And doesn't changing the ppi end up changing the image size? Example, a 72x72px image at 144ppi is actually 144x144px, right?
By @james45091159
It’s not a stupid question, but it is one of those questions where the answer is “It depends…”
Everything depends on the idea that ppi is pixels per inch, which means pixels divided by inches. The basic formula is “pixels / inches = pixels per inch (ppi)”.
72 x 72 pixels, then, can mean only one size at 144 ppi, and that is 0.5 inches.
Because at 144 ppi, 72 pixels covers half an inch (72 pixels divided by 144 ppi equals 0.5 inches).
A 72 x 72 px image can become 144 x 144 px at 144 ppi if it is not only resized, but also resampled. One way to do that is to enable the Resample option in the Photoshop Image Size dialog box. Resampling allows pixel dimensions to change (from 72 px to 144 px on a side), but image quality can be reduced because pixels are either being made up or merged together to reach the new pixel dimensions.
But if you are keeping pixel dimensions constant (that is, not resampling), then resizing an image for print always goes through “pixels/inches = ppi”. So…
72 x 72 px printed one inch wide = 72 ppi, or…
72 x 72 px printed at 2 inches wide = 36 ppi, or…
72 x 72 px printed at 300 ppi = 0.24 inches wide, and so on…
…all from plugging the numbers into that formula of “px/in = ppi”.
But for on-screen graphics like a LinkedIn profile picture, ppi does not apply, because dimensions of on-screen graphics are rarely expressed in inches. And again, without inches, there is no ppi. On-screen graphics, such as for web, video, and social media, are expressed in pixel dimensions only (no inches), such as 400 x 400 pixels. (Although some do say “72 dpi,” that doesn’t really make sense for the screen. 72 dpi displays have not been common for many years.)
The problem with stating ppi for an on-screen image is that it isn’t like a printed image where the size stays the same. On screen, an image’s actual real world ruler-measured size changes for each viewer, depending on whether you’re viewing it on a big computer screen, a laptop screen, a tiny phone screen, or a huge projector screen 10 feet wide. And if someone increases the zoom level in their web browser, that real-world size changes again. So there can be no single ppi number for an on-screen image.
If LinkedIn asks for a 400 x 400 pixel image, you resize, correct, and sharpen for 400 x 400 px without even having to think about ppi. Then save that in PNG format, or JPEG format with High Quality compression. (Maximum Quality is not needed or recommended because the visual improvement over High Quality is hard to see, but the file size goes up dramatically.) It looks like a LinkedIn profile page actually shows the profile picture at 128 x 128 pixels, and uses other sizes in other places like comment threads. They probably just want 400 x 400 px to have enough pixels for various display sizes throughout the site.
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