Image Dimensions

Digital raster images have two kinds of dimensions; pixel dimensions and linear dimensions.

Of course, the pixel dimensions refer to the actual numbers of pixels in the image. Changing pixel dimensions involves resampling the image and creating new pixels. Resampling always reduces the quality of the image.

The linear dimensions are in inches, or centimeters, or millimeters, etc. and represent the size that the image should have when it is printed. Linear dimensions are easily changed, as no resampling is needed. A 10 MegaPixel photo remains 10 MegaPixel no matter what the linear dimensions are.

These two types of dimensions are related by image resolution, which may be in pixels per inch, pixels per centimeter, etc. We can change resolution and linear dimensions at the same time without needing to resample.

Websites ignore the linear dimensions and use the pixel dimensions, as computer display sizes are in pixels, not inches. Some websites automatically resample to a smaller size.

Microsoft Word uses the linear dimensions when pasting an image into a document.

Adobe Photoshop uses the linear dimensions when printing an image.

When you make prints at a camera shop or drug store, they usually ignore both types of dimensions and enlarge the image to fill the print, even if some of the image has to be cropped.