Scanning Help - File Size
1: How does file size differ from image size?
File size refers to the amount of memory needed to store a given image document. File size is directly proportional to the number of pixels in an image; the more pixels, the greater the file size. File size also depends on the kind of pixels that comprise the image; e.g., since a full-color pixel needs more memory than a black & white pixel, a 1200 pixel "color" image will consume more memory than a 1200 pixel "grayscale" image. The file format of an image document can also affect its file size.
Image size refers to the actual dimensions of the image, as measured in inches (or centimeters). The dimensions of an image are independent of its file size: e.g., a 6" x 8" 2400 pixel will print out at the same size as a 6" x 8" 1200 pixel image (although the 2400 pixel image will have a higher resolution). Conversely, a 3" x 4" 1200 pixel image will print out at one quarter the size and twice the resolution of a 6" x 8" 1200 image, even if the images have the same file size.
While it is possible to scale an image during the scanning process, it is usually easier to do so after the image has been scanned. In order to scale an image without losing image quality, it is important to understand to relationship between image size and image resolution. When scaling an image, remember this basic formula: Total Number of Pixels = Image Area (sq. in.) x Resolution (dpi) x Resolution (dpi).
When scanning a very small image, such as a color slide or film negative, it may be necessary to greatly increase the scale of an image after scanning. Accordingly, you will notice that a slide scanner will typically allow you to scan at resolutions as high as 2700 dpi (printers and flatbed scanners rarely go above 600 dpi). Such a high resolution may seem excessive (not to mention imperceptible), but is necessary to capture all the information contained in a 1" x 1.5" slide. Also, the small image size would make for a relatively manageable file size (see above formula). Increasing the image size to 6" x 9" without adjusting the resolution, however, will probably consume all available memory in your computer! Fortunately, Adobe Photoshop allows you to scale your image while maintaining the same file size. A 2700 dpi 1" x 1.5" image is therefore easily translated into a 300 dpi 6" x 9" image without change in file size or image quality.