Video: Camcorder, Compact or DSLR? It all depends on the depth of field you want

The useful article The Definitive Camera Guide in the new magazine, Digital FilmMaker, together with a discussion in Amateur Photographer on whether to buy separate video and still cameras, prompted me to remind myself, in the headlong rush to use full-frame and APS-sized cameras for video, why a small sensor, as found in most amateur, 'prosumer' and some professional camcorders, is preferable for the type of video photography that I do – travel, wildlife and documentary. Film makers which I define as those setting out to make a movie with actors or a staged documentary, appear to be fixated on getting a film-like appearance. Since the appearance of film can be more easily achieved with the depth of field range of a larger sensor (as with 35 mm cine cameras) some follow the fashion and use a large-sensored DSLR. The fact that we see in such videos endless focus shifts that the dramatic effect is expected and therefore ruined is neither here nor there. Such is the world of arty film making.

For travel, wildlife, family and documentary videos there is one great disadvantage in using a DSLR for video. The depth of field is simply too shallow. I have seen some awful efforts. Even if the camera can be focused, the slightest movements lead to the autofocus hunting to regain focus; objects slightly in front or slightly behind the point of focus that are needed to be sharp to provide context are fuzzy. DSLR video is fine for controlled shooting. The many rigs now available can be used; focus can be pulled manually after rehearsing the shot.

Camcorders are not thought cool any more. But they do work and work very well, especially those in the prosumer ranges of the major manufacturers (even though the gimmicks, like the projectors now added by Sony for example are a real pain and very silly). The quality of the full HD video is superb. I have sold wildlife footage from my Sony camcorder.

So what is the advantage of using a small sensor for video? For stills, the bigger the sensor the better, particularly for the prevention of noise. The advantages of a smaller sensor in terms of depth of field are very great for travel and wildlife. In general, with the same final image size, depth of field increases inversely to the size of the format. That is because for the same picture size, a shorter focal length lens can be used with the smaller format. Shorter focal length equals greater depth of filed. Even though the circle of confusion has to be reduced as the format size decreases, the depth of field is still increased.

The first graph illustrates the point. I compare five formats: 1. Full-frame 35 mm; 2. APS (I know the various sensors vary in size a bit but I have taken one for comparison); 4/3 format as used on many modern non-DSLR cameras; 2/3 sensor as used on many professional camcorders; 1/2.88 sensor as used on many amateur, prosumer and some professional camcorders. I calculated for each the equivalent focal length of the lens and, using the circle of confusion appropriate to each format, read off the hyperfocal distance at f/11 and f/4. The graph shows the near point of acceptable focus (i.e. half the hyperfocal distance), the far point of course being infinity.

The difference in depth of field is clear and dramatic. At a small aperture and at focal lengths shorter than the equivalent of about 50 mm, it is difficult to be out of focus using a camera with a small sensor.

I used the same data to calculate the depth of field with the camera focused at 3 metres, again at f/11 and f/4.

It can be seen clearly why footage from a small-sensored iPhone (1/3.2 – similar to 1/2.88) often produces better results in casual photography than a DSLR.

These calculations were all for the equivalent of a 50 mm lens. At 200 mm the effects are even more marked. Imagine a subject moving (e.g. a child playing or a group of otters) in the frame of full-frame DSLR where the depth of field is only a total of 14 cm at f/11 compared with a camcorder with a depth-of field of 100 cm.

Shallower depth of field can be obtained with a small sensor (even if not to the extent possible with a large sensor) by using neutral-density filters (built-in in some professional camcorders) or a polarising filter.

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.