The only reason I 'got into' photography was because I was pig sick of sourcing stock photo's for my art........... Photography has now become my main interest, but I still see art as art, no matter where it originated. As long as the artwork has a photographic base.... its photography.
It 'may' come into some sort of subclassification of that genre, but as the OP said 'Where do you draw the line'? Film choice? Aperture setting? Rearranging the pixels? Adding new elements? (thats been done ever since the first first film was developed)
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"I meant," said Ipslore bitterly, "what is there in this world that truly makes living worthwhile?"
Death thought about it.
"Cats," he said eventually. "Cats are nice."