Sunday, 30 June 2013

Lens Depth of Field Scales. 1. Hyperfocal Focusing and Circles of Confusion

I know it is difficult to incorporate depth-of-field scales in modern zooms but even on modern prime lenses (like my Nikon f1.4 50 mm) they are rudimentary to the point of uselessness). And I miss them. One of the great advantages of a proper depth-of field-scale is the rapid setting of the hyperfocal distance without having to remember what the hyperfocal distance is. All you have to do — and it seems that very few photographers know this — is decide what the near point of acceptable focus is (by focusing on that point and reading the focus scale), swing the focus ring so that the ∞ mark lines up with a particular aperture (e.g. f/8) on the depth-of-field scale, then look at the other side of the scale to see if at that aperture the near point shown on the focus ring is within the range of acceptable focus. If it is, then set that aperture for exposure. If it is not then move the ∞ mark to larger aperture number (i.e. smaller stop, e.g. f/11) and try again until the near point is within the range. The whole process is so rapid that it is easier to do than describe.

These photographs of the focus ring and depth-of-field scales of an f/2 50 mm Pancolar lens illustrate the point. In each case the ∞ mark on the focusing ring is swung to the point indicated by the aperture (f/8 in one photograph, f/11 in the other). It can then be seen easily that the range of acceptable focus is from about 12 feet (about 3.5 metres) to infinity at f/8. The focus mark is at the hyperfocal distance but you didn’t need to remember that or look it up in tables or even in a depth-of-field app to get there.

Similarly, in the second photograph, using f/11, the depth-of-field extends from about 8 feet or 2.5 metes to ∞.

Here, the aperture is f/8 (lower ring). the focus (upper ring) is swung so that
the ∞ mark is opposite f/8 on the depth-of-field scale (middle). The point of
acceptable near focus can be read from the focusing ring at the f/8 mark on
the depth-of-field scale. The point of focus (orange arrow) is at the
hyperfocal distance.

Same as above, this time using f/11

Now, the interesting point is that you will not get the same result using a modern depth-of-field calculator for a full-frame 35 mm lens. Instead of 12’ to ∞ at f/8 we get 17’ to ∞, and instead of 8’ to ∞ at f/11 we see 12’ to ∞ on a calculator. The explanation is simple. The depth-of-field scale markings on an individual lens depend on the value taken for the circle of confusion used by different manufacturers at different times. For the Pancolar made by Carl Zeiss Jena in the 1960s shown in the illustrations above, the circle of confusion was taken as 1/20 mm (i.e. 0.05 mm). This is what Werner Wurst had to say in the 9th edition of his Exakta Manual (2nd English edition, 1966):

...a circle of confusion of 1/20 mm is still recognized as sharp for the 24 x 36 mm format. The tolerances of unsharpness in which the depth of field scales of the EXAKTA Varex lenses made in Jena and Görlitz are based are derived from these data. However, other optical manufacturers reject as unsharp anything larger than 1/25 mm [0.04 mm]. In actual practice these variations can be ignored; however, bigger variations in either direction can no longer be tolerated. Thus, the assumption of a still smaller circle of confusion (e.g. 1/30 mm [0.03 mm] would unduly restrict our possibilities during focusing while larger diameters (e.g. 1/10 mm [0.1 mm] will be immediately be recognised as unsharpness.

It is interesting that the current practice is to use 0.03 mm for the full-frame (24 x 36 mm) format. Thus, depth-of-field is taken to be shallower than was once regarded as perfectly acceptable by a major lens manufacturer. Whether we can discern a difference is another matter.

What we can do is to compare the reading for near acceptable focus and hyperfocal distance obtained with a depth-of-field scale on an old lens with those values calculated using an iPhone app and the like. By using different circles of confusion in the calculation, it is easy to determine what value was being used by a particular lens/camera maker at the time of manufacture. You can then decide — provided you are using the lens with a sensor of the same format for which it was designed — whether to stick with the depth-of-field scale on the lens or move to a smaller circle of confusion and a shallower calculated depth of field.