Thursday, 1 August 2013

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.