JPEG is abbreviation for Joint Photographic Experts Group and is the most common used digital image format. It was developed by the MPEG association and is using lossy image compression for reducing the number of pixels by simply removing the colours that the human eye can not see. This action reduces the image’s file size but also lowers the image’s quality. JPEG can achieve compression ratios by the factor of 10 to 20 compared to TIFF format with a very little visible loss in image quality. JPEG compression divides the image into blocks of 8 x 8 pixels, then each of these blocks is analysed independently in order to find pixels that can be eliminated. It is important to remember that once the compression has been made the pixels will be permanently lost.
By delivering small file sizes the JPEG has become the “De Facto” image format in the digital image industry and can be found in all digital cameras. It is the optimal format for transferring your images over e-mail or put them on the web. It is also compatible to most of the browsers, viewers and image editing software on the market.
In contrast to the RAW and TIFF formats, which are easy to calculate their file size, the calculation of a JPEG file size is very complex. Therefore determining a JPEG’s file size in advance is difficult, if not impossible. It is also impossible to have the some file size for every image taken by the same digital camera since each image file size will be unique for a digital image when taken in JPEG mode. The complexity of the scene that is being photographed is the most important factor behind the JPEG’s variations in file size. A complex image will create a larger data file than a homogeneous image. It is easier to eliminate more pixels from a clear blue sky image before becoming noticeable for the human eye.
For calculating the JPEG’s file size in our digital image storage table we have used a compression ratio of 5 compared to TIFF format. This considers being the approximate file size as a JPEG digital image taken on the digital camera’s highest, 100 %, JPEG mode. Digital camera manufactures are using different JPEG quality settings, such as FINE, NORMAL, and BASIC or 100%, 95%, 90% etc. It is important to notice that some digital cameras will compress more than others, even at their highest JPEG quality setting. For example a file size of an image taken by a 2 megapixel digital camera can be calculated as follows: First we calculated the TIFF file size and then we divide it by the factor of 5 to receive the JPEG file size.
2 000 000 pixels x 3 colour channels x 8 bits per colour channel
=48 000 000 bits = 6 000 000 bytes = 5.72 MB/5 = 1.14 MB
We always recommend to saving in the digital camera’s highest available JPEG quality setting. Still if the complexity of the scene that is being photographed has a lot of uniform surfaces the compression ratio could be higher without noticeable quality loss.
The most important thing to remember is that JPEG compression is performed each time you save your image. Therefore it is possible to keep losing data from your image each time you save it. Therefore if you are going to work with one of your JPEG images in a image editing program like Photoshop you should save the file in some other format such as TIFF or RAW. These formats do not use image compression and therefore will preserve your photo over time.
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