Instead of film, a digital camera has a sensor that converts light into electrical charges.
CCD image sensor – charge coupled device is the most common image sensor found in digital cameras
Pros: Traditionally, the highest image quality, pixel for pixel. Current sensors include innovative chip architectures designed to enhance dynamic range or speed.
Cons: Most expensive. Most power-hungry.
A CCD sensor Photo courtesy DALSA
CMOS image sensor – complementary metal oxide semiconductor
Pros: More light sensitive and higher quality than CCD.
Uses less power. Less expensive to produce.
Cons: CMOS sensors tend to be bigger than their CCD equivalents, resulting in larger cameras.
A CMOS image sensor
Complimentary Metal-Oxide Semiconductor sensor.
This is less widely used, the main proponent of this technology being Canon, which uses it in its EOS range of digital SLR cameras.
CCD and CMOS image sensors convert light into electrons
A sensor is composed of millions of light sensitive
areas or pixels
These can be thought of as a group of buckets, into which the light falls and is trapped. The number of light rays falling into each bucket determines the brightness level at each pixel. Once the bucket is full, the light level is said to be ‘blown’. By placing a different coloured primary filter over each bucket, only light of that colour is captured. Each line of pixels has only two of the three primary colours, either red and green or blue and green.
Once the sensor converts the light into electrons, it reads the value (accumulated charge) of each cell in the image.
- CCD sensors create high-quality, low-noise images.
- CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality pixels, and more of them.
- CMOS sensors are generally more susceptible to noise.
- CMOS light sensitivity is lower.
- CMOS sensors traditionally consume little power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.
Digital Camera Resolution
The amount of detail that the camera can capture is called the resolution, and it is measured in pixels. The more pixels a camera has, the more detail it can capture and the larger pictures can be without becoming blurry or “grainy.”
Some typical resolutions include:
- 256 x 256 – Found on very cheap cameras, this resolution is so low that the picture quality is almost always unacceptable. This is 65,000 total pixels.
- 640 x 480 – This is the low end on most “real” cameras. This resolution is ideal for e-mailing pictures or posting pictures on a Web site.
- 1216 x 912 – This is a “megapixel” image size — 1,109,000 total pixels — good for printing pictures.
- 1600 x 1200 – With almost 2 million total pixels, this is “high resolution.” You can print a 4×5 inch print taken at this resolution with the same quality that you would get from a photo lab.
- 2240 x 1680 – Found on 4 megapixel cameras — the current standard — this allows even larger printed photos, with good quality for prints up to 16×20 inches.
- 4064 x 2704 – A top-of-the-line digital camera with 11.1 megapixels takes pictures at this resolution. At this setting, you can create 13.5×9 inch prints with no loss of picture quality.
- High-end consumer cameras can capture over 12 million pixels. Some professional cameras support over 16 million pixels, or 20 million pixels for large-format cameras.
Image sensor size
Image sensors come in a variety of sizes with the smallest ones used in point and shoot cameras and the largest in professional SLRs. Consumer SLRs often use sensors having the same size as a frame of APS film. Professional SLR cameras occasionally use sensors the same size as a frame of 35mm film—called full-frame sensors. (Large format cameras use even larger sensors).
Larger image sensors generally have larger photosites that capture more light with less noise. The result is pictures that are clearer, brighter, and sharper.
Because the size of photosites is so important, a large 6 Megapixel sensor will often take better pictures than a smaller 8 Megapixel sensor. Not only is noise a problem but smaller sensors also require better, more expensive lenses, especially for wide-angle coverage. Here are some typical sensor sizes: