For Screen: Classroom Use and Web sites.
For images that will exist only on screens, it’s better to think in terms of pixel dimensions only.
For classroom use, the guiding factor is the presentation equipment. Your monitor might be able
to show 1800 x 1440 pixels, but you won’t be able to project that. The Hitachi CP-X430W
projectors we have installed in the Schermerhorn classrooms project an image of 1024 x 768
pixels (what’s known as XGA resolution). This is pretty standard for high-end digital projectors
these days. Any image you’re showing that’s larger in pixel dimension will be resampled down
by the projector. So if you’re saving an image for use in the classroom, there’s no need to
make it much larger than 1024 pixels wide. (Of course if you’re going to zoom in on a detail of
the image, you’d need it that much larger.)
If you use PowerPoint to project your images, you might notice that a 1000 pixel wide image
looks tiny on the PowerPoint workspace (or perhaps unexpectedly large). That’s because
PowerPoint measures its images according to the Document Size, not the Pixel Dimension as
PowerPoint is made to work at the highest resolution possible for whatever device will ultimately
display the slide show. (See below for more about Document Size and Pixel Dimensions.) The
PowerPoint slide is 10” wide and 7.5” high. So an image with a Document Size of 10” x 7.5”and
a resolution of 50 PPI will fill your PowerPoint screen, but when it’s projected it’ll look fuzzy (in
pixel terms, that image is only 500 x 375 pixels). Conversely, if you have an image that’s 4” x 3”
at 300 PPI it will import into PowerPoint as rather a small image on the 10” x 7.5” field, but
since the image is actually, 1200 x 900 pixels you could scale it up to the full width of the
PowerPoint slide without any loss in image quality when it’s projected. (You could think of it as
the projector having an effective resolution of about 102.5 PPI.)
It’s best to look at the pixel dimension of your images as you’re making them. As long as they’re
at least about 1024 pixels wide (for a horizontal image) they should be fine for teaching.
The standard resolution for web images is 72 PPI (often called “screen resolution”). At that size,
the pixels you see on the screen are all the pixels there are; an image that’s 4” long at 72 PPI will
take up about 4” of your monitor. (Obviously there’ll be a lot of variation here, as most monitors
have a range of resolutions they can be set at.)
Most web sites are built to be visible on many different kinds of monitors. Usually a web site
would be about 700-800 pixels wide. That means an image that’s about 400 or 500 pixels wide
will take up a good chunk of the web page, and look pretty big on a monitor. You might want a
bigger image on your site, but remember, some users might only have screens that show 800 x
600 pixels.
For Print: The dot and the line.
A bit about printing: images are printed using a halftone screen, made up of a mesh of tiny spots
of varying sizes. In the old days, these patterns were formed by exposing a photograph through
screens etched on glass, which were measured by counting the number of parallel lines to the
inch. Thus the traditional measurement for the resolution of a printed image is still “lines per
inch” or LPI.