Comparing digital and film-based cameras

In terms of gathering and focusing light, analogue and digital cameras are nearly identical. Both must let in the proper amount of light for the proper time, then focus it, an operation that requires a carefully coordinated combination of the following features:

Shutter

• Analogue
  The shutter on a camera keeps light from entering the camera except at the desired time (sometimes just thousandths of a second). If a camera lets in light for too long, the image is over-exposed (resulting in photos that are too bright or too white). If it's open for too short a time, the photo will be underexposed (too dark).
• Digital
  Some digital cameras don't use shutters, but combining digital technology with a mechanical shutter tends to yields higher-quality images.

Aperture

• Analogue
  The aperture of a camera functions by widening and narrowing according to the overall level of light. A narrow aperture lets in less light so that the film is not over-exposed.
• Digital
  Digital cameras have mechanical openings to adjust the iris size.

Lens

• Analogue
  The lens takes the light that enters the camera and focuses it onto the camera's film through a process called refraction. Adjusting the focus of the camera actually moves the lenses.
• Digital
  Some digital cameras can override the fixed focus for special shots such as close-ups.

Film Speed

• Analogue
  A film's speed is a way of describing its sensitivity to light. The more sensitive (or faster) the film, the faster it reacts when it comes into contact with light.
• Digital
  Instead of film, digital cameras have an image sensor built with a single overall sensitivity to light, equivalent to ASA 100-speed film in most cameras. The camera's built-in computer can enhance images by removing the blur and thus effectively raising (or lowering) the light recorded in the image sensor.

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Some or all of these functions may be fully automatic in some digital cameras, much like film cameras. However, professional digital cameras are designed to allow for manual adjustments to shutter speed, aperture and focus.

Recording Light

• Analogue
  Black-and-white film is coated with what is called an emulsion layer, which when exposed (i.e., the shutter opens and lets light hit the film) changes the halide crystals chemically. The developing and printing processes translates this into an image. Colour film has three emulsion layers, each one reacting to a primary colour of red, green or blue light. Coupler dyes mix to approximate the actual colour of the light that first hit the film.
• Digital
  The image sensor in the digital camera is made up of thousands of photosites which turn light energy into digital information. By combining information about hue and intensity, the camera assigns a specific colour to each pixel (short for picture element). A pixel is the smallest unit that makes up an image.

Creating an Image

• Analogue
  In the development process, film is bathed in chemicals to form pure silver. The parts with the least exposure are the most transparent, and the parts that were most exposed to light are black or opaque. This same process is true for colour film, except that the dye couplers are also included in the process. The film is "fixed" to prevent further chemical reactions, creating a negative. To print, a very bright light is shined through the film onto the photographic paper, which is covered with an emulsion layer very similar to that of film. The negative image now becomes a positive creating the photograph.
• Digital
  A digital camera records light electrically. This "information" then becomes an image. A microchip inside the camera converts the digital reading from each individual sensor and combines it with information from the surrounding photosites. A colour is assigned to a particular pixel. Thousands or millions of pixels are combined into a single computer file which can then be downloaded.

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