When I am teaching my kids at school I ask them when does the editing process start, "In Photoshop, sir" is the normal, and understandable reply. Finally, they get to understand it is the moment you put the camera to you eye (or look at the screen if you're under 25).
Then they get to understand that this is one of the areas where photography differs from the other art forms. It's what I call subtractive, you start with everything in front of you and you select what to include in your frame. Henri Cartier-Bresson allegedly never cropped his negatives in the darkroom feeling that the frame of the image as defined by the camera is sacrosanct. I am not so precious and have no particular problem with cropping in post-production. Indeed, some of the greatest images are the result of some drastic cropping, Arnold Newman's portrait of the composer Stravinsky is as case in point.
Igor Stravinsky by Arnold Newman
Before photography came along most paintings (with the exception of Bruegel and his ilk) were self-contained with no real suggestion of life beyond the frame. In the late 19th century artists like Degas started to paint people and objects half in the image, photography was influencing it's older artistic disciplines.
After many years using an SLR, and more recently as DSLR, I have now switched to a rangefinder style of camera (not a Leica, alas). Here one is looking through a window alongside the lens and inside this window, depending on the lens being used, is a bright frame indicating the image area. One is therefore observing life beyond the frame during the process of actually taking the photo, and it is possible to anticipate a person as they enter the frame itself. I have the option on my camera to switch to a digital view which will show me only what the lens is seeing, but I am trying to shoot in the way that the photojournalistic greats of old did.
It's taking some getting used to.
Oxford 'trashing' June 2017