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Digital Noise

By Nick Thomas - United Kingdom

Comparison of digital noise in smooth tonal areas in images from CCD and CMOS cameras.

After using a Canon D30 for some time I was struck by how smooth the images were in areas of even tone such as clear blue skies

Most comparisons between digital cameras focus on the pixel size of the chips. The S20 uses a 3.3M pixel chip, the D30 has a 3.1M pixel sensor; though in practice the actual difference in recorded images is minimal; 2048x1536 pixels (3.15Mp) for the S20 and 2160x1440 pixels (3.11Mp) for the D30.

CCD and CMOS sensors

However, pixel count and resolution isn't everything in digital imaging and both the chip type and circuitry contribute to the final recorded image, not to mention the physical size of the chip. The S20 uses a small CCD (charge coupled device) to record incident light while the D30 uses a much larger CMOS (complementary metal oxide semiconductor) imaging sensor. The large chip size in the D30 means that individual pixel elements on this chip are very much larger than on the much smaller CCD in the S20,

The major difference between CCDs and CMOS sensors is in the way the stored charges are converted into a usable signal. A CCD sensor scans its pixels consecutively. Stored charges from each row are moved down to the next row and at the bottom of the array the charges are output in a serial stream to an analog-to-digital converter. The resulting signals are then converted into an array of bytes to form the stored image. In a CMOS sensor each pixel has its own amplifier circuit, and can be read directly. CMOS sensors also support analog-to-digital converters and JPEG compression processors directly on the chip. In addition to on-chip conversion circuitry the D30 images benefit from dual sensor scanning; the sensor is read once before the shutter opens, and again while the shutter is open. Subtraction of the resulting images removes electronic sensor noise from the recorded image.

Image comparisons

A clear blue sky is just about the most uniformly coloured natural object that it is possible to photograph. There may be variation in the tone or intensity of colour across the sky, but blue skies do not have any small scale variations in tone so are a good subject for comparing image noise. 

To compare image noise between the C20 and D30 I selected at random from images taken with the two cameras that contained areas of clear blue sky. Both images were recorded at 100ASA using the highest resolution and lowest compression available in each camera. 

Both images were imported into the same Photoshop file as separate layers and the edges butted against each other. I chose images that have different tones and colours to make the edges of the images easier to see. Both images are taken directly from jpeg camera files with no post-processing.

The side by side comparisons of the two images below are taken from screen grabs using different magnifications between 100% and 1600% within Photoshop.

 

S20 D30 S20 D30

100%

400%
800% 1600%
The pixels forming the image are clearly visible in the magnified C20 image at 400% and above and can be seen as texture in the image at 100%. In contrast the D30 images show smooth tones across the board with no visible pixels even at 1600% magnification.

This is not a particularly scientific comparison, but it's very heartening to know that you do get what you pay for by investing in a high end digital camera.



Nick ThomasNick Thomas lives in Wales and has been a photographer for 17 years using 35mm and medium format systems. Since buying his first digital camera three years ago Nick’s work has been exclusively digital.

Nick’s current activities concentrate primarily on landscape photography in a variety of environments which can be viewed in the galleries at Land & Sky ( http://www.nickgallery.com/ ).

Nick is an Associate of the Royal Photographic Society.

 

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