It can be difficult to determine the quality of a digital image based on its file size.
Digital services like scanning are often priced by file size in megabytes. If you have a 30MB
file, how many megapixels is that? What is the difference between compressed and uncompressed
file sizes? How many megabytes do you need to make a large print? This page attempts
to answer these questions.
Image file size formula: (UNCOMPRESSED image formats)
|8 bit Grayscale (each pixel = 0 to 255)||1|
|16 bit Grayscale (each pixel = 0 to 65536)||2|
|24 bit RGB (8 bits for each R,G,B) "8 bit color"||3|
|32 bit CMYK (8 bits for each C,Y,M,K)||4|
|48 bit RGB (16 bits for each R,G,B) "16 bit color"||6|
Each calculated file size is the uncompressed size.
When discussing the quality of digital files based on file size, comparisons
should only be made based on uncompressed sizes. Compression algorithms
will modify each image differently depending on the subject matter of the image.
Therefore it is impossible to accurately compare the file size of two digital images
once they have been compressed.
There are two types of file compression, "lossy" and "lossless". Lossy compression actually changes some of the original pixels and some details are lost. The most common type of lossy compression format is JPG. While the original JPG image out of a digital camera is fine, every time the file is saved again, detail is lost. If the same file is saved as a JPG a few times, significant quality is lost and cannot be recovered. Valuable originals should always be saved in a lossless format, like TIFF or RAW. TIFF files can be edited and saved any number of times without loss of detail because the compression does not alter any pixels. The trade off is that TIFF files do not compress as well as JPG.