Digital Camera (Pixels and Beyond)

Cells and Pixels

In digital cameras the light-sensitive medium is a silicon chip covered with a regular pattern of very small light sensitive circuits. When a few photons hits one of these it causes a few electrons to flow in a circuit. These electrical signals are then amplified and converted into a digital format.
Each cell in most sensors is covered by a red, green or blue filter (RGB), so that only light of that colour can reach it. The 'raw' image that contains the digital values for these separate RGB sites is then mathematically converted into pixels - values for red green and blue at a particular point.


Technical Quality

Assuming that the camera is used properly, the major factors that affect the technical quality of digital camera images are:

Number of pixels
Size of sensor
Optical quality of lens
Correct design, build and operation of focus, exposure and processing systems.
Pixel count alone gives a very poor idea of a camera's capabilities.

Pixel count

Many images used professionally have been taken on cameras with only 2-3 Megapixels. A greater number of pixels will not improve image quality if they information they provide is less reliable. A 5 or 6Mp cameras should be able to produce decent prints at up to A3 size.

Sensor size

Consumer cameras use small sensors, only a few millimetres in each dimension, making the individual pixels very small. Professional cameras use larger sensors - so that the individual cells are perhaps 5-10 times the area and are also better separated. The larger cells capture more light for the same exposure, giving electrical signals that need less amplification and have less random noise. The better separation also means there is less interference between the signals from neighbouring cells.
For small sensor consumer cameras, the increase from 6Mp to 8Mp or more seems to result in little if any improvement in image quality. With current manufacturing techniques, 6Mp is probably the optimum size for consumer cameras.

With the larger sensors in more professional digital single lens reflex cameras - such as the Nikon D70 and Canon Digital Rebel (EOS 300D) - 6Mp gives noticeably better quality images. With this size sensor, an increase to 8Mp or 11Mp does give an improvement in quality, with sensors around 18x24mm giving 8Mp probably likely to become a standard. The current high-quality 6Mp cameras are as good as 35mm film for most purposes. dSLRs are generally fairly large, bulky and expensive compared to consumer (or prosumer) cameras. You can find more about the different digital camera types in the separate features on them.

Lens quality

Lens quality also has an important influence on the image quality - there is little point in having a high-resolution digital conversion of an unsharp optical image. Lens quality can only really be assessed by practical tests, but the better-known camera and lens makers - Canon, Nikon, Olympus, Konica Minolta, Pentax, Leitz as well as Sigma, Tamron, Tokina and others - can generally be relied on to produce good quality.
Design, Build and Operation
These are qualities that can only be assessed by a combination of detailed camera testing - as carried out for the better reviews on line and in print - and also a long-term user assessment.

Viewfinder

Most consumer digital cameras also have an LCD screen that can be used for taking pictures. Some have a digital viewfinder where you look through an eyepiece at a digital image. Current models are slow to update and have poor resolution. The best viewfinders are those on dSLRs where you see an optically produced image through the lens on a viewing screen.

Storage Type

Most digital cameras use memory cards of one kind or another to store the images you take. These can be taken out of the camera and replaced by other cards. Several different card types are in use and are not interchangeable. You can read more about storage in the feature on Memory Cards.

Image Download

Digital camera images can be downloaded from the camera to a computer or printer using the cable which is supplied with the camera. Older cameras mainly used serial cables, but almost all newer cameras use a USB connection. When plugged into a computer running Windows XP or other recent operating system, the images will appear as files on another hard disk. You can also remove the storage card from the camera and read the images using a card reader. Some printers allow you to print directly from the storage card.