Last night I sat in a movie theater and watched the HD broadcast of the Leonardo da Vinci in Milan exhibition from the National Gallery in London. It is better to see the paintings live, but it is already too late. The entire exhibition is sold out.
Included in the exhibition is a copy of the Last Supper by one of Leonardo's students. They explained how slowly Leonardo painted, thus providing the reason he didn't want to do fresco to paint the Last Supper. For fresco you have to paint very fast. You must finish the painting before the plaster dries.