GIF (Graphics Interchange Format) is an 8-bit format, and can therefore only cope with 256 colours. The compression algorithm is non-destructive, and compressed images are the same quality as the original. Files can be compressed to about 25% of their original size.
JPG (or JPEG - Joint Picture Experts Group) will support True Colour (16.7M colours). The compression algorithm works by selectively removing detail from the image. At up to 90% compression this is hardly noticeable, but above this image quality starts to suffer. Note also that the loss of detail is cumulative, i.e. the more a JPG image is loaded and saved, the more detail will be lost.
Other formats include PNG (Portable Network Graphics) which is similar to GIF but supports true colour; WIF (wavelet compressed) and FIF (fractal compressed) which are variants of JPG but provide a more pleasant loss of quality. Currently browsers do not support these formats directly and requite plug-ins; a shame, because I was very pleased with the quality of FIF files.
I scan all my pictures as TIF files and keep these as master copies. They can be anything up to 800K. When I want to post a picture I convert it to a JPG file using Xat Image Optimiser. This allows you to select the quality of a JPG file, and also to selectively compress different areas of an image at different qualities. I have settled on 75% quality, which gives compression ratios of between 90% and 97% depending on the complexity of the original picture. By doing this I can post pictures which download in seconds, and still retain a reasonable quality. I scan photos using my trusty Epson GT5000 flatbed scanner; it is sturdy and reliable, and with an optical resolution of 300dpi is adequate for most jobs. I now use it mainly for OCR work, having acquired a Canon FS2710 transparency scanner with which I have re-scanned all my available negatives. (see below for comparisons).
A word about scanning photos. There is no point in scanning at more than 100 dpi if the result will only be displayed on a computer screen as screen resolution is only 96 dpi. I have seen people recommending scanning at 300 dpi and then resampling to 100 dpi. Rubbish! - there is absolutely no difference in the quality of the resulting picture. There is also little pont in scanning above the optical resolution of your scanner; you then use software interpolation which can produce unpleasant results.
If, however, you intend to print the resulting image file then scan at a sub-set of the printer resolution. For example I currently use an Epson Stylus 1290S (1440 x 2880 dpi). I achieve better results by scanning at 240 dpi or 120 dpi than I do at 300 dpi, and there is no noticeable improvement when scanned at 720 dpi.
When it comes to scanning 35mm transparencies it's a different matter. Scanning at an output resolution of 96 dpi doesn't give very good results. The reason for this is simple; a 6"x4" photo is scanned 1:1 to produce an image of about 600x400 pixels; a 35mm negative needs to be scanned at a far higher input resolution to produce an image of the same size and quality. With my Canon transparency scanner I scan at an input and output resolution of 1360 dpi and then re-sample down to an image width of about 640 pixels and a resolution of 96 dpi.
The following images were all processed from the original TIF master whose size is 36K, even for this small picture!
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This 95% quality JPG is almost indistinguishable from the original, yet compression ratio is 93%. A full screen image would still be unacceptably large however. Size 2.8K |
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This 75% quality JPG represents a good compromise between size and quality at 96% compression. A small loss of image quality can be seen along the boundary between mountain and sky, but I think it's quite acceptable. Size 1.7K |
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This 50% quality JPG shows considerable loss of quality, both of image and colour. I consider it to be quite unacceptable considering the small saving involved. Compression 97%, size 1.4K |
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This GIF image had to be reduced to 256 colours, and for this one I used Photo Paint's dithering and palette optimisation. The results are pretty good, but it's still quite a large file. Size 8K |
I have just acquired a Canon FS2710 negative scanner, and I am astounded at how much detail can be lost when a photograph is printed, especially if it is done by a high street chain (SupaSnaps in this instance). Look at the example below, and notice especially the extra detail in the sky, and the more realistic colours. You will also notice how much of the original image is cropped when the photo is printed.
