Hockney vs Sontag

I've a great deal of time for David Hockney and I unfairly took against Susan Sontag before reading her because of his remark: 'The point that Susan Sontag ... does not deal with in her book ... is the fact that the invention of the chemical process was simply added to a Renaissance drawing machine.'  (Hockney, Joyce, 2008 p.71 ).  Yet Sontag says that the use of photographic slides: 'goes back even further into the camera's pre-history, for it amounts to using the photographic camera to do the work of the camera obscura.' (Sontag, 1978, p.125-6).
I'm enjoying and learning from Sontag at least as much as Hockney, though I find this remark at least delightful: 'Nobody has been able to make a photograph that moves you in the way that a Rembrandt does.' (Hockney, Joyce, 2008 p.72 ).

Hockney, D & Joyce, P. (2008) Hockney on art. London: Little, Brown.
Sontag, S. (1978) On photography. London: Penguin Books