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Digital cameras - how they work

The basics


Digital cameras capable of high resolution can produce remarkably-detailed, sharp images, as shown by this bird's-eye view of an iris.
Digital cameras capable of high resolution can produce remarkably-detailed, sharp images, as shown by this bird's-eye view of an iris.

IF THERE IS NO FILM, WHERE IS THE IMAGE CAPTURED?

Light enters both digital cameras and traditional (film) cameras to record a scene onto a light-sensitive material. A digital camera's image sensor or picture-recording sensor, a type of light-sensitive computer chip, replaces film as the image-capturing medium.

The most common sensors are the CCD (Charge Coupled Device) and the CMOS (Complementary Metal Oxide Semiconductor) sensors. Both are chips containing a flat, grid-like field of extremely small, light-sensitive cells (photodiodes), each of which emits an electrical signal in proportion to the intensity of the light striking it. These cells each react to the different amounts of light that strike them through either a tiny red, blue or green filter that covers each cell. They then output different electrical voltages which the camera converts to digital data -- that is, numbers that directly correspond to the brightness and color of the light striking each cell. This process is known as analog to digital conversion.

The camera translates these numerical values through a process called interpolation that results in an image's pixels each being a color value that corresponds to colors in the scene. The image file (composed of all the pixels) is electronically stored on the camera's memory storage card.

A CCD or CMOS chip may be as big as one-inch by one-and-a-half inches - about the size of a 35 mm negative frame, but most are smaller than a 35 mm negative. Consider that each tiny, individual sensor cell produces one pixel of the image, and you will have an idea as to how many cells there can be in a CCD or CMOS chip, and how tiny they really are.

When you hear of a digital camera having mega-pixel capability, it means its CCD or CMOS chip has a million light-sensing cells. And if a camera has, say, 2.4 mega-pixels, it has 2,400,000 pixels, and can produce very high resolution image files. It is no longer uncommon to find consumer cameras that have 8,000,000 or 9,000,000 pixels on store shelves. These 8 and 9 MP (mega-pixel) cameras deliver extremely high resolution images. Some cameras, generally those intended for the professional photographer, have many more pixels. As examples, the Nikon D2X, which has more than 12,400,000 pixels in its CCD; the compact Samsung NX10 with 14.5 MP, or the newer Nikon D3X DSLR with 24.5 MP. Interestingly, Nokia markets a cell phone that has a 41 MP camera.

The CCD (charge coupled device) from a Nikon D2X digital camera is roughly 0.9 X 0.6 inches (23.7 X 15.7 mm) in size and has 12.4 million effective pixels.
The CCD (charge coupled device) from a Nikon D2X digital camera is roughly 0.9 X 0.6 inches (23.7 X 15.7 mm) in size and has 12.4 million effective pixels.

The amount of detail in a digital image is dependent on the number of cells, or light sensors, there are on the CCD or CMOS chip and also on the size of the chip itself. Generally, the more pixels a digital camera has, the sharper the image. Strange to say, the number of pixels in a given CCD may be slightly smaller than in another CCD, yet image quality may be better. That is because the size of the CCD itself may be significantly larger than the CCD in the camera with the greater number of pixels. The larger size of a CCD leads to higher sensitivity, and can therefore provide a higher-quality image technologically. The size of a CCD chip is usually reflected in the price of the camera.

HOW DOES DIGITAL RESOLUTION COMPARE WITH FILM?

A 35 mm negative has tens of millions of light-sensitive grains of silver halide. If you equate one pixel with one grain of silver halide in the emulsion of film, you can see that film is capable of producing more finely-detailed images than most sophisticated consumer digital cameras costing hundreds of dollars. However, you would need a very big enlargement in order to detect the difference, and most digital camera users are not seeking huge enlargements when they take pictures. Digital cameras with multi-mega-pixels sensor chips that are intended to be used by professional photographers, though, rival the quality of film.

IMAGE QUALITY AND RESOLUTION ARE DIRECTLY-RELATED

Many new users of digital cameras begin by taking as many low resolution pictures as possible and loading them into the camera’s memory card. Then later, when they download the image files onto their computer, they are disappointed at the small size of the images, or how much the image deteriorates when it is enlarged on the monitor.

The more-sophisticated professional digital cameras may offer as many as eight options for image quality and file size, but most digital cameras permit pictures to be taken at a choice of three resolutions - high, medium (or normal), and low (or basic). Some have an extra selection mode that may be called super-fine, with resolution high enough to print out sharp 8" X 10" color prints.

SHOOT AT THE HIGHEST PRACTICAL RESOLUTION FOR GOOD QUALITY

Consider your camera’s memory capabilities when selecting the resolution you intend to use. Together, image quality and size largely determine how much space each picture occupies on the camera's memory card.

The more megabytes of memory a digital camera or its memory card has, the more images you can store in the camera. If you have lots of memory or if you are using a number of removable memory cards or have a very large-capacity memory card, you should take most of your pictures at high resolution.

When you take pictures at the highest-quality (highest resolution) that your camera is capable of, you will have more image data to edit and manipulate, permitting you to print in larger sizes and also to crop images without fear of them becoming too small. You can always reduce the size of an image file, but you can’t enlarge a file without some trade-off in image quality. An image resolution of 640X480 pixels is good for internet web page viewing.

Images are stored in the camera or on removable media, such as this 16MB SmartMedia memory card.
Images are stored in the camera or on removable media, such as this 16MB SmartMedia memory card.

MEMORY CARD

Pictures taken on a digital camera are stored in the camera's memory, which is commonly a removable memory card. An image memory card is a removable image recording media developed for digital cameras. Memory cards come in varying sizes (capacities) and with different characteristics suited to a particular make and model of digital camera. Click here or on the "Memory card" link in the left-hand column for more detailed information on image memory cards.

If your digital camera has macro capability, you will be amazed at how sharply small subjects can be rendered. This is a macro picture of the center of the iris shown at the top of this page.
If your digital camera has macro capability, you will be amazed at how sharply small subjects can be rendered. This is a macro picture of the center of the iris shown at the top of this page.

VIDEO CLIPS

Many digital cameras have the ability to also capture video images with sound. A digital camera is not meant to replace a video camera. Its video clips are typically short and have low resolution, generally suitable for use on a web page. Video captures also need larger amounts of memory than are available in most cameras primarily designed to take still digital images.

 
Further information...
CCD versus CMOS sensors
Memory card
Shooting delay
Two kinds of zoom
Downloading images to your computer