I should really elaborate on my posts a bit more.
So here it goes.
You want your image to be 300 dpi at the desired dimensions in your project. So if your image is going to be an 11x17 inch spread, then your image needs to be 300 dpi at 11x17 inches. This would make the pixel dimension be 5100 x 3300. If you just want your image to be a 1 inch square thumbnail, then you would need your image to be 300 dpi at 1x1 inch. This would make the pixel dimension be 300 x 300.
This is incorrect.
When you place your image at size it will have what’s known as an Effective Resolution.
For example, placing a 72 PPI image into your layout and scaling it to 24% would mean that effectively it’s 300 PPI.
72/24*100 = 300
And the 300 PPI for print is completely false.
This is based off a Litho Printing algorithim that has taken the globe by storm for over 30 years.
In Litho Printing LPI is the lines in halftone or screen. The higher the number of lines the smaller the screen, the lower the number the larger the screen.
Computers are great - but did you know that mathematically they cannot rotate objects outside of 45 degree - or at least they couldn’t not so sure now - so long since I checked.
However, the LPI for haltones meant a Square of dots would be rotated 45 degrees for the screen angle.
When we measure a square from side to side - say it’s a 1x1 square.
If you then measure from the corner to corner it’s 1.41.
It’s basic trigonometry.
It’s this rotation number, 1.41 - that is multiplied by the LPI.
And someone somewhere had their LPI set to 150.
And that 1.41 number got rounded up to 2 for ease of mulitplying.
150x2 = 300
And this number has now stuck with everything.
Where it should really be 211.5.
However, the LPI would change for a newspaper - so that could be 80 - 120.
so you’re looking at 112.8 up to 169.2
For RAG magazines you could be looking a LPI of 120-150
For higher end gloss magazines you’re looking at 150-200.
So even if the LPI was 200 - and you multiplied by 1.5 - that would be 300 PPI.
And all of the above is perfectly fine for printing off Litho printing using certain LPIs.
Digital Presses are not like this - in fact can be a lot more forgiving (and you don’t need 300 PPI even for most Litho printing given a LPI of 150).
You then have many more different printing methods:
So the 300 rule is fake.
I 100% never ever ever resize images in Photoshop to exactly the size required at 300 PPI.
That should never be done - unless for a very specific reason.
With this - you’d be left resizing your images for every type of printing - because only Litho would require the faux 300ppi rule.
Bottom line is to ask your printers what way they want it setup and what image resolution is best for their printing method and output devices.