It’s at the bottom right of the krita window. I have no idea what this does.
It’s a good question. It’s not mentioned in the reference guide.
I thought this was a hangover from a time when some displays did not have square (1:1 aspect ratio) pixels. Modern LCD displays do have 1:1 pixels (I know my 15 year old monitors do because I’ve just measured it with a ruler on the screen) so it’s not something that most of us need consider.
What I do notice is that if I press that button then the zoom percentage changes.
It seems that it’s to do with dpi adjustment or accomodation or … something.
There may be someone who knows what it’s all about and is capable of explaining it.
By messing around with this button I found this -
My monitor resolution is 2560 x 1440, So I created a document with same dimensions. When I enable this button and hit 100% zoom the document gets filled edge to edge to my screen which means it matches my screen pixel size. When I disable this the document 100% zoom gets adjusted to real life document dimensions in cm or mm or inches. i.e is when you measure the document by holding a ruler against your monitor it will show the exact value that is shown in the canvas size dialog. I disable this button to see the actual print size of my painting.
If you want to have more fun set the dpi while creating the document to what your monitors native dpi. For example if you monitor has 96 dpi set the document to 96dpi, so now your document should have 96 pixels for 1 inch. Now when the documents dpi matches to that of your monitors there is no different in real life size and size on the monitor so this button should not affect the zoom on canvas
TLDR: Disabling this button option shows you the actual physical size of the painting on screen,
Good opportunity for some beginner volunteers to start contributing to the manual. If anyone is interested please add information about this in the manual.