You will need to adjust an image's dots per inch (DPI), to create a thumbnail, adjust the size of an image for the web or to determine if an image's quality is good enough for professional printing. You don't have to a master's degree in graphic design to make these simple changes to your images, just alter the DPI using Adobe Photoshop.DPI is a measure of resolution for printing and the web. According to the National Park Service's promotions office, DPI "is the number of individual dots of ink and spaces between the dots that a printer produces within a linear inch space of paper." (Reference 1) When printing, use photos that have a DPI or resolution of 300 or greater. When altering photos for the web, use 72 DPI.
Adjusting the DPI
Launch Adobe Photoshop and open the file you want to manipulate. Go to the file drop-down menu and select "Open," then browse through the file directory to select the file you want to adjust.
Adjust the DPI by going to the "Image" drop-down menu and selecting "image size." Be sure to leave constrain proportions, scale style and resample size checked. Depending on your particular version of Photoshop, you may or may not have the options named above. Regardless, constrain proportions should remain checked on all versions to avoid warping the image.Next, go to the "document size" section. Here you will find your image's width, height and resolution. If you want to make and image smaller, say to be used on the web, change the resolution to 72. If you want to be able to the print the image, change the DPI to 300. After you've made the changes, press "OK".For the best results, stick to reducing the DPI rather than increasing it. According to Photoshopessentials.com, "it's usually okay to make your image smaller without losing much in the way of image quality. However (and this is a big "however"), you will absolutely lose image quality if you try to make your image larger." (Reference 3)When Photoshop makes an image smaller, it removes pixels from your image, but when you increase the DPI, Photoshop adds pixels to your image. The program automatically adds pixels in places where it thinks they should go. Photoshopessentials.com warns that "the larger you try to make your image, the worse your image is going to look. It will have a very soft and blurry appearance, not the crisp and sharp image you'd normally want." (Reference 3)
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Save the file. Go to the "File" drop-down menu and select "Save as." Save the image as a .jpg file for maximum compatibility among computers.