Adobe Photoshop : File size

Managing file size is important when working with digital images. If you don't keep track of your file sizes you may end up with very large images, and this will undoubtedly cause problems somewhere down the line: large files eat up disk space quickly, crash printers, slow down web browsers, crash email and take much longer to edit in Photoshop. Do everything possible to keep your file sizes as low as possible!

The size of a file (measured in bytes) is related to its physical dimensions (the size at which it prints); when you decrease the physical size of an image, the amount of memory used by that file also decreases (all other things being equal).

You can check the amount of memory a particular file uses by using Windows Explorer on the PC, or by opening the folder in which the file is contained on the Mac. You'll see the name of the file and its size in bytes.

To check the physical dimensions of an open Photoshop file:

Choose Image>Image Size.
A dialogue box like the one below will appear with the size and resolution. Size is usually given in both pixels and inches (you have the option of changing these units). Note: Pixel units are useful when sizing images for the web.

Notice that there are two sections to this window. The upper section allows you to adjust the number of pixels used for the image. This is how much data is available in the image. Imagine an image of a face with only 10 pixels of information per row and column, you probably would be able to tell that it is a face, but it would be unrecognizable. Now imagine an image with 1600 pixels by 1200 pixels, this is what most 2.1 megapixel cameras can record. There is a lot more information captured.

The lower section of this window allows you to adjust the print size. This is how you adjust the physical size of the image. You can print out the 10x10 image at 11 inches by 17 inches or .2 inches by .2 inches. Changing the print size does not adjust the amount of information, and therefore the size of the file, it only adjusts the size that it will be displayed.

The resolution is the pixels/inch. This ratio relates image size to print size. 72 dpi is the default resolution used for online graphics, 150-300 dpi is used for printing.

You can change the image size by changing either size or resolution. Just type a new value in each box; if you want to resize the image proportionally, check the "Constrain Proportions" box.

If you have Resample Image checked, you will be able to change the total number of pixels in the file.