Photographs. In the past if you wanted a portrait you needed to sit with an artist and wait for them to paint it. That limited the amount of time an artist had to get everything exactly right because the subject can’t sit still forever waiting to be painted.

Now you can take a high quality photograph and the artist can work from that taking as much time as is necessary.

Also we have much better equipment for drawing painting portraits.

As an artist I can add that painting from life or copying a photo are two different skills.

And I don’t think the materials matter that much in this instance, many people nowadays paint with terrible tools lol, mostly it’s just that it’s not noticeable when represented via digital media such as a screen :D

Consider also that in former times, rules and techniques on how to properly render perspectives were unknown. Bringing a 3D scene to a 2D drawing is not so trivial.

More on that topic: http://www.essentialvermeer.com/technique/perspective/history.html#:~:text=In%20its%20mathematical%20form%2C%20linear,De%20pictura%20%5BOn%20Painting%5D

The question assumes that accuracy and realism were the end goal of the artist or the subject, or at least would be a desirable outcome. I'm not sure that's a fair assumption. We live in a world influenced by photography. Portraiture can imitate photography because photography exists to imitate. Prior to that development, capturing the "essence" or displaying the beauty of a subject may have been more desirable.

Let's say you're painting a portrait of Washington. The man was known to be stern, but fair. He was generous to friends and supporters but also had a fierce temper if pushed too far. He carried himself with the surety and sense of nobility that a planter from a FFV would have. But those aren't physical characteristics. Still, the artist would be expected to convey those qualities through their work. So maybe you broaden his shoulders, soften his eyebrows to emphasize his eyes, reduce the swelling in his jaw, get rid of a few wrinkles in his brown, straighten his nose to be more reminiscent of Roman emperors (Idk if that was actually done except for the jaw thing. Just examples). You convey the essence of the man and not just his image.

In art class, we learned that sometimes men posed as women, so sometime the girls look muscular. And sometimes the painter would edit the buyer’s features out of either courtesy or to avoid being executed hahahahahahahaha.

She said the general assumption was that a lot of ladies were hairier, especially in the older ones.