If you want an image to look good on the screen in PowerPoint, scan it at the screen resolution: 72. dpi. Higher does you no good--it only makes the files bigger. HOWEVER, you may find that 72 dpi images are a bit too low-res for printed output, so you might want to scan at 150, which makes a nice compromise. THEN AGAIN, if you are outputting to 35mm slides, you're talking a whole 'nother ball game. 35 mm slide output devices are very high resolution, and you'll want 300 dpi scans. These will make your presentation gigantic, but anything lower resolution will look really bad.
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