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Scanning Images

Scanning for Printing

When scanning images for printing in papers, posters, etc., the most important thing to think about is pixels per inch. When scanning images from a book, magazine or other print source, first ask yourself: How large do I want to print this image? Ideally, an image should have between 200 and 300 pixels per inch (ppi, also known as resolution or dpi) in order to be reproduced well.

Example

You're creating a poster for your thesis and you'd like to include an image of a painting. On your poster the reproduction of the painting will take up approximately a 3 x 5 " space. What size digital image would be ideal? Consider this calculation for an image that has a resolution of 250 ppi.

3" x 250 = 750 pixels
5" x 250 = 1250 pixels

You need an image which is at least 750 x 1250 pixels at 250 ppi.

Let's say you find a reproduction of this painting in a book, which is an 8 x 10 inch image. Scanning an 8 x 10 inch image at 250 ppi results in...

8" x 250 = 2000 pixels
10" x 250 = 2500 pixels

Great! This one will be more than big enough.

After scanning, open the image in photoshop and resize it to 750 x 1250. Or, leave the image larger than what you need (either exactly 2 or 4 times larger is ideal) and scale the image down within your document.

Remember: better to start with an image that is too big than too small, you can downsize later.

Scanning for Projection

When acquiring images for projection (PowerPoint, Keynote, etc.), the most important thing to think about is pixel size. 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. When scanning a 3 x 5 inch image at 250 ppi, you'll end up with an image which is 750 x 1250 pixels.

How big should my image be?

Ideally, your digital image should be exactly as large as the space you want it to occupy. A standard 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 the Colgate Visual Resource Library's Theory on Using Digital Images presentation to further explore this concept.

And what resolution should it be?

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.

One can 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). 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.

What is native resolution?

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. 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.

(credit: Colgate Visual Resources Library)

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