Sizing Digital Images For Powerpoint
One of the most prevalent uses of digital imagery is in PowerPoint Presentations. PowerPoint has become the most used slide show program fror teaching and training and most everyone in science will have to use it at some point. Inserting digital images and movies into PowerPoint is easy, but care must be taken to optimize the file size of the images to keep the program from hanging up or crashing during the presentation. To optimize the file sizes of your images, it's important to understand how to use the Image Size function of Photoshop.
Open an image in Photoshop and then open the Image Size Dialog Box under Image in the menu bar. The upper part of the box, Pixel Dimensions, tells you exactly how large the image is in pixels and how much space the image requires on the hard drive if it is uncompressed. In this case, the image is 10.1 Megabytes (Mb) in size. The lower box, Document Size, tells you what size the image would print if it were sent to a printer and what the resolution of that print would be. In this case, the print would be at 72 ppi (pixels per inch) at 32x21.3 inches. For some unknown reason, when you download an image from a digital camera it always sets the ppi to 72 at a large print size. These are somewhat awkward dimensions, so I try to make the following changes as soon as I download an image:
Change the resolution to 300 (this is a standard resolution for printing and publishing photographic images) and in the pulldown menus next to Width and Height choose percent. Set the percent to 100 and click OK. Be sure that you have the Constrain Proportions box at the bottom checked, this will make any changes proportional to both the height and width.
When you open the Image Size Dialog Box again, you see that the image will print at 7.6x5.1 inches at 300ppi. Also note that in this process the Pixel Dimensions stayed exactly the same. We didn't add or subtract pixels, we just told the printer that we wanted a smaller, higher resolution print of the image.
Another way to change the Document Size without changing the Pixel Dimensions is to uncheck the Resample Image box (red arrow) at the bottom of the Image Size Dialog Box. Now when you change the values of either the Width, Height, or Resolution, the other values will automatically change while retaining the same Pixel Dimensions. In this case by changing the Resolution to 300ppi, the Width and Height automatically changed to 7.68 and 5.12 respectively. (Thanks to Paul Chaplo for this method.) When you resize an image to place in a PowerPoint presentation, you will be changing the pixel dimensions as detailed in the folowing section.
Image Size for PowerPoint
Open the image you wish to insert into a PowerPoint presentation in Photoshop and open the Image Size Dialog Box. Make sure the Scale Styles, Constrain Proportions are checked and set Resample Image to Bicubic. Note that this image requires 10.1 Megabytes of space (uncompressed) to save. This is a very large file and inserting only a few images of this size into your presentation will require alot of memory and likely slow down or even crash the computer. Images do not need to be this big to appear well when projected so we want to reduce the pixel dimensions before inserting into PowerPoint.
PowerPoint defines the area of one slide (in On-screen Show setting of Page Setup) as if it were a piece of paper 7.5 " high and 10" wide at 100 ppi. This gives an area of 750 x 1000 pixels. Compare this to the initial pixel dimensions of the image to be inserted. All those extra pixels are not necessary, as PowerPoint will not use them. Lets go back to Photoshop and change the size of the image to more economically fit into PowerPoint. If the image is horizontal, change the Resolution to 100 ppi and the Width to 10". If Constrain Proportions is checked, the Height will automatically change proportionately. Note that at this new size, the image is only 1.9 Megabytes, a fifth of the original size. Click OK and at this point it is a good idea to use the Unsharp Mask filter to sharpen the image. Now save the resized and sharpened image as a Jpeg. I would set the Quality to the highest number in the Medium range (7) or the lowest number in the High range (8). Also make sure the Format Option is set to Baseline ("Standard"). When saved at these settings, the image now is only 60 Kilobytes (Kb), about 1/160 of the original file size! Make sure you save this Jpeg in another place than the original file, as you wouldn't want to accidentally overwrite the original file with this much smaller version. When you insert this image into PowerPoint it will fill the slide horizontally. If you don't want the image to fill the slide you can proportionately reduce the image size in Photoshop before inserting or resize the image in Powerpoint. For vertical images, resize the original to 100 ppi at 7.5" in height, sharpen, and save as Jpeg.
When you insert this resized image into PowerPoint, it will fill the slide vertically.