ppi and real size confusions

Alright, you all really helped me out on this one, it's way clearer now.

Thanks a lot, and i wish you a merry holiday season.

Note, the earlier post is from Conrad Chavez, one of the important Photoshop educators. He has written a number of excellent guides, the latest that has just been published is: 
https://www.peachpit.com/store/adobe-photoshop-classroom-in-a-book-2021-release-9780136904731

Well worth all of us all getting it - it’s available as a printed book, a digital ebook, in various formats, and a matching website that includes video tutorials..

What 100% and Actual Size mean can vary across applications. In general, and even in non-Adobe applications:

• Applications based on print units (inches, centimeters, picas…) tend to define 100% as “real world size.”
• Applications not based on print units, such as those for video editing and web design, as well as most photo/image editing applications, usually define 100% as “one image pixel to one display pixel.” This means how big something looks at 100% changes depending on the pixel density (pixels per inch) of the display.

Current versions of Illustrator and InDesign use the first definition. In Illustrator 2021, 100% should equal real world size if the option “Display Print Size at 100% Zoom” is enabled in the General pane of the Preferences dialog box. In InDesign, you get real world size on screen if you choose View > Actual Size or set the view magnification to 100%.

Photoshop and all Adobe video and web applications use the second definition. But because that second definition has gotten increasingly inconsistent as display ppis have both diversified and dramatically risen, Photoshop recently added a new command, View > Actual Size. Choose that if you want a print document to display at the same size as in the real world; it works as long as the document Resolution (ppi) is set correctly for final output in Image > Image Size. If Actual Size is the most useful magnification to you in Photoshop, you might want to assign a keyboard shortcut to it using Edit > Keyboard Shortcuts.

Things get tricky when you move between those two types of applications, since they define 100% differently. If you place a Photoshop image in an InDesign layout, for it to display at the expected size on layout such as 6 inches wide, what matters is that it was set to be 6 inches wide in Photoshop. Because PPI resolution alone doesn’t tell you the real world size: It’s not enough to say you need a 300 ppi image; you have to say you need a 300 ppi image at 6 inches or whatever real world size is required. That’s why the Effective PPI readout in the Info panel in InDesign, that Derek Cross mentioned, is so important — it reports the ppi for the image at its real world size on layout, accounting for any scaling.

In the example below, the images below were created in Photoshop at very different ppi values, but import at the same 6 inches wide in InDesign because both were set to be 6 inches wide in Photoshop (Image > Image Size). This works because InDesign, being primarily for print, is based on inches, not pixel dimensions or ppi.

If the two images were going into a video or web application instead, where inches and ppi don’t matter but pixel dimensions do, then the second image would import appearing over 3x wider than the first, because the first image has pixel dimensions of 575 x 384 px and the second is 1798 x 1200 px.

Also — To accurately show a document at its real world size, the ppi value of an image has to be compared against the ppi value of the display it’s on. Current applications get the ppi of the display from the operating system (OS), and I think the OS gets that info out of the display hardware somehow. If the display ppi information handed to Photoshop, InDesign, etc. is slightly off, that might be why you see tiny differences between a document’s real world size and how big it is on screen at an application’s real world magnification.

BTW Export in Photoshop strips the ppi number altogether, because it isn't relevant for screen use (which is what Export is for). A file out of Export doesn't have a ppi number at all. It's just so many pixels w x h. When you reopen into Photoshop, it assigns a default ppi of 72, just because it needs some ppi number for other reasons.

The principles in my first post still apply.

Actually 100% means different things in PS and in Id/Ai - reflecting the fact that PS is a raster image editor, working with pixels, and the other two are vector applications, working with mathematical formulae.

In Photoshop, 100% means one image pixel is represented by one screen pixel. It has nothing to do with physical size.

In InDesign/Illustrator, 100% means print size (as long as the real resolution of the screen is reported accurately to the application).

The two are connected through the ppi number. That's the translation from one to the other. Ppi stands for pixels per inch. If you read that absolutely literally, it explains itself: pixels per inch.

When you place a rasterised image in InDesign the resolution depends on the size of the image in InDesign. Look at the Effective PPI in the InDesign Links panel. Experiment by changing the size of the image so you can see what I mean. Aim to have an Effective PPI resolution for documents to be printed of around 300PPI.

I suggest you take some of the excellent online video tutorials in LinkedinLearning.com to learn these applications (you can get 30-days free access)

Of course resturn to the forums for specific questions.