Well - bluntly - this sounds ridiculous.
This approach sounds quite misguided.
DPI (dots per inch) is determined by the output device and the printing method used.
For instance, in lithographic printing, different LPI (lines per inch) settings lead to varying results. Newspapers typically use coarse paper, which absorbs more ink.
To calculate DPI for lithographic printing, many people mistakenly use double the LPI. However, this isn't entirely accurate because halftone dots are rotated 45 degrees, impacting the effective measurement.
Using the Pythagorean theorem, where A^2+B^2=C^2 , the diagonal measurement yields a factor of about 1.41.
The actual number to multiply by is 1.41 - which is harder than just x2 which is why people use x2
For Newspaper printing - the LPI is set quite low - which is around 80-120
For newspapers then it's 80x1.41 = 112.8 dpi up to 120x1.41= 169.2
And you'd need to ask your newspaper what their LPI is
Magazines could use 120-150 = again multiply the LPI by 1.41 to get the optimal DPI
This is where the magic 300PPI or DPI comes in for print It's a simplification of the 150 LPI output at x2 = 300
And that's why people say it must be 300!!!!
High end coffee magazines etc could use 175-200 LPI
So you're limited by the output device LPI in Lithographic Printing For digital printing, it's a bit more forgiving in a sense You could go below 225 as low as 200 or even 175
When dealing with various types of images such as grayscale images, line art, and bitmaps—different DPI considerations apply. Line art, for example, typically requires higher settings in around the 1200DPI to maintain sharpness, while grayscale images might vary based on the desired print quality.
You could have a foggy scene or photo and where your LPI might be 150 on the output device - and you set your image to 211.5 DPI - that might not be necessary - you could go lower for foggy images, or images which are not very sharp - even to 150 or 125 DPI
Then you have large format printing - you typically don't need 300 DPI for billboards - usually 30DPI is fine for billboards.
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All that being said - you won't know what DPI to make anything until you ask the printers what LPI their devices use (if they even know) - given for Litho it's usually 150 - then you are safe around the 225 mark or 250 or 300 -
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Finally - INDesign you can select your DPI of images when exporting In the Compression Settings for the PDF
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In short Ask your designated printer what DPI is optimal - and set your compression settings to suit the PDF export.
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