| Product: |
Digital Cameras in general |
| Date: |
16.01.02 (247 review reads) |
| Rating: |
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Advantages: No film used, Pictures can be revied instantly
Disadvantages: Expensive
Digital cameras have been around for several years and their picture quality can now rival that of traditional 35mm cameras. The digital camera is quite different to a 35mm camera in its workings but with the exception of the LCD display it looks the same. The two main differences are that a digital camera captures the image using a CCD or a CMOS sensor and then stores the image on memory rather than film. The main progression in digital camera technology has been the resolution. A few years ago 300,000 pixels was the norm but now a good quality camera will record 3M Pixels with the top of the range models being capable of 6M.
The way the digital camera captures the image is very different to that of a 35mm camera, rather than focusing the light onto light sensitive film which can only be used once, the picture is captured by a CCD. A CCD is a chip covered with light sensitive cells, used to capture an image. The CCD was first invented in 1969 as a way to store data however in 1974 Fairchild Electronics created a CCD which could capture an image at a resolution of 100 X 100 pixels.
The CCD is the heart of the digital camera and it's quality and resolution is one of the most important factors when buying a digital camera. The CCD captures the image by using millions of photodiodes on the surface which react to light. When a picture is taken each photodiode passes a voltage across the CCD, the higher the voltage the more light there is on that tiny area of the CCD. A typical CCD will have between 1M and 3.3M photodiodes. This voltage level is an analogue value and it must be converted to a digital value before it can be used by the camera. An analogue to digital converter (ADC) is used and each pixel is converted to a binary number.
The CCD is colour blind, it can only detect light levels and not colours, there are several ways to combat this. The first is to place a filter over each photodiode on the CCD so as only one of the primary colour
s is allowed through, this is called a Bayer filter. The image can then be put together by working out which colour each pixel should be.
Another method is to rotate a disk, with 3 filters, over the CCD. Although this means that each photosite is used to capture each colour the 3 images are recorded at slightly different times so it is not suitable for action photos. The third method is to split the light into the 3 primary colours and record each one on a different CCD. This provides a very high quality image but due to the expensive of 3 CCDs it is only used on top of the range cameras.
The number of pixels that the CCD an record is the biggest decider between different cameras. The greater the resolution the smoother the picture will look (see last page of report). The number of pixels used also effects the file storage required because more detail must be stored about each image if the resolution is higher. Some people may only need a low resolution like 640 X 480 if they only want to e-mail photos and put them on their website however people who want their printouts too look like developed photos will need 2M-3M pixels.
The more pixels used, the greater the amount of memory required. Fortunately memory sizes have kept pace with CCD technology. While most cameras have a small amount of internal storage (1MB-8MB) this can normally be supplemented by additional storage. The most popular form of storage is flash memory cards. These are small, solid state memory devices which have flash memory inside. They can be written to and rewritten to but they are non-volatile memory.
CompactFlash cards look like credit cards but are thicker. They were invented by San Disk Corporation in 1994 and use 50 pins to connect to a PC card adapter. Type I cards are 3.3mm thick and currently hold up to 192MB while type II cards are 5mm thick and can hold up to 300MB. Smartmedia was invented by Toshiba in 1996 and are smaller and lighter that Compact
Flash, they are only 0.78mm thick and weigh just 0.48g. They use a 22 pin connector which can be used in PC card slots. At present they have a maximum capacity of 128MB. There are several other formats such as memory stick (128MB), Floppy disk (1.44MB) and IBM Microdrives (1GB) but these are less popular.
Most cameras have a choice of file formats and compression options for image storage. The most common is JPEG which is a lossy compression format with varying levels of compression. TIFF is another commonly used format which is not compressed and subsequently has much larger file sizes then JPEG and slightly higher quality. A high res photo would use on average 6MB-10MB as a TIFF, 500KB-1MB as a high quality JPEG and 100KB-200KB as a very low quality JPEG. The file size is directly proportional to the resolution. The majority of cameras use JPEG and TIFF but some may use less popular formats such as FlashPIX too.
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