Cameras do not see a scene the same way the human eye does. The camera is an unflinching tool which sees each and every object in the field of view with the same clarity as every thing else. It is not selective. It does not care one whit for anything it sees. The human eye, on the other hand, scans a scene and picks out specific things on which to focus, things it likes and things it dislikes. It spends more time dwelling on the things it likes and passes over the things it dislikes. As an artist working from photos or in plein air, I get to be the eye of the viewer. It is my selection that determines which objects will be your focus in the end piece and I carefully eliminate the things I determine you will not like. The green artist takes the photo and tries to replicate everything they see. They fail to understand that a camera is a tool and presents no truth. In my mind, there is more truth in the artist's carefully manicured result which selectively emphasizes some things and eliminates others.
Cameras also have a nasty habit of mangling color relative to what the human eye perceives. I think this is particularly true of digital cameras. While the modern camera may be able to capture a much broader gamut of colors than many humans can see, this does not capture light in anywhere close to a manner similar to human eye/brain system. The image formed in your brain is dynamic and each rod and cone produces a response to an individual photon of the wavelength it is designed to detect. Meanwhile, a digital camera is like a vast array of tiny buckets, with each individual bucket being a pixel in the final image. Photons of various wavelengths from many many different directions strike the focal plane array (or bucket array) of the digital camera during the time the shutter is "open" and the energies of each photon is captured and stored in the bucket that it fell upon. A pixel bucket that depicts part of a tree in the final image will have collected photons from many more sources than just the tree. The camera will take all of the wavelengths and average them together to get a final color that is NOT what the eye sees when looking at the same tree. Professional photographers know this too and that is why all of the good ones post process their results to some extent in software! There is nothing inherent in paint that makes it capture a better image of what the mind reads from the eyes more readily than film or a focal plane array. It is the studied mind of the creator behind the process that allows pigments to be assembled into a selective construct which we will call art. The green artist will again look at the photo and try to replicate the colors the camera made up. As an aside, I think it's very fascinating to think the manner in which our eyes detect color and the mysterious manner in which our brain assembles those nerve impulses into an image is probably not a very good approximation of the truth of reality either! Think of all the data we are missing because of the response time of our rods and cones or what types of data our brains may just simply make up to bridge gaps that our eyes cannot process! How different the truth of the world may indeed be from our limited perception.
If there IS one thing that cameras tend to do well, it is to capture value information. Value is, and I suspect few people would argue with me, the backbone of realist painting. Without proper values relative to one another, the whole illusion of space and three-dimensionality falls apart.
Working with photographs is an excellent method, but only when one realizes that the picture can only provide so much information and the mind of the creator must make up for their limitations.