The image size is the amount of data contained in an image and the file size is the amount of storage space consumed by an image file. File size is related to image size, but the two are not necessarily equal.
File size is also expressed in bytes, but it is a measure of how much space an image file takes up when it is saved to a storage card or disk.
If we want to figure out how many two million pixel images we can store on an 8 MB Compact Flash card, we start by calculating image size - in the case of the two million pixel example, 6 MB. Without compression we could store only one image, but a 6 MB file with 1:8 JPEG compression is reduced to 0.75 MB. So we can fit about ten images on the 8 MB card.
We can safely compress images to about one tenth of their original size with no noticeable difference in quality. Maybe even to as much as 1/20, but beyond that we can notice the degradation.
Fig. 20.11. a JPEG compression: 1.2 MB JPEG image compressed from a 2 MB image (Fig. 20.10a). b When we enlarge the aggressively compressed image, we can see a difference (cf. Figs. 20.10b, 20.12b)
For example, the 2 MB image in Fig. 20.10a) may be saved as a 1.2 MB JPEG image (Fig. 20.11a) or a 172 KB JPEG image (Fig. 20.12a). Looking at these images like this, we find it pretty difficult to see any difference, but when we look at the aggressively compressed image, however, we can see a difference (Figs. 20.10b, 20.11b, 20.12b.
Was this article helpful?