This is a brief introduction to some of the concepts and technical details of reducing the size of image files so that they can be imported into PowerPoint without creating a 40Megbyte monstrosity.
Graphics files captured using NIH-Image, or scanned using Photoshop can easily grow to upward of 10Megbytes in size. If you have a PowerPoint presentation with 10 such images your PowerPoint file will be over 100Megs. Since the entire presentation must be loaded into the computer's memory (which is rarely larger than 24Megs and must hold the operating system, and PowerPoint as well as your presentation) one finds that it's fairly straight-forward to create a presentation that either takes a very long time to load, or won't run at all. The solution to this problem is to squeeze each image down in size so that the images in your presentation are each as small as possible. Note that once the images are in PowerPoint it's too late to shrink them. Some forethought is required.
First a quick reminder about file sizes on computers. File size is measure in bytes (a collection of 8 binary digits). The two most common metrics are kilobytes "k's" or kB (thousands of bytes) and megabytes "Megs" or MB (millions of bytes). Files range in size from a few kB for small text files to several MB for programs (like PowerPoint), to many MB for large graphics and data files. One way to figure out how large a particular file is on a Mac is to select the file (by clicking on it once) and then choose Get Info from the File menu. Now back to the original question.
Well, a full screen, true-color image-the best picture you can get on a computer screen-is about 1.5 Megs uncompressed. If your images are bigger than that then they contain more information than they need to for screen viewing. Also, most graphics format (such as PICT) compress the images in some way so that the odds are your images can be smaller than 1.5 Megs and still be the "ultimate" in resolution and color.
Most images don't need to be the "ultimate" in terms of resolution or color depth. My recommendation is that if your image file is larger than 200kB, think about reducing it's size. A presentation with 20, 200kB images won't be much larger than 4MB which is a perfectly reasonable size for a PowerPoint file. If it looks like the sum of the sizes of all your images is approaching 10MB you really need to start thinking about shrinking some or all of the images. (or cutting out unnecessary ones?). In the end the maximum size for a PowerPoint file depends on the memory setup of the machine you're using and how long you're willing to wait for things to load.
There are two main variables that control the size of a graphics file: resolution and color depth. You can increase and decrease either of these to increase or decrease the file size. Lower resolution gives you less detail (NOT necessarily a smaller image). Lower color depth gives you less accurate colors. Resolution is measured in pixels. For instance one standard size for a computer screen is 640 pixels by 480 pixels. Note that by varying the size of the pixels one can change the size of the image without increasing the resolution. This can cause some confusion since the pixels that make up the image on the actual computer screen are fixed in size. If you've made the pixels in you image file large each one may be represented by several actual pixels on your computer display. So, you can vary the size of your image by changing the number of pixels (resolution) or the size of pixels. Only changing the resolution will change the file size. Note that some programs express resolution in dot per inch,dpi-As with pixels, more is better (and larger).
Color Depth is measured in bits/pixel. If you store one bit of information per pixel you'll be able to have 2 colors (usually black and white). With 8 bits/pixel you'll have 256 possible colors. With 24 bits/pixel you'll have millions of colors-generally this is enough since the human eye can't distinguish many more than that.
So, there are three strategies for shrinking your files. The most obvious is to crop them. Cut off the parts that aren't relevant. Although it's often overlooked, most of the stuff that you happened to capture in your original image probably isn't relevant to your presentation. Cut off the extra stuff that you don't need. Next, you can reduce the resolution. How far you can cut this down depends on the what sort of image you're dealing with and how much detail you can afford to loose. Finally, you can decrease the color depth. Again, how far you can go with this depends on the particular image, and what you're trying to convey with it. Play with the color depth and resolution until you get an image that is as small as possible while still showing what you wanted to show.
There are a wide variety of tools that can serve this purpose. I'll focus on one tool called Graphic Converter that's available in the Graphics & Design folder in MacLab folder on public lab Macs. It allows you to make adjustments to the resolution, size, color depth, and cropping of an image and gives good feedback about file size.
The first step is always to make a safe copy of your original image. Many of the file shrinking processes are irreversible. Once you've reduced the resolution or color depth of an image, that information is generally lost. Make sure you've got an original (large) image file in case your efforts at reducing file size go a bit too far.
Once you're fired up Graphics Converter open up your image using the standard file open box. Graphics Converter can open and display most of the common graphics file format. Next you want to save your image as a PICT. Since your final image that you'll bring into PowerPoint will be a PICT you want to make you adjustments in file size on the image while it's in this format. To convert simply choose Save As and set the format to PICT. Then close the current image and reopen the newly created PICT file.
Now you can modify the Size, Resolution, and Color using the appropriate menu items in the Picture menu. Remember "size" doesn't effect how much file space an image takes up. The best resolution of a computer screen is "72dpi" so anything larger than that (like 300 dpi) is just unecessary information. To crop a picture click on the lasso, or box tool in the tool bar to the right of the image. Select the region you want to keep and then choose the Trim Existing option from the Edit menu to get rid of everything else. Remember, most changes are NOT reversible (though there is an Undo option under the File menu) so keep a copy of the original around until you're sure you've made all the changes you want. Finally, keep an eye on the memory row in the Information box-that's the bottom line.
By thoughtfully applying the tools in Graphic Converter, you
should be able to reduce the size of most or all of your image files
to a point where your finished PowerPoint presentation will load and
run reasonably, if not fit on one floppy.
Maintained by Sean
Fox, sfox@carleton.edu
Last updated: Thursday, 31-Aug-2000 12:26:08 CDT