As a lifestyle photographer, authenticity is something I truly value in my work. I love to show the genuine emotion and "realness" of a moment. To do that I try to alter the setting as little as possible. What this means in lifestyle sessions is largely letting the subject take the lead and photographing what happens. In posed sessions, I look for moments when the subject isn't necessarily trying to "look good for the picture." But does "authentic" mean keeping something exactly as it is without influence? Or is it more of a concept?
Definitions
"Authentic" as defined by a quick Google search means "of undisputed origin; genuine." Merriam Webster defines it as "not false or imitation: Real, Actual." Merriam Webster also defines it as "worthy of acceptance or belief as conforming to or based on fact." Basically, authentic means the real deal.
So how much can you change before it's no longer the real deal? The latter Webster dictionary includes "based on fact" in their definition. That means there must be a certain amount of leeway. If that's true then authenticity can't only mean taking a picture without changing anything. It has to be something more than that.
In my opinion, authenticity comes down to the emotion of he subject. Is what was happening when you took the photo the same thing that is happening in the photo? If so, that's authentic. It might seem like kind of a restricting definition but it applies in all different kinds of photography . . .
Photojournalism
Photojournalism prides itself on essentially changing nothing in an image. The photojournalist tries to remain separate from what they're documenting so they don't change anything going on around the subject. Following this thought process, moving objects around the subject would change the authenticity of the photo. What I argue is that authenticity has more to do with the subject itself than what's going on around it. This doesn't mean photojournalism isn't authentic, it is! My point is that authenticity isn't about capturing strictly what you see through the viewfinder.
For example, I once did a senior session in Concrete City in Nanticoke, PA. All the buildings there were covered in graffiti, some of which was pretty profane. When I edited the photos from that session I took out all the profanity I could find but I didn't change what was happening in the photo. The authentic emotion from my subject was still preserved.
Studio
Studio is the opposite example from photojournalism. Where photojournalism tries to control nothing, studio photography tries to control everything. In a studio environment the photographer can control the lighting, background, and subject. This begs the question, is anything in studio photography authentic? Just like with photojournalism, it can be since the important part is the subject, not how they got to the picture.
In college, I did an internship with a studio, newborn and maternity photographer. She set the lighting, backgrounds, props, and even outfits sometimes. Yet all of that only served to enhance the authentic moments she captured of newborns, moms, and families.
Lifestyle
Lifestyle has parts of both photojournalism and studio photography. It tries to capture the moment as it is but allows for some artistic discretion. This is my chosen style of photography so I'm definitely biased when I say it captures authenticity in the best way. It can freeze a moment as it is but through the artistic eyes of nostalgia. Enhancing the picture to focus the viewer on the message is encouraged as long as what it's trying to convey isn't changed.
But really, there is no "best" way to be authentic because each style has a different purpose. Photojournalism is meant to document what's really going on without interference, studio photography is meant to create art, and lifestyle is meant to do both. At the end of the day, authenticity is realness in an image. Being authentic means capturing the emotion that was really there. Each photo genre takes authentic photos, just in different ways.
"Photography takes an instant out of time, altering life by holding it still."
—Dorothea Lange