If you're an amateur who photographs because you love to do it, there are many reasons to shoot a picture. But it seems to me you're always trying to do one of two things: you're either trying to record something you found interesting or you're reacting to an emotion you got from a scene and hoping to pass on that emotion to others who will see your picture.
In the first case you're reporting, and to do that effectively your picture has to include enough information to get its point across. A picture of flowing water doesn't make the point that the dam is leaking and is about to break. You need enough of the dam and its surroundings to make clear it's a dam and that it's leaking. Beyond that you need enough technical perfection — sharpness, contrast, color relationships — to make the details clear: the hairline cracks that show the dam may break.
But it doesn't require a completely understandable picture to convey emotion. What's missing may enhance or even pass on an emotion. A ship doesn't need to be in the picture of a woman on a widow's walk staring out to sea as the storm gathers. A ship might even detract from the effect. You could include a sinking ship, but the viewer's emotion probably would be hilarity. A picture of the woman, the sea and the storm has ambiguity and will, as Eliot's Prufrock put it, "drop a question on your plate." An unresolved question often leads to an emotional reaction.
At this point I'll go out on a limb and for the rest of this article define things created by humans that convey an emotion as art, and things that simply convey information as reportage. The distinction applies to the results of human creation whether it's photography or painting or poetry or any other creative genre. Obviously the two categories can overlap; reportage sometimes includes art, and art — especially visual art — often contains reportage. What makes the difference is the presence or absence of something that strikes you emotionally.
Let's look at an example of the difference. Consider two of Ansel Adams's famous pictures: "Moonrise Yosemite Valley" and "Moonrise Hernandez." The picture of Half Dome is dynamic, striking, desolate. It has stark cliffs, snow, trees, a rising moon. Hernandez, on the other hand, is subdued. It has mountains near the horizon, clouds, the moon, a desolate plain. But it also has human habitation and a little graveyard in the foreground with crosses flashing white in the setting sun. The isolation of the village and the white crosses are things with which you, as a human, connect. You can't help wondering what it must be like to live in this tiny, isolated, momentarily beautiful community under a rising moon.
Stories about how Ansel shot the two pictures are interesting and instructive. Ansel shot the Half Dome picture with a Hasselblad mounted on a tripod. He carefully metered the moon with a spot meter that had a very narrow angle of view, and he made several exposures with various lenses and an orange filter to darken the sky. Everything was carefully planned and executed, and he made notes about the various exposures regarding what he planned to do with them in the darkroom.
Moonrise Hernandez's birth was more dramatic. Ansel was driving home from a New Mexico shoot when he saw what was happening in Hernandez. He says he "almost ditched" the car getting it stopped, and rushed to set up his 8 x 10 camera. Once he got the camera ready he realized he couldn't find his light meter. The moment was passing quickly and the scene was changing. His explanation of what he did then is complicated, but what it amounts to is that he made an educated guess at the exposure and made the shot. He wanted a second negative for insurance, but in the time it took him to pull the film holder and reverse it for a second shot, approximately three seconds: ". . .the sunlight passed from the white crosses, I was a few seconds too late."
I'd say that Moonrise Yosemite is the more conventionally "beautiful" of the two pictures. But Moonrise Hernandez is Ansel Adams's most famous and most sought after picture. The difference, I think, is that Moonrise Yosemite is reportage while Moonrise Hernandez is art.
Consider the difference in approach. Ansel had made other pictures of Half Dome and he knew when the moon would be where he wanted it to be for Moonrise Yosemite. He planned ahead, set up carefully and made a series of exposures. His emotions certainly were involved. He'd been struck by the beauty of the rock in moonlight, and he wanted to capture it. He did of course capture it, and the result is a lovely photograph of an object in moonlight. But it's a photograph devoid of human context: a beautiful object that's in front of your eyes, but that doesn't echo in your heart.
But look at how Moonlight Hernandez came to be. Ansel had been over Highway 84/285 and past Hernandez many times before. I'm sure he'd always glanced at the sparse little town of Hernandez as he went by. In the story about Moonrise Hernandez you can find in the online Ansel Adams Gallery Ansel says: "We were sailing southward along the highway not far from Espanola when I glanced to the left and saw an extraordinary situation — an inevitable photograph!" At that point he screeched to a halt, jumped out, and made a snapshot, if you can call it a snapshot when you're using an 8 x 10 view camera on a tripod.
It's pretty obvious that what made Ansel slam on the brakes and shoot Moonrise Hernandez was a strong emotional reaction to what he saw. He captured the scene that gave him that reaction, and considering the fact that he couldn't make copies of the picture fast enough to keep up with the demand it's obvious there are a lot of other people who have a similar reaction to the picture.
Nowadays if you were driving down Highway 85/285 past Hernandez in the evening and got smacked with the kind of emotional jolt that hit Ansel you could hop out of your car with your Nikon D810, lift the camera to your eye and make your shot. No fumbling with tripods, film holders, light meters or any of the other paraphernalia you need with large film cameras. And before the sun moved off the crosses you could go click, click, click, click, and bracket your shots. Later on you could even merge your shots into an HDR picture if you were so inclined.
As Cartier-Bresson said, "Photographing is nothing. Looking is everything." To make art — to capture the kind of artistic success Ansel Adams captured in Moonrise Hernandez — you can't plan ahead. You go out with your camera and you look! And when a scene grabs you emotionally you raise your camera and make art. That's the most important reason to click the shutter.