Viewing Distance

When you stand in front of the Nightwatch, an enormous painting of 4,5 meters wide, to see the whole picture you would have to stand at quite a distance.
Looking at a gravure - in this case 'the Beheading of Jane Grey' slide 01-03a with a size of 12,6x17,7 cm.- your distance to the artwork would be 60 - 70 cm. at the most, To appreciate the incredibly fine details slide 01-03b, you like to be a lot closer still.
How convenient is that on a smartphone, luckily you can (and often will) enlarge pictures on your phone quite easily.

Calculation viewing distance

The Execution of Lady Jane Grey by Paul Delaroche Link: (not a very good reproduction)