Computers store information digitally so it is natural to think
that a digital camera would be the ideal way to make pictures
that can be edited and printed with a computer. There is nothing
like the convenience of a digital camera to produce pictures quickly.
The advantages in producing illustrations for presentations are
enormous but the advantages of speed of production and convenience
of handling soon fade when high quality images are required.
The sensitivity of a digital camera's sensor unit is not very high. It suffers from the same constraints as film does to a degree. The amount of output from a sensor cell is determined by its basic sensitivity and its area. The larger the area of a cell, the more light it can collect in a given time. This directly compares with grain size in films. The compromise usually reached is a cell size that has a sensitivity of around ISO 100 film.
The digital sensor has two disadvantages when compared with film. A colour film is made with the three colour sensitive parts arranged in layers, one on top of another. The layers are thin so light can pass through the upper layers to the lower ones. A digital sensor has to arrange the colour cells side by side in triangles. Three cells have to be used separately to collect the red, green and blue light for one digital pixel. This decreases the maximum possible resolution available. The second disadvantage is the fact that all the data from all the cells in a sensor has to be collected and stored in the correct order. This makes the cost of sensors increase according to the square of the number of cells. With film the cost is more or less related to the area of film. These factors combine to make digital cameras with higher resolution sensors disproportionately expensive.
Computer screens are usually organised to provide a level of resolution that is proportionate to the size of the sceen. A small screen can display 640 by 480 pixels. A medium size screen can display 800 by 600 pixels. A large screen can display 1024 by 768 pixels or more. All computer screens are organised to provide the optimum quality of picture within their resolution limits. Pictures produced by digital cameras always look good on a computer screen because the screen resolution is comparable to the camera's resolution.
Computer printers have to produce text and images that look as sharp as possible. A resolution of between 600 and 720 dots per inch (dpi) is adequate for text and basic graphics. A resolution between 1200 and 1440 dots per inch is needed to provide photographic quality images. A printer can put dots one on top of another to produce the right colour and density. Each dot can represent a pixel. 1024 pixels represent a distance of 0.71 inches on a 1440 dpi printer. However, printer makers strive to increase the number of dots per inch to get the smoothest tones possible. Fewer dots mean a grainier picture. If two or three dots are used to represent one pixel from the original image, the sharpness is noticeably degraded with a high resolution printer. A digital camera with a resolution of 1024 by 768 pixels can produce an acceptably sharp picture up to around one and a half inches wide. A camera resolution of 2048 by 1536 (3 megapixels) is OK for pictures up to three inches wide - at a quality that matches good film prints. If a lower quality can be tolerated, the pictures can be printed larger. Only a few digital camera makers offer the three million pixels needed to match the quality of a typical colour print - at smaller than postcard size.
A 35mm colour print negative can provide 5.5 million pixels. A professional 35mm colour slide film can provide 22 million pixels. A Hasselblad negative can provide 33 million pixels and a Hasselblad transparency can provide around 66 million pixels. A Pentax 6 X 7 negative can provide 42 million pixels and the transparency can provide over 80 million pixels. A 5 inch by 4 inch plate camera negative can provide 130 million pixels and the transparency can provide 260 million pixels.
The super pictures provided on CD with Epson printers look as if they were taken by a 5 X 4 plate camera using a film capable of providing at least 130 million pixels. A 9 inch by 7 inch picture printed by an Epson Photo 750 printer has 9 X 1440 X 7 X 1440 = 130,636,800 pixels
Digital cameras have a long way to go to catch up with professional film cameras.
Wilf James March 2001