Lowering Image File Size

When you put captured images into a Microsoft Office document (for example, Word or PowerPoint) for reading on a computer screen, they should not be of a higher quality than can be seen on a computer screen. By lowering the quality, you lower the file size and make the resulting document easier to upload and download. Here are some ways to lower image file size.

(1) Don't make the original image of higher quality than is necessary.
(1A) If you scan the image, set the resolution at 72 d.p.i. (dots per inch), which is the maximum resolution visible on a typical computer monitor.
(1B) If you take a digital photo, with most cameras you should set the camera to low quality, which will still be higher than 72 d.p.i.

(2) In Photoshop, crop the image so that you keep no more of it than you need. If you don't know how to crop, use Photoshop's Help and then enter Crop in the search box. Photoshop has excellent built-in help.

(3) In Photoshop, adjust the Image >> Mode to the least information necessary. If the image is black and white, adjust to Grayscale to discard unnecessary color information. If the image is color, make sure it is RGB color.

(4) In Photoshop, adjust the Image >> Size. When you select Image >> Size from the menu, a dialogue box will appear. Make sure that “Constrain Proportions” and/or “Resample Image” are checked. These options will create icons of chain links on the dialogue box indicating that some characteristics are linked together. For example, if Width and Height are linked and you lower the value in one, the value in the other will automatically lower by the amount necessary to preserve your image's aspect ratio. Near the top of the Image Size dialogue box is an estimate of the file size. When you make a change, and before you click OK, the estimate will change. Play around with the check boxes and the values to get the best image with the lowest file size. As long as you don't File >> Save, your original scan won't change. If you want to preserve your resized image but not change the original scan, save the resized image with a modified file name.

(5) In Photoshop, use File >> Save for Web. When you do, you'll find yourself viewing a new window. If your image looks bigger than necessary, click the Image Size tab to open the Image Size dialogue. Make sure the Constrain Proportions box is checked, which will chain link Width and Height, and then adjust either Width or Height to produce what seems to be a good visible size. If it does look like a good visible size, notice the file size number just below the image. Adjust the graphic format to JPEG or GIF. (Which one you might prefer is a technical matter that is irrelevant if your only use is for insertion in a Microsoft Office document to be accessed through the web and viewed on screen.) Play around to get the best file size. If you choose JPEG, try adjusting Quality (0% to 100%) to see how low you can go without unduly compromising necessary visual quality. If you choose GIF, the easiest way to lower the file size is to lower what is called color depth, which is accomplished by lowering the number in the Colors box. There is no need to use more colors in your palette than the original image uses. If, for example, the original image is straight black and white, 2 colors will capture all available information, so why make a place in the image file for more? If you are scanning most comic books, there are only four colors used in the original, so a setting of 4 or 8 (for overprinting) will often capture all the color information there is. Whether you use JPEG or GIF, play with the settings to get a good image that is no better visually than you need and no bigger in file size than you need.

For the purposes of online viewing, it should be readily possible, when dealing with, say, children's picture books, to have a Word document of about 10 pages of text and up to 30 pages of images with a total file size of less than 3M.

Copyright © 2005-2007 Eric S. Rabkin This page was last modified