Can PPI be compressed into jpg?

[daeyeth]

this is more of a general question but I have something I was really confused on. I downloaded two jpegs from DA, one has a height of 1500px and in PS its document size is listed as 5 inches. The other has a height of 1000px and yet its document size is 15 inches. Wtf? How is that possible? Is this an error or does this have something to do with PPI or something else?

[daeyeth]

also, outside of PS, the 15inch image is only 300kb and yet, in PS, its document size is listed as 2.8mbs. I am so confused.

[Metsys]

Yeah, there's meta data in the JPG format that says what the PPI should be. All the PPI data does is tells what size the file should print at, or how big it should be when you place it in an Illustrator, InDesign, Word document, and so on.

In fact you can change the PPI without doing anything to the image. Just go to Image > Image Size, un-check "Resample Image" and then change the PPI (in the Photoshop dialog box it's marked as Resolution). That allows you to change that PPI value without doing anything to the image whatsoever. That's useful if you want an image to be placed smaller in another program so it's more manageable. It can be pretty annoying, for example, when a photo is brought in and the PPI is set to 72, and because it's a high-res camera the photo when placed is huge on the screen. Even with illustrations an artist, when creating a new file, will leave the PPI at 72 instead of changing it to 300 or 600, depending on what they are doing with the image.

As for the size difference, I believe the 2.8MBs is Photoshop telling you how big the PSD file would be if you saved it as a PSD, or how much the image file is using in system memory. Not sure which. Of course the JPEGs are compressed so it'll be the small 300KB file size.