How Digital Cameras Work
Over the past few years the digital camera has become very popular with everyone from the amateur photographer to the professional. The digital camera provides the ability to instantly view a photo and either delete or keep it. How is a digital camera able to take and store a photo without the use of film? This post will explain how a digital camera is able to do that.
The Image Sensor
Digital cameras include a digital sensor that converts light into electrical charges. There are two types of image sensors found in digital cameras: charged coupled device (CCD) and complementary metal oxide semiconductor (CMOS). Most digital cameras use a CCD image sensor so we’ll focus on that type.
The CCD has millions of elements that are light-sensitive. Each one of these elements represents a single point in the picture, which is called a pixel in computer terms. For example, a 6 megapixel camera would have approximately 6 million pixels. A megapixel is equivalent to 1 million pixels.
When light hits one of these elements an electric charge is created with the strength of the charge determining the brightness of the pixel. No charge produces black while a full charge produces white. As you can see, a CCD produces shades of gray and not colour. Colour is produced using a filter within the digital camera.
There are many kinds of filters that can be used within a digital camera to generate colour, but the most common one is the Bayer filter pattern. This type of filter contains a series of rows with one row alternating between red and green, while the next alternates between blue and green. You’ll notice that green is in all rows, why is that?
The human eye is more sensitive to green colours than to red or blue. To ensure that the images appear as "true colour", there are more green pixels than red or blue. In other words, there is the same amount of green as there are blue and red combined. The image below shows an example of a bayer filter.
When a picture is taken, the raw output from a bayer filter is a mosaic of red, green and blue pixels. The digital camera then uses a demosaicing algorithm to convert this image into true colours. Each pixel in the filter can be used more than once, and the colour of each pixel is determined by averaging the values from the surrounding pixels.
Once the image is generated from the CCD and coloured by the filter, the camera than applies any white balancing, sharpening, contrast, and other settings specified by the photographer. This step is skipped if the picture is to be saved in a RAW format.
Storing the Image
Early digital cameras stored the images on memory within the camera. As digital cameras evolved, new types of removable memory became available. Digital cameras today can use one or several types of memory including:
- Secure Digital (SD)
- Memory Stick
- CDs or DVDs
Some cameras can also accept two types of memory, such as CompactFlash and SD, although most only accept one type. No matter what type of memory your camera uses, you will need a lot of room to store the pictures, especially when storing the images in one of the uncompressed formats.
The most common file format used by digital cameras is JPEG. This is a compressed format commonly used for photographs. JPEG files use a lossy compression, meaning that data in the picture is discarded when the file is compressed. The more a file is compressed, the more data is lost and JPEG artifacts appear. This also means that the file size is smaller. Your digital camera has built-in settings that can be used to control the amount of compression.
Another common format used by digital cameras is the TIFF format. This is usually an uncompressed format which results in much larger file sizes. Unlike JPEG, however, a picture in TIFF format doesn’t lose any data. You trade memory space for quality.
Some of the higher-end cameras can also save pictures in a format called RAW. In this format, no processing (white balance, sharpening, and contrast) is done by the camera. This format allows complete control for those that like editing images in photo editing software. These files are larger than JPEG but are much higher quality. One of the biggest differences between RAW and the other formats is that RAW is proprietary. This means that only specific photo editors can be used to modify a RAW file.
Once a picture has been saved to the memory card, the next step is transferring the pictures to a computer.
Digital cameras can store hundreds and possibly thousands of images on a memory card. That’s great, but you would probably also like to delete the old images to make room for the new ones. This is done by transferring the images to a computer.
Included with your camera is either a USB (Universal Serial Bus) or Firewire cable. These cables are used to transfer data between devices. Your computer will have either a USB or Firewire port, or both. Simply plug in the cable into your computer and then into your digital camera. Turn on you camera and your computer will recognize the camera and allow you to transfer the files.
I use Windows XP, and when I plug in my camera it instantly becomes a drive letter under Windows Explorer. This allows me to copy the images to my computer just as I would with any file.
An alternative to using your camera to transfer pictures is to purchase a card reader. Card readers require little power (they get their power from the USB or Firewire port), and they may transfer the files faster than your camera. Without using your camera, you can also save battery power. The one big difference between the card read and the camera is that a card reader may support many different types of memory cards. This allows someone with a CompactFlash card and someone else with a SD card to use the same card reader.