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Young adults with 20/20 vision can often focus at 4 to 6 inches. At 6 inches, they can see detail at 600 pixels per inch or 300 line pairs per inch. As you age, your lenses become less resilient and the focus distance increases. At about 50 years, the average minimum focus distance is about a foot corresponding to 150 line pairs per inch or 300 pixels per inch. These numbers reflect good light and high contrast scenes. Generally, under more typical conditions, people don't see this well. --.
It is a common mistake to describe normal lenses by comparing focal distance to sensor or film diagonal - a normal lens shows the proportions like the human eye. Tele lenses compress them, and wide angle lenses expand them, as you take the proportions of far and near objects. 50mm is about the average for the human eye and it changes nothing if your camera crops or not.