My personal belief is that the typical consumer has swallowed the bath water about resolution, in spite of what was written a decade ago about NOT really needing much more than about 25MPixel to run into Physics and diffraction.
The human eye only can resolve about 1 minute of arc, so
- in a 4" x 6" print viewed from 78" away, we need not many pixels at all; we only need 4.5MPixels and any more our eyes are not able to detect!
- in a 8" x 10" print viewed from 12" away, we need not many pixels at all; we only need 8MPixels and any more our eyes are not able to detect!
- in a 16" x 24" print viewed from only 12" away (instead of at 24" viewing distance), we only need 32MPixels and any more our eyes are not able to detect!
And things have been published on the web about billboard size images being perfectly presentable with source images having only 2MPixel.https://www.youtube.com/watch?v=UCaZt5ndRW0
About 'ratio'...there is no ratio per se which is related to 'resolution'. The ratio I think about is 'aspect ratio' which tells the relative size of one side vs. the other side.
1" x 1" is 1:1 aspect ratio
8" x 8" is 1:1 aspect ratio
4" x 6" is 1:1.5 aspect ratio
8" x 10" is 1:1.25 aspect ratio
the APS-C and the FF image is 1:1.5 aspect ratio
If you start with APS-C image in camera, and want to print an 8 x 10" print to be framed for the bookshelf, you have to either:
- cut off about 17% of the original image length so it fits the 1:1.25 aspect ratio
- fit the long dimension into 10", leaving 1.3" of white space in the height of the 8" x 10" tall paper.
...most commercial print making services do the first, and you have little control usually about what gets cut off!