At the heart of the cost equation is chemistry. Traditional, 150 year old photography depends on the effects of light on compounds of silver both to capture the image and to print it. Taking pictures digitally uses silicon chips to capture the image and iron oxides for storage. The difference in price between silver and iron oxides (exotic forms of rust, really) makes the per picture cost of digital far lower. (Provided, naturally, enough pictures are taken to justify the cost of the computers involved.)
There are lots of other arguments for going digital. It is far easier to adjust a digital image - to fiddle with the colour balance, contrast and brightness. Once it is in a computer and loaded into a suitable program, a photograph can be blurred or sharpened, given a halo effect or jazzed up in dozens of other ways. If the first effect is unsatisfactory it can be done and redone endlessly without recourse to a negative, an enlarger, baths of expensive chemicals and even more expensive paper.
Even the seedier side of photo adjustment: painting out a former colleague from a group photograph in the Soviet fashion, or making a new housing development look larger by populating the pictures with three quarter size humans and cars, becomes much easier. It was never true that the camera does not lie, but a digital picture and a copy of Adobe Photoshop puts Olympic class fibbing within reach of anyone prepared to learn the program. And once a picture is perfect it can be reproduced again and again with no loss of quality.
More and more photographs are being reproduced on digital media - published on the Internet or used in disk based catalogues. Even if they are published on paper, most are handled in computerised prepress systems before publication. In either case, including a traditional photograph means scanning it - one extra step which costs money and possibly quality.
So why aren't all photographs taken on digital cameras instead of captured on film? This is a contest of old and new technology - and the new has yet to become superior in all regards. If a picture is to be passed around a family gathering, or sent by post, or greatly enlarged, then it is difficult to beat the range of tools available to traditional photography. Only very specialised and expensive colour laser printers can recreate the colour saturation, tonal range and resolution of a print from a film negative.
Until very recently digital cameras were expensive, cumbersome, limited in capability - or all three. They are getting cheaper, litter and more useful, but they still have not evolved as far as the traditional camera has. Even digital cameras intended for the professional have limitations. Many have a refractive period of a second or several seconds between shots - something totally unacceptable to a press photographer trying to cover a sports event, for example. Others add too much weight or cost to the tools a photographer is accustomed to.
Gradually, bit by bit as it were, the digital camera is moving into new areas.
. An estate agent taking pictures of dozens of houses for a large advert can save time and money by eliminating the developing, printing and scanning of images on a roll of film.
. Those who need instant pictures of limited quality for a personnel or student database have a cheaper and faster alternative to Polaroids. An added advantage is that the resulting pictures can be distributed over a company or campus network far more readily than prints can.
. Someone illustrating a Web page with shots of products, places or people would be crazy to go through the extra steps traditional photography imposes to end up with a digital image displayed at a computer screen resolution which overcomes any differences in quality.
In areas like these, digital cameras are now providing serious competition to their film based ancestors. Companies like Kodak and Casio are improving the features and cutting the prices of digital cameras with their latest models and it is now likely that in 20 years' time a camera with film will have the same quaint associations as a car with a sorting handle. .