Today’s Question: Magazines are asking for images at 300 ppi resolution, even for images that will only be presented online. Doesn’t this mean I’m providing “wasted” resolution? Also, when I’ve resized images and displayed them alongside each other in Photoshop, sized to fit the screen, the higher resolution image (at about 30% zoom) appears less sharp than the lower resolution image (at nearly 100% zoom). What is going on here?
Tim’s Quick Answer: The pixel per inch (ppi) resolution is essentially meaningless for images that will be displayed digitally. Also, the closer an image is sized to a 100% scale on a display, the sharper that image will generally appear.
More Detail: A reference to “300 ppi” for an image suggests that the intent is for the image to be printed. While optimal resolution settings vary based on the specific output, 300 ppi is a common setting for printing. For an image that will be displayed digitally (such as online or in a digital slideshow), all that matters are the pixel dimensions, which should be set to match the intended presentation size.
Of course, you could still describe the pixel dimensions for an image that will be shared digitally using a pixel per inch value. For example, you could specify that the image needs to be sized at 6 inches on the long side at a resolution of 300 pixels per inch. But it is much simpler to just say that the image should be 1,800 pixels on the long side.
Regarding the image with larger pixel dimensions not appearing as sharp or detailed as the image with smaller dimensions, this relates to the scaling of the image on the display.
An image will look its best in terms of sharpness at a 100% view, referred to as the Actual Pixels view in Photoshop. This is why it is recommended that you view an image at 100% scale when evaluating settings for sharpening a photo.
With the scenario described in today’s question, the higher resolution image is being scaled down to fit within the display area, which causes a loss of perceived sharpness in the image. If you viewed both images at 100% scale, they would both appear equally sharp, though you would obviously be seeing a smaller portion of the image that has larger pixel dimensions.
Photoshop does a good job of scaling the preview for images, but viewing at a 100% scale still provides an advantage in terms of sharpness. Just because an image has larger pixel dimensions does not mean that image will appear with better quality than an image with smaller pixel dimensions. What matters most is that the pixel dimensions are optimal for the intended output.
What that translates to when sizing images for a digital display is that it is best to resize the image to the exact pixel dimensions that will be used to present the image. For example, for a digital slideshow you would want to resize the image based on the resolution of the digital projector to be used.