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This simply means that some clever maths is performed on the image and some data is eliminated. This doesn’t necessarily mean a loss in quality though. If you think of a photograph, it is easy to see that some area are all one colour, as in the sky. It is not hard to imagine how the area of the sky could defined as a shape and assigned a single color. This would take much less space than it would to describe each pixel in detail.
Picture Formats
When the picture is displayed, the file is decompressed and the full detail is then visible. There are three main compression types, JPEG, TIFF and RAW.
JPEG (pronounced Jay- Peg) is the most popular format and stands for Joint Photographic Experts Group. The JPEG algorithm operates with small blocks of the image, each of size 8*8 pixels. It uses a lossy compression technique, which is another way of saying that some content is lost in the process. The good news is that you will probably never notice. The really great thing about JPEG is that it is so universal that not only can you view the picture directly from your digital camera’s memory stick, but if you sent it by email to a friend on the other side of the world, they too could view it with no problem.
TIFF (rhymes with stiff) stands for Tagged Image File Format and has fallen out of fashion over the years. It is still a major force in the world of Macs and is also popular when using mainstream desktop publishing programs. However it is not that common in digital cameras.
RAW is digital photography's equivalent of a negative in film photography: it contains untouched, "raw" pixel information straight from imaging chip. The RAW file format contains just one red, green, or blue value at each pixel location. The RAW format is unique to each manufacturer and possibly each model. It requires software to interpret the image but has the advantage that the image can be processed in its source state, with no loss of quality. It is the original image that is being processed, not a compressed version of it.
Picture Memory
The obvious reason for compressing a picture is to reduce the storage space required, so how big is a digital picture. It is easier to appreciate if you take a 1GByte memory stick and ask how many pictures can it hold. It will depend on how many pixels the picture uses.
If you use the best quality JPEG to store the photo, then the picture storage in Megabytes will be about half the pixel count of the digital camera. So, a 8 megapixel digital camera will produce a 4MByte picture.
A TIFF format photo will require three times the pixel count, so an 8 megapixel digital camera will produce a photo that takes up 12MBytes of space. A RAW format will take about twice the pixel count and therefore 16 Megabytes. So our 1GByte memory has 1,000 Mbytes and can store 250 JPEG’s, or approx 40 TIFF’s or about 60 RAW files.
This article has been written for www.TopDigicam.com by Roger Titley and all copyrights are reserved. It may only be reproduced if this copyright notice and the site link are included.
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