Digital Image Technical Standards

Before you consider how large your digital images need to be, you should first ask yourself, where are my images coming from, and what is it I want to do? Are you scanning images from print sources, or downloading them from the web? Do you plan to publish or print the image, or use it in a projected presentation?

Online images and printing

Rule of Thumb

We'll start off with the rule of thumb and the explanation to follow:

pixel dimensions ÷ 250 = largest size in inches at which you can print

This is regardless of the resolution of the image.

Explanation

When searching the web for images for printing, there are a few extra things to keep in mind other than pixel dimensions (width by height). For printing, an image should ideally have a resolution of 250 pixels per inch. This means that for every inch of space you want your image to occupy on the printed page, there should be 250 pixels of information. For example, an image which is 1024 x 768 pixels would look best if printed no larger than 4.1 x 3.1 inches (roughly 4 x 3). How do we figure this out??

1024 pixels ÷ 250 ppi = 4.096 inches

768 pixels ÷ 250 ppi = 3.072 inches

Photoshop and image size

A common mistake that many people make is misinterpreting the information they read about the size and resolution of an image once they've downloaded the image and open it in Photoshop. When in Photoshop, under the Image menu one can view Image size. This dialog box will tell you the pixel dimensions of an image, and below this, one can also see the document size in inches. Many people look at the document size and immediately read this as the size at which they can print the image and they assume it will turn out looking good. This is only the case if your resolution is 200-300 ppi.

For example, compare the two images below:

The difference in the document sizes of these two images is almost 3x. However, according to our rule above, we don't want to print the second image at 4.5 x 6.9 inches because we have only 72 pixels of information per inch and our image won't look so great. Ideally, we wouldn't print this image any larger than 1.3 x 2 inches. This way we'll have 250 pixels of information per inch. An image of this size is okay for a small illustration within a text, but if you're seeking to print larger images to accompany your text, we suggest finding a larger image online or scanning an image from a book.

Using digital images in projected presentations (PowerPoint, Keynote, Artstor OIV, etc.)

These are the main ideas to remember regarding using digital images for lectures successfully...

When images are resized appropriately, they will display at their best

• • Keep a copy of the largest version of an image (maybe a 1-5MB image which is at least 1000 to around 3000 pixels on the longest side, if possible) as an 'original' from which you can make smaller derivatives.

  • Make smaller derivatives from your larger one based on need (consider whether you are using one, two or three or more images per PPT or Keynote slide).

  • Look into Using Photoshop for resizing images.

  • Avoid having to 'drag' the corners of the image once it's uploaded into PPT or Keynote. It's not the end of the world, but it is one cause of blurry projected images

Know that a standard PPT, Keynote or ARTstor OIV slide is 1024x768.

• • Think of a slide as a 2D layout of a room, and the images are all little rugs used to fill up the room. What size do they each need to be to fill up the room (if that's what you want to do)? View a presentation on our Theory of Using Digital Images. Print it out and keep near you as you get more comfortable working with digital images.

    • University of Texas at Austin also has a great video explaining this idea.

Start organizing images right away

• • Start naming images in a way that makes sense to you and adhere to a naming convention, such as by class. A few recommended naming conventions which could be used:

    • Name the largest version of a file with _full as a suffix or full_ as a prefix.

    • Name derivatives with suffixes (or prefixes) such as _halfvert, _halfhoriz, _qtr, _print, or _web

    • Use a suffix with pixel dimension for future reference, such as _1024x768

  • Organize folders on your computer or in other online image hosting sites by classes or presentations.

Scanning images to be printed

The first question to ask yourself when scanning images from a book, magazine or other print source is: 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 images to be projected

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.

How big should my image be?

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.

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, 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 200 ppi image doesn't look any better than a 72 ppi image. One caveat to keep in mind, however: Each image will only look as good as its original source. Even if you scan 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, 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.