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In the early 1980's the future of conventional silver-halide photographic systems was of great concern due
to the potential introduction of electronic imaging systems then typified by the Sony Mavica analog
electronic camera. The focus was on the quality of film-based systems as expressed in the number of
equivalent number pixels and bits-per-pixel, and how many pixels would be required to create an
equivalent quality image from a digital camera. It was found that 35-mm frames, for ISO 100 color
negative film, contained equivalent pixels of 12 microns for a total of 18 million pixels per frame (6 million
pixels per layer) with about 6 bits of information per pixel; the introduction of new emulsion technology,
tabular AgX grains, increased the value to 8 bit per pixel. Higher ISO speed films had larger equivalent
pixels, fewer pixels per frame, but retained the 8 bits per pixel. Further work found that a high quality 3.5"
x 5.25" print could be obtained from a three layer system containing 1300 x 1950 pixels per layer or about
7.6 million pixels in all. In short, it became clear that when a digital camera contained about 6 million
pixels (in a single layer using a color filter array and appropriate image processing) that digital systems
would challenge and replace conventional film-based system for the consumer market. By 2005 this
became the reality. Since 2005 there has been a "pixel war" raging amongst digital camera makers. The
question arises about just how many pixels are required and are all pixels equal? This paper will provide a
practical look at how many pixels are needed for a good print based on the form factor of the sensor (sensor
size) and the effective optical modulation transfer function (optical spread function) of the camera lens. Is
it better to have 16 million, 5.7-micron pixels or 6 million 7.8-micron pixels? How does intrinsic (no
electronic boost) ISO speed and exposure latitude vary with pixel size? A systematic review of these issues
will be provided within the context of image quality and ISO speed models developed over the last 15
years.
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