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The Digital Domain: File Formats
In the previous section on Basic Camera Settings, we looked at the JPEG file format for storing photographs as well as touching on RAW formats. Let's take a look at these different formats in more detail.
The amount of data in any photograph is at least 3MB per megapixel - therefore a modern 10MP camera will create a 30MB (or greater) image if not compressed. It's obvious that this is not always convenient and so in the early 90s, the Joint Photographic Experts Groups (JPEG) got together to define a new format that could compress photographic images to a fraction their original size.
Existing formats at the time could compress drawn images very well, but not photographs. Due to the huge variation of colour in photographic images, the only way to achieve compression more than a few percent is to discard some of the information. Of course this should be done without affecting how the eye perceives the image. The clever bit is the amount of this compression can be controlled by the user - you can choose between low compression (high quality) or high compression (lower quality). Up to 80% compression can be achieved with no perceivable loss in quality (to most people anyway).
A word of warning though - every time you save a JPEG image, the quality degrades further. So if you are working on an image, save intermediate files in a lossless format such as TIFF, or at the very least, select the very highest JPEG quality setting.
Also note that if the compression is set too high, the effects (known as JPEG artefacts) can become particularly unpleasant - especially if you intend to do any image manipulation which can exaggerate the appearance of these further.
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Compressed in Paint Shop Pro (compression = 8 ; File size = 30KB).
Acceptable quality although I personally always use the best quality setting. |
Compression = 25 ; File size = 17KB.
The JPEG artefacts are starting to become apparent around the high contrast edges. |
Compression = 50 ; File size = 8KB
Very poor quality. |
The above image would be 134KB uncompressed.
Note that the compression number is arbitrary – different software packages use different scales. The scale above is from Paint Shop Pro version 11. Be careful which values you select to make sure you are not selecting high compression when you actually want to save as high quality.
- Paint Shop Pro: Scale from 1-99 1 is highest quality
- Adobe Photoshop CS2: Scale from 1-12 12 is highest quality
To professionals and serious amateurs, only the very highest quality will do - therefore many cameras support a format called RAW. Unlike JPEG, this is not a standard format - each camera manufacturer has his own RAW format. In fact different models from the same manufacturer often have different formats (although be aware that they sometimes have the same file extension).
See the section on Shooting in RAW format for more information as well as a comparison of JPEG versus RAW.
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