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Thanks jno, interesting links, particularly the pinterest - amazing selection.
Couple of thoughts from that - one, that the usual portraits of Henry VIII's women were painted by inferior artists (I know, I know, but just LOOK at the technical skills of the others). Most of Henry's unfortunate wives and others look flat, and not very good looking. Perhaps they weren't. Inbreeding in Europe's royal houses was/is rife.
Two, that your last point is almost certainly true - that even the great Leonardo got better in 30 years. Unlike most modern 'celebrity' artists, who start off poor, and end up enriched, but no better as artists.
I still don't think that giving an average (or in the case of BritArt's mob a poor) artist an iPad as a visual aid does them much good.
Unless you're the one in a hundred who is 'born with it', then only very hard work and practise, practise, practise, gets you as good as the example you and others here have shown me.
I'd love to have seen one of Zeuxis's works...
BB