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“Dear Digit, there are so many terms and technicalities associated with digital images, I’m lost. Do I need to know the difference between JPG and GIF, DPI and PPI, resizing and compressing, etc.? If so, how can I keep them straight?”
There are three categories of terms that are essential to producing, using, and sharing digital images: file type, file ‘quality’ and file size. Let’s take a closer look at each category.
Files types are identified by their extensions. If you are using or dealing with RAW (.raw) images, you will want to have a more nuanced understanding of file types than we can address in a brief article. For the other types, some general rules apply. The following table is a useful way to make sense of which specific uses each file type is designed for.
Click here to download printable PDF version of the table.
FILE QUALITY: Overview
The terms DPI (dots per inch) and PPI (pixels per inch) can be misleading and confusing. PPI is typically represented/referred to as DPI in photo metadata (EXIF files) and in most photo editing programs. In both cases, the concept deals with how much visual information the image contains per inch. The higher the number, the more information there is per inch of your image.
DPI and PPI can be reduced, which results in a blending of pixels and smaller file sizes. While DPI and PPI can technically be increased, the computer automatically generates extra pixels by matching them to nearby existing pixels. Doing so enlarges the file size and can result in very strange and unwelcome shifts in color and detail in images. As a result, it is advisable to create original images at the maximum DPI/PPI that may be needed; in practice, DPI and PPI are only reduced.
FILE QUALITY: Printing and viewing
PPI is technically a hypothetical digital number that can be manipulated with little concern as long as the image is only used digitally. DPI, however, is a real number that makes a big difference when printing images.
An image printed at 300 dpi means 300 dots will be printed per inch to create the color in that spot on the print. So, 1200 dip would mean 1200 dots per inch printed to create the color. Not enough dots or pixels = fewer dots per inch of color being printed = prints with less smooth blending of colors and edges.
DPI for most print jobs should be at least 300 for 5″ to 10″ images, and typically does not need to be higher than 400. If you print larger images, you may need to decide whether you need higher DPI (appropriate for fine art prints) or a lower DPI (appropriate for billboards and other large ad images which will be viewed from a distance). A handy calculation can help you determine how large your image can be at a given DPI before it becomes pixelated, and can also help avoid having the picture cropped incorrectly by the printer.
FILE QUALITY & TYPE: Scanning
If you are creating a digital image by scanning it, the absolute minimum DPI you want to use is 300. Scanning at higher DPI settings means you are capturing more information. This takes longer, but can be important if you anticipate enlarging and printing the scanned images. Scanning at resolutions between 600-1200 dpi might seem like overkill in some situations, but might be exactly the right quality level for others. If you have the option, it is advisable to scan images at TIFFs, as that file type has the greatest capacity to capture information. Once you have a high-resolution file, you can edit a copy and adjust DPI and file type to suit.
FILE SIZE: Shape/Dimensions
There are two ways of thinking about file size, and both are relevant for online and print uses: ‘size as shape’ and ‘size as data’. ‘Size as shape’ = dimensions. Most compact (point-and-shoot) cameras produce images with 4:3 dimension ratios while most DSLR cameras and 35mm film cameras produce images with a 2:3 ratio. These ratios can be written as pixel ratios or as inches or centimeters. For example, 1200 x 1800 pixels = a 4″ x 6″ inch print at 300 dpi (as in the calculation outlined above).
There are lots of ways of changing the dimensions of your image: cropping, resizing, resampling, changing the pixel ratio, etc. Your final purpose for the image will help you decide how to change the shape. If you are uploading images to a social media or website platform, the platform’s help forums should provide the information you need to decide what dimensions are ideal for your image. If you are having your images printed, collaborate with your printer to determine the necessary dimensions. If you are printing your own images, use the information above to determine the dimensions and pixel ratios you need.
FILE SIZE: Image data/Bytes
Generally, each pixel = 3 bytes of data for 8-bit RGB color (the amount of information provided to produce a color) and 6 bytes/pixel for 16-bit color. You may need to change the image data size for a host of reasons. For online viewing or computer-only viewing, you may need high-quality or low-quality images at certain dimensions. For printing, you may need to have a high-quality image at a certain DPI. As with dimensions, your software, platform, and/or printer should be able to help you determine how large of a data file is needed
Digital images are fairly easy to distribute, but it is important to make sure the file quality and types are appropriate for your needs.
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Dear Digit was a “question-and-answer” column that provided resources, ideas and tips to address digital communications questions from an arts perspective. I wrote this column for State of the Arts, a bimonthly newspaper published by the Montana Arts Council.
Although the column emphasizes artsy digital communications, the topics we address are widely relevant, with application for communication in science, education, sustainability and many other sectors.