better for 6x7. if you used a 500 mm lens then the f-stop would be 500/10 = f50, which would be well and truly diffraction limited to apply to a 35 mm film size and so you would not use this lens.
This consideration is only optical and not aesthetic and so you would have to use all your normal judgements and procedures. This procedure will give you much better results than DOF lens scales or tables and is about the best you will do with a camera that has fixed lens and film planes. It is not as complicated as it appears and once you get used to it, it will become second nature and you will hardly have to think about it. I have used a Linhof 617 for some time and it has no range finder and no way to look through the lens, consequently you are forced to use techniques such as these to get decent shots with anything in the foreground.
If you are a perfectionist and want to have better sharpness than this gives you, then I suggest that you buy a view camera and the possibilities of photography will open up to you. Just ask Alain Briot or read his “The Agony And The Ecstasy” article on Michaels web site.
This is the best I can do in a relatively small space and I hope it helps.