This opinion is common and untrue. If you've ever seen Tim's Vermeer, David Hockney, a well-known and very successful 20th painter, claims that Vermeer absolutely could not have achieved a particular gradation without some sort of optical aid. Then, check out https://www.instagram.com/grandcentralatelier/. Buncha 19 year olds with proper instruction developing near-absolute control over the kind of gradation Hockney claims is impossible. Hockney just can't paint, like most 20th century painters.