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Introduction to digital photography:
Differences between analogue and digital

For over 150 years, photography has been a chemical process. Images are captured on photographic film. This is made up of layers of light-sensitive silver halide emulsion coated on a flexible base. Film is exposed to light in a camera. This creates a latent image, which is made visible by immersion in a solution of chemicals called a 'developer'. Prints are made by projecting the image from the film on sensitised paper and processing the material in a series of chemical baths. Much of the processing of both film and paper must take place in darkened rooms to avoid extraneous light reaching the sensitised emulsions.

Digital photography has changed all this. There is no need for film, chemicals or darkrooms. Images are captured with arrays of photo sensors and are processed by computer software. Prints are made by firing tiny jets of coloured ink or dyes at paper.

But the real future of digital imaging lies in the way it integrates with other technology. You can send images to clients using e-mail. You can post a gallery of your photographs on the internet. You can import images into a range of computer applications to create presentations, newsletters, magazines, and much more. And this is just the start. Already it is possible to have your images sent automatically to a picture desk or photo agency as you shoot. Who knows what the future will bring?

Digital advantages

Although there are still photographers using film, most professionals have switched to digital. There are good reasons for this.

Digital cameras provide real instant photography. Within a second or two of the exposure, you can see the captured image on the built-in LCD screen (or even before the exposure with the Live View mode of cameras like the EOS-1D Mark III). You can decide there and then whether you want to keep or erase the image. You can also use histogram displays to determine if the exposure is correct. If not, it may be possible to re-shoot the subject.

Images are captured as digital files and stored on removable media cards. Unlike film, the cards are reusable. Once the files have been transferred elsewhere, you can erase the images from the card and reuse it again. This cuts out all the film and film processing costs.

A digital file is data, no different to any other computer file. It can be saved to any computer storage media. The file can also be copied and recopied without any loss of quality. Copies can be kept in more than one picture library, or in other locations, all giving high-quality images.

Image files can be opened on a computer using imaging software such as Adobe Photoshop. This allows dust spots and other minor blemishes to be removed quickly and easily. It is also possible to make more significant changes to an image, but this may not be acceptable in areas such as news, sport and wildlife.

Almost all newspapers, magazines, books, brochures and other printed materials are now created on computers and use digital image files for their photographs. To meet this demand, most picture libraries now only accept digital images. Although it is possible to scan film transparencies to create digital files, it is more convenient to shoot digitally in the first place.

Analogue photography

It has become fairly common to describe film photography as 'analogue', to differentiate it from digital photography. In the sense being used here, analogue refers to a signal where the output is proportional to the input. A light meter is a good example of an analog instrument. Light falling on a photocell generates an electrical current which moves a needle across a scale. The brighter the light, the greater the movement.

On this basis, the sensor in a digital camera is also analog. Each of the many millions of pixels which make up the sensor is a light-sensitive photocell which generates a tiny electrical current in response to light. The brighter the light, the stronger the current. Digital only comes into play when the brightness levels are coded into the binary system (the language of computers) to create an image file.

To avoid the confusion of analogue and digital, the French use the word 'argentic' to describe non-digital photography. Argentic means silver and is used because of the silver halide crystals that make up the film emulsion.