When acquiring images for projection (PowerPoint, Keynote, etc.), the two most important things to think about are pixel size and descreening. Pixel size will be covered in detail on this page. Visit our page on descreening to learn more about it.
Pixel size (or pixels per inch (ppi also known as dpi)) matters only in regards to how many pixels per inch one needs to scan an image in order to end up with a desired pixel size. For example, when scanning a 3 x 5 inch image at 250 ppi, you'll end up with an image which is 750 x 1250 pixels. Ideally, your digital image should be exactly as large as the space you want it to occupy. The default size for a PowerPoint slide is 1024 x 768 pixels (but you can work with larger slide sizes than this). Let's say you want to use one image alone on a standard PowerPoint slide and you'd like that image to fill up as much of the slide as possible. In this case, a vertically oriented image should be 768 pixels high and, at most, 1024 pixels wide. A horizontally oriented image should be 1024 pixels wide and, at most, 768 pixels high.
See our Theory on Using Digital Images presentation to further explore this concept. Resolution is a tricky term. Some use it to refer to pixel dimensions and some use it to refer to pixels per inch. Here, we are thinking of it as ppi, pixels per inch.
If you have the chance, compare a downloaded image from ARTstor (which is 1024 pixels wide with a ppi of 72) with an image scanned from a book (let's say it's also 1024 pixels wide, but with a ppi of 200). You'll notice, that on a screen (or projected) the 72 ppi image doesn't look any better than a 200 ppi image. One caveat to keep in mind, however: Each image will only look as good as it's original source. Even if you scan in an image at 600 ppi, if the reproduction in the book is low quality, it will never look as good as a 72 ppi digital image photographed in situ & supplied by vendors via ARTstor* or via MDID, for example. When people talk about native resolution specifically, there is less ambiguity as to what they are speaking of compared to resolution alone . Native resolution refers to fixed dimensions which constitute the "area" of computer monitor displays and digital projector displays.
When teaching in a classroom on campus, it's good to know what the native resolutions are of the computer and projector you'll be presenting with. Most older projectors have a native resolution of 1024x768, but newer projectors are made with higher native resolutions, such as 1920 x 1080. That's a lot more real estate! A computer and projector which are working together don't have to be set at the same native resolution, but the computer display should be equal to or lower than what the projector is capable.
Why is it helpful to know about native resolutions? PowerPoint, Keynote and ARTstor's OIV each offer slide sizes which are larger than 1024x768, even though this is the standard default. When creating a presentation with a larger slide size, you can use larger images and have more 'real estate' for arranging information on your slides. Choose a slide size that is equal to or less than the native resolution of the computer from which you'll be presenting. When traveling to other institutions, the safest bet is to build presentations at the default slide size, 1024x768.
Check out the native resolutions for computers and projectors in Little Hall.
*ARTstor contains over 1,000,000 digital images supplied by various institutions and vendors. Not all of the content in their database is at the same quality level. Their collection also contains many poor quality scans from books and slides.