There’s more to choosing images than finding a pretty picture. It takes a combination of creative and technical ability to understand not only what to choose, but which type of image is appropriate for your needs. That’s why we’ve put together this guide to help you understand the difference between delight and disaster for your next project.
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Types of Images
All images fall into two basic categories: raster and vector.
Raster images are for photography and scanned documents.
The quality of a raster image is called resolution. Resolution is measured in ppi (pixels per inch) or dpi (dots per inch). PPI measures pixels on a screen, while dpi measures drops of ink on a printed page. The higher the ppi or dpi, the higher the quality.
While dpi is expressed as a single number (most photos are a minimum of 300dpi), ppi is usually written as a set of dimensions (400x1080ppi).
Commercial printing for projects like textbooks or marketing material requires high-resolution images of 300dpi or greater. In web and mobile applications, the resolution requirements are much lower. Because high-resolution images have larger file sizes, web and mobile applications use lower resolution images for faster loading times.
When you increase the physical size of a raster image, you lose quality. When you decrease the size, quality increases.
If you double a two-inch by two-inch 300dpi image, it becomes 150dpi. If you decrease the size by half, it becomes 600dpi.
These images are much more difficult to manipulate than vector images.
Color modes available for raster images vary, but the most common are CMYK, RGB and Index color.
Photoshop is the most popular program for manipulating raster images.
Vector images are most commonly found in commercial printing applications and work well for illustrations, logos, graphics with a lot of text, charts and graphs.
Unlike raster images, vector images do not depend on resolution for their quality. They retain their quality at any size, making them a popular choice for large format printing.
Windows Bitmap (BMP): Raster
Developed by Microsoft for their operating system. They are high quality and uncompressed, making for larger file sizes and they are not web friendly. They don’t work on the web and quality is compromised when you make them larger or smaller.
Joint Photographic Expert Group (JPG): Raster
Most common format, many cameras shoot in jpg. Compression means that every time you manipulate and save the image you will lose quality.
Graphic Interchange Format (GIF): Raster
The grandfather of web graphics, this format has been around since 1989. Its small file size and support for both transparency and animation makes them popular for web work. It’s recently been popularized by the animated gifs of popular movie clips from Will Farrell and others. An 8-bit color palette limits this format to only 256 colors. The ability to save with interlacing means faster loading online. The image will come up quickly with less detail but once fully loaded all detail is present.
Tagged Image File Format (TIF) Raster
Older file format developed in 2009 by Aldus for use in high-end printing. Also popular for commercial printing, scanning and computer fax applications. It does not lose quality like a jpg, and has several compression options. Touted for its flexibility even though it cant be used for web graphics.
Encapsulated Postscript (EPS): Raster or Vector
Crisp lines work well for text, info graphics, and line drawings. Can easily manipulate vector images and apply PANTONE or other spot colors for commercial printing applications. The exception is the Photoshop EPS, which is essentially a TIF file with an EPS extension.
Great for photography because it captures the most data from your camera meaning you have the best images possible. One drawback are the options to manipulate photo formats are limited to Photoshop and whatever software comes with your camera.
Portable Network Graphics (PNG): Raster
Small file size works well for internet work. Supports transparency and only uses indexed color.
Portable Document Format (PDF): Raster or Vector
While the PDF is not really an image format, it is the universal file format of choice today for top professionals. A PDF file is usually created in a different format and then converted. It locks most users from being able to edit them, making it a file format of choice for ebooks, forms, and large images. The quality can be low or high depending on how it is made.