I only use other peoples photographs in class as an exercise but painting from my own photographs, I use them with the sketches I may have done. Its not the same as being there and painting from life, the environment can play such an important part as to why you are doing a painting in the first place but it shouldn't be totally ignored as a recording device. Set yourself a time limit if you like, but I tend not to add every detail because it is done in the comfort of my own home. Richard Schmid notes that they are, 'technically smart but life-dumb instruments'. I only had a few hours in the city before moving on so it was time well spent sketching and photographing work that could be used at a later date rather than searching out one definitive painting.