Reducing Image File Sizes
On of the most common e-mail failures we see here appear when a user has sent an image via e-mail and exceeded the recipient's mail limit. The vast majority of these are preventable... If you have the right information. To that end, let's examine the six main factors that contribute to graphic file sizes. They are:
- Compression/Format - Many graphics programs save images as .tif, .bmp, or some other program specific format that have no compression. These are good formats for manipulating raw images because no data has been lost. There are even instances where, for acedemic or research purposes, there is a legitimate need to keep those images in that format. However, for most purposes, a certain amount of compression will not cause a noticeable decrease in quality. Saving these file types as a jpeg (.jpg) can reduce file size by 90% - making them much more easily sent through e-mail.
- Dimension - Most people have their computer screens set at either 1024x768 pixels or 1280x960 pixels. Many digital cameras take pictures that are 2180x1620 pixels or larger. That means that most monitors can only view 1/4th of a picture that size at a time and have to scroll to see the rest of the image. That means that at on many computers, about 2.7 million pixels (77.7% of the total) are off the screen at any one time. A good rule of thumb is anything over 1024 pixel wide or 768 pixels high is generally wasted. Another problem we see in this area is when someone adds a large image to a Word document or a PowerPoint presentation. The image is then resized to be smaller to fit the space in the document or presentation. Unfortunately, Word and PowerPoint will retain the original image size. Combine that with an image out of context (see below) and you have the recipe for a bloated file.
- Color Depth - The more colors you use in an image, the more information it takes to store that image. The only time you should worry about color depth is if you have an image that is already black and white. Then you might want to consider setting the image to Gray Scale. It _might_ save a little file size.
- Chaff - This includes centering and cleaning up white space (in scanned documents). If the subject of your picture is off-center, you can crop the image to just the good parts. Not only does this decrease file size, but it also makes you look like a better photographer. You can employ a similar strategy with scanned documents. If you crop the margins from a scanned document, it will reduce file size AND if someone wants to reprint what you scanned, it will fit on the page better. Also with many scanners, documents don't always scan with a uniform background. Many "white" backgrounds will actually be predominantly white with intermittent light gray specks. These specks can increase the "complexity" of the image; increasing the file size and decreasing the quality of the image. Most graphics programs have an eraser tool. You can use the eraser to clean the background.
- Resolution - This pertains mostly to scanned images - particularly documents. Resolution is the number of pixels per inch. A higher scan resolution makes a better quality scan. Too high a scan resolution, however can ballon the image size. For text documents, a good resolution is 300 pixels per inch. Or, if you are more interested in the text itself, many scanners will allow you to scan the text into another program like Microsoft Word and allow you to edit or change the font of the text like you would if you had typed it in yourself. Now THAT would decrease file sizes.
- Context - There are a number of programs that will accept images, but that is not their primary function, so there are times they do not handle them perfectly. Notable among those is Word (If they had planned on it being a graphics program, they would have called it Picture). When a large number of large images are inserted into a Word document, computer performance drops significantly and saving the file can take a very long time. If, however, you think ahead and size your graphics appropriately, clean out the chaff and save them as a jpeg, then you can insert a number of graphics without the associated problems. The same thing applies to PowerPoint.