Image File Sizes

Starting with an original JPEG file scanned in at 778 by 1078 pixels, the file was 442KB - a very large image indeed, the file was then manipulated to decrease its size while not changing the dimensions of the image.

Effects of JPEG compression

JPEG files can be compressed using mathematical algorithms to reduce the file size. Unfortunately, this compression is not totally reversible, so there is some loss of image quality. The trick when working with JPEG files is to experiment to see just how much compression you can apply without measurably affecting the quality or usability of the image.

With a compression factor of 22, the resulting image now weighs in at 163KB. The effect of the compression is hardly noticeable and the image is definitely as usable as the original image. Attempting to further reduce the size, I then applied a compression factor of 35, at which point the quality appears to start deteriorating rapidly. The result is still usable, but only slightly smaller at 135KB.

Effect of different file type

In addition to JPEG format, all browsers support the GIF format for images. GIF lends itself most handily to line images or images with large areas of the same color. So, my first task was simply to convert the JPEG file to a GIF file to see what the resulting quality and file size would be. As a GIF file, the image is quite readable and is only 197KB, not any smaller than the compressed JPEG files. I wanted to see if I could get the image down to a more reasonable size, so I applied GIF compression by reducing the number of colors from 256 (the default for a GIF file) to 16. The resulting image is a bit grainy where the grey discolorations appear, but otherwise is a very usable image. And, it is only 72KB.