Posters are inherently graphic ways of communicating information. When dealing with digital imagery, it's important to start by understanding the technical basics, and how they help you obtain a quality result. Please watch or read the following documents about pixels, resolution, and file types.
"Pixels" means "picture elements," and they are little, monochromatic squares that make up all bitmap images. Watch this video to understand how pixels work.
When making a poster, you need to have print quality resolution. Image size and resolution are some of the basic elements of a bitmap image, but ones that frequently trip up the uninformed user.
Resolution is a function of the information, the amount of pixels, in an image, measured per inch (dots per inch or pixels per inch). This matters most when it comes to printing, as most printers create the best images, with photographic quality, at 300 dots per inch (dpi). If you need to raise the dpi for printing, you have to decrease the size in inches the image prints at. Since in most programs one pixel is equal to one dot, it's a simple formula: the number of pixels divided by the number of inches is the print resolution; put another way, the number of pixels in an image divided by the desired dpi is the number of inches the image will print at that dpi. If you want it to print at 300 dpi, you'd divide the number of pixels of the width or height to find the size the image will print at 300 dpi. If you want to print a 10 inch wide image at photographic print quality, it will need to be 3000 pixels (3000p/300dpi=10in). If your image is only 1000 pixels, the largest it will print and still be photographic quality is three and a third inches.
Lucking for us, Photoshop or Preview can do the math for us—and change to the appropriate size in inches for us at the same time. Watch this video to learn how size and resolution are related, and how they can be manipulated without distorting the image—one of the most common needs from patrons making large format posters. While the tutorial uses Photoshop, the process is identical in Preview.
Digital Image File Types Explained
This information is very useful for assisting patrons: what's the difference between JPEG, PNG, and TIFF? What's Camera RAW mean? And when should a project be saved as what file type? Follow the links above for an explanation of the different types of files you can save images as, and when to use each one.
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