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« Last post by Les Olson on Today at 06:05:53 »
Leonardo objected to outlines, which were and are widely used in painting (here is Giotto using them in the Lamentation in the Scrovegni Chapel and Picasso using them in his portrait of Gertrude Stein), because we don't see outlines, we just see the end of one colour and the start of another. Leonardo didn't object to the boundaries between colours being sharp - at least in good light and for near objects (he also knew that atmospheric effects cause distant objects to be less sharp and paler than near objects).
Leonardo thought the eye worked like a camera obscura. He studied the camera obscura carefully, and he knew that in the camera obscura image blur is not related to subject distance. Leonardo had been dead for 100 years before anyone (Kepler) understood that what we see is an image formed by a lens, for which image blur is related to subject distance. Basic lens concepts like focal length were only worked out in the late 17th century and the optics of depth of field were only worked out in the 19th century.
Until the 20th century being aware of objects being "out of focus" was relatively uncommon. Although everyone who is old enough to have lenses in their eyes that have lost the capacity to change shape and has to wear corrective lenses for close work with a slightly different correction for reading a book vs a computer screen is seeing the effect of depth of field, that is not how they understand their vision problem or how it is discussed at the optometrist's. The hyperfocal distance of the human eye is about 6m, so even someone like me with no accommodation to speak of only needs correction for short distances, and young people with 10 dioptres of accommodation don't see depth of field even if they have refractive error, because accommodation happens in the brainstem, isn't represented in consciousness and can't be suppressed. It is important to remember that short-sightedness was relatively uncommon until recently because children spent more time outdoors, and in a world where there are no phones or televisions and relatively few people can read, presbyopia is not a problem.
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Now, that is more rare around here - and probably for more daring individuals like those in your image 