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The Digital Cameras Are Coming

One of the major problems with video compared to traditional film, is that it looks ‘flat', and there are very clear technical reasons for this. A frame of 35mm film is 35mm wide - hence the name, although the actual picture area is smaller, as this includes the perforations along the edge and audio track. But the sensor used to collect light in a camcorder is much smaller. Cheap consumer camcorders often use CCDs as small as 1/6in in diameter, and the largest generally in use with high-end professional models is 2/3in. This equates to 3.4mm across for the consumer models, and still only 13.5mm for the professional models, assuming a 4:3 aspect ratio.

In other words, 16mm film beloved of cinematography students is still larger than a professional camcorder sensor. Even the 8mm variety popular in amateur film cameras during the 1960s and 1970s has a much greater area than cheap consumer camcorders. In fact, it's over twice the width, and therefore over four times the area.

This has important optical implications. At the most basic level, the larger something is, the more photons will fall on it from a light source. So larger sensors are more sensitive than smaller ones, and therefore perform better in low light. This doesn't translate directly from film to CCD or CMOS, because the chemical technology of film is so different from the electronic one of camcorders, and generally less sensitive.

But there is an implication which does translate, and that is depth of field. Briefly, for those not conversant with photography, this is the range which is in focus. A shallow depth of field means that just the thing you are focusing on is sharp - objects closer or further away are blurred. Conversely, a deep depth of field is where much more of the frame is sharp.

There are a number of factors effecting depth of field, some of which are fixed for a given camera and some of which can be controlled. Aperture can be used to alter depth of field, with a smaller F-stop (wider aperture) providing a shallower focus. But the size of the sensor is one thing which cannot be changed, and the depth of field is inversely proportional to the sensor size, at least approximately. So, for the same aperture, one sensor a quarter the size of another will have four times the depth of field.

This is the main reason why video tends to look flat - a much wider band is in focus, thanks to the small sensors. Professional camcorders are often fitted with built-in neutral density filters for this reason, so that a wider aperture can be used in bright light, for a shallower depth of field. But this can only go so far, and video cameras can't hope to replicate the sharp, shallow focus possible with 35mm film, which makes artistic effects like focus pulling easier to produce.

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