One of the most frequently asked questions from phone calls relates to the relationship between file size and quality. People seem to think that the quality of a photo scan is determined by the size of the resulting file. Bigger file, better quality.
To re-hash what I've said before the key factor in picture scan quality is the dpi at which the original is scanned, then the cleverness or otherwise of the program which compresses the resulting image. So I was surprised when one of our clients took me to task - we had scanned a large number of 35mm slides, all at 4,000 dpi into jpg files. Why, he asked, did the file sizes vary? After a little thought and file comparison the answer related to the slide mounts.
First, jpg files are compressed. Clever maths enables the program to remove some data points, for example in uniformly solid black areas. If one image has a lot of solid black, while another is more varied, jpg compression will give you a smaller file. In theory that goes some way to answer the question but in practice the file size variation isn't that great.
Second, as I found after some digging, look at the amount of space the slide mount creates. Yes the maximum image size on a 35mm negative or slide doesn't vary but even within the same type of film the bit in the middle varies in size. By a significant amount. It was noticeable, and maybe understandable, how much difference there was between different slide mount manufacturers but I was surprised to find variation between different Kodachrome slide mounts.
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