These are just details of the face - there is a lot more to each picture that bears looking into. The assertion of The Mona Lisa Foundation is that the picture on the left was painted by Leonardo eleven years earlier than the picture on the right, and that he painted both pictures. They seem to have a lot of evidence which they say supports this; far more than I care to dig through. That is why my opinion must remain uneducated. However I live in the Kooks Museum and our shelves are full of books by people with something to prove and the one thing they all have in common is that whatever it is they have to prove, whether it is perfectly sensible or completely crazy, they all prove it, at least to their own satisfaction. I am also a painter and I have found that people who set out to prove things with lots of supporting evidence usually think in a very different way from the way a painter thinks, as I have pointed out in the past.
Here is what my opinion is. It is my opinion that this is not necessarily a person painted by same artist eleven years apart. One painting is copied from the other painting. I am an artist and I am going to paint the same person eleven years after I first painted her. I have painted a really good portrait of a cute little gal and eleven years later I say I want to paint you again as a kind of stodgy matron only everything else will be EXACTLY the same except for the background. I will even give you the same hands and not paint the hands you have now which are eleven years older. And she and her husband are going to say okay that sounds good. What gets me is the line of her hair on the left side. I might be
able to cajole a woman into wearing the same costume and same hairstyle
and into striking the same pose eleven years later - which itself does not
make much sense except in a 'conceptual art' context which I don't think
even existed then - but her hair is not going to do the exact same
thing as it did eleven years before. It isn't even going to be the same
hair. Her hands are not going to be in exactly the same position, the folds in the sleeves are not going to be exactly the same. Or maybe I would just have a couple of assistants copy the previous painting and I will just step in and do the face and make her look kind of flabby and grumpy compared to the peppy little doll in the old painting. Then I have to make sure they don't see the first painting so they don't go what the hell you made her look like her own mom, that's not what we are paying you for.
One of those pictures is copied from the other one, so side by side comparisons of the proportions of the two paintings don't prove anything because all you need to do is use any one of three simple mechanical devices to copy the forms and proportions exactly, which any apprentice could do. They both seem to be painted with a great degree of skill and they both look pretty old. As I said I have never been able to grasp the romantic ideal of the Mona Lisa so I don't really get what a great painting it is, but it seems to be well executed. Both of them. I am not qualified to make any statements on how much of Leonardo's paintings were done by assistants, how often he used what kind of pigment on what kind of board or canvas, etc. but I will tell you this as an artist - I am not the only person on earth who has looked at that painting and said, "That would be a pretty good picture if you fixed up her face a little, because frankly she is not that great looking a gal." I think that is what we have got here. But what do I know.