No reciprocity failure

Unlike film, digital sensors don't suffer reciprocity failure. This gives them a strong advantage for photographing galaxies, nebulae, and other faint objects.

Film is inefficient at recording dim light. The silver crystals "forget" that they've been hit by a photon, and revert to their unexposed state, if another photon doesn't come along soon enough. For details, see Astrophotography for the Amateur (1999 edition), pp. 180-184.

Figure 2.3 shows the estimated long-exposure performance of an old-technology black-and-white film, a modern color film, and a DSLR image sensor, based on my measurements. The advantage of the DSLR is obvious. Astronomical CCD cameras have the same advantage.

A practical consequence is that we no longer need super-fast lenses. With film, if you gather twice as much light, you need less than half as much exposure, so film astrophotographers were willing to compromise on sharpness to use f/2.8 and f/1.8 lenses. It was often the only way to get a picture of a faint object.

With DSLRs, f /4 is almost always fast enough. You can either use an f /2.8 lens stopped down, for greater sharpness, or save money and weight by choosing an f/4 lens in the first place.

2 Web address: http:llwww.canon.co.jpflmaginglastrolpagesxl09x.html.

Exposure time (seconds)

Figure 2.3. Long-exposure response of two kinds of film and a digital sensor.

Exposure time (seconds)

Figure 2.3. Long-exposure response of two kinds of film and a digital sensor.

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Figure 2.4. Left: Bayer matrix of red, green and blue filters in front of individual sensor pixels. Right: Dots mark "virtual pixels," points where brightness and color can be computed accurately.

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