P.H. Emerson was adamant that one should never manipulate the photograph after shooting it. You must, he felt, shoot from nature with at most modest interference, and then you can manage tonal placement through development, and that's it. No burning and dodging, no erasures, no enhancements, nothing. He did ask people to pose, but used only actual people doing the things they actually do. Emerson felt that manipulating the negative or the print made you simply a bad painter.
He's got a point. Gursky is a painter, and a pretty good one. Between Emerson and Gursky, well, pretty much everyone else falls in there. So, of course, it's a spectrum. I manipulate pretty heavily, and am ok if Emerson chooses to dub me a poor painter from beyond the grave.
More specifically, though, Emerson felt that manipulations sucked the life, the juice, the vitality out of a picture. A photograph begins with a certain potent reality, due to being a true and accurate transcription of certain phenomena. Manipulating that may well improve some things, but it seems to me, usually does suck some of that juice from the picture.
We're probably all seen walkthroughs of "my post processing process" where some idiot starts with a pretty terrible picture, and then photoshops it gradually but determinedly worse and worse. Each stage of this bozo's process makes the picture palpably worse.
What's more interesting are the walkthroughs where each stage appears to make the picture better, but the result is worse. Our hero might straighten the horizon, and then clone out a little group of trees that is distracting or out of balance, and may warm up the colors to make the picture more appealing, and so on. Comparing each stage with the previous stage, we think "yeah, I see, that is an improvement" but when we finally compare the end result to the original, we are unsatisfied. The original feels better.
This is partly due to the fact that at each stage we're focused on the thing the artist is doing. Why yes, the little group of trees was distracting, and now they are not there, so we're not distracted -- but the whole picture, which we are ignoring -- has suffered. So we lend too much value to the change.
Then again, I think there are optical effects in play here. The group of trees may have altered the light falling on another object nearby, and now the light feels off. There are, at least some of the time, subtle optical effects that are rendered incorrectly by the process. We miss, in the step by step, any actual damage that gets done to the picture.
Finally, there can be end up being an unreal sense of too much perfection. This is most obviously seen in overly 'shopped portraits. Each effect is not too much, sometimes skin really is that smooth, sometimes eyes really are that brilliant, sometimes faces really are that symmetrical, but the overall effect is that of falseness. No face is that perfect, the sleekly plastic. We sense that it cannot be real, because it lacks the tiny flaws and imbalances that the world is filled with, made out of.
Somehow, this all adds up to a sense of wrongness, of discomfort, a feeling that the juice is gone from the picture.
Me, I paint, but I either paint a lot, or a little. If I'm painting, I'm painting. Work the picture until it fits with what my imagination loves. If I'm not painting, I'm not painting. Clean up a little this and that, but don't go nuts. Keep some of the juice.