Photo Fixing

Scanning Resolution

Since professional printing is at resolution of 300 PPI, the optimal scanning resolution for printing at the same size as the original photo is 300 PPI as well.
In most cases it is unnecessary to scan at a resolution greater than 300 PPI.

However, it is necessary to scan at a resolution greater than 300 PPI when you scan a very small item with purpose to enlarge it.

If you wish to double the original size of your picture (thus 4 times in area), double the resolution of your scanning i.e. scan at 600 PPI.
In the same manner, if you wish your picture to be 4 times the original size (thus 16 times in area) scan at 1200 PPI.

If you do have to scan at higher resolutions such as 600 or 1200 PPI or higher, be aware that the scanning time increases at a non-linear rate.
Therefore your computer must be powerful enough to handle such enormous amount of data.
Furthermore, the greater the resolution is, the more disk space it will occupy, as described in the following table:

File size per area of scanned photo (jpg format)*
24-bit color
Scanning resolution
[PPI] File size
[Kb per sq. inch]
300 55
600 150
1200 630
8-bit grayscale
Scanning resolution
[PPI] File size
[Kb per sq. inch]
300 35
600 110
1200 440

* Appoximated values only, real values may vary depending on the photo being scanned, type of scanner, and more.

Examples:

1. You have a 3 X 4.5 inch color photo that you want to enlarge to a 4 X 6 inch. Although it's not double in size you still need to scan it at 600 PPI instead of 300. The file size will be approximately:
3 x 4.5 x 150 = 2025 Kb ~ 2 Mb

2. You have a 2 X 3 inch B&W photo that you want to enlarge to an 8 X 12 inch. It is 4 times larger which means that you have to scan at 1200 PPI.
The file size in this case will be 2 x 3 x 440 = 2640 Kb ~ 2.6 Mb

For most personal computers it is not recommended to scan any photo at more than 1200 PPI.