The Middle is not Where it's At
Lens tests confirm that the sharpest aperture tends to fall somewhere in the middle. As the old saying advises "f/8 and be there," I suppose. What with aperture and focus, exposure and composition, photography is filled with variables and choices. But sticking with the middle as your default is rarely your best choice.
Exposure meters are calibrated to optimize results to medium toned, or more specifically, eighteen percent reflectance. Whether it's the one built into your camera or a hand-held one next to it, there's really no way for a meter to know how bright something is supposed to be. That subject you are contemplating shooting may be dark gray lit by very bright lights, or it might be pure white but in a dimly lit alcove. There's really no way for your meter to know. Maybe someday we might have meters with artificial intelligence capable of interpreting the likelihood of such alternatives, but not any time soon at least. For now, camera meters are only able to show brightness on a more objective scale.
Failure to understand this little detail has resulted in more over and under exposed images than any other cause. While it is true that the world, on average, is by definition "average" hued, a list of subjects that refuse to play along and abide by this rule would be long indeed. "Program" metering modes may try to guess when something should be rendered brighter or darker than average, but more often than not, they'll default to medium toned. Somewhere along the way, aspiring photographers lean to open up a bit when shooting snow or stop down a tad to keep a black cat from coming out gray.
The correct exposure isn't always medium toned. And the correct aperture isn't always the one in the middle if you need something else get the right exposure or to achieve the desired depth of field. What works as a default choice isn't always your best choice when shooting in the real world. There tend to be other factors to be considered. And you need to set things as they should be to meet those real-world challenges.
Auto white balance works by neutralizing any detected color cast. This works great if you don't want to deal with compensating manually but can be a real bummer if you expect your sunset images to have that same warm glow you remember from being there. Some things are supposed to have a color cast. Relying on automatic white balance may be an easy default, but it won't always give you the best results. Whether you prefer to handle white balance in-camera or during post processing, don't just settle for the middle, default choice.
Being in the middle is often the default choice. This may make things easier in some ways, but it can't be relied on to give you the best results.
When shooting a subject, the default choice would be to bullseye that subject in the middle of the frame. Doing otherwise might risk cutting part off part of your subject if it veered too close to an edge of the frame. Even talking in terms of "aiming" a camera at a subject would tend to promote the center of the frame as being some sort of goal. And if you did avoid placing your subject dead center, what about the horizon line? See, gotcha. Defaults are easy. And because this compositional default is so pervasive, a whole industry has built up around teaching the rule of thirds as being the central gospel of good photography.
The default choice of the beginner is to place the subject in the center of the frame. The default choice of the more experienced photographer is to place the subject directly on "power point" at the intersection of two of the grid lines. But as with all other choices to be considered when shooting, the default choice in the middle isn't always your best choice if you stop and think about it. And while defaulting to those thirds lines may get you in the vicinity of a great image, it's worth pausing for a bit to check if you can do better still before you press that shutter release button. Stop and look at what you have to work with. Place the subject where it should be to help you create the best image you can.
This whole middle thing as default goes further still. Most cameras are available as a kit together with a 50mm "medium" focal length lens. I guess this would avoid disappointment for those new to photography who realize only after their new camera arrives that all they have is a body and can't take any pictures without paying for a lens too. That would indeed be a bummer. But as a first lens, 50mm strikes me as being unfortunate even if it is understandable. 50mm provides a good approximation of the angle of view for human vision. They are easy to design and generally require few glass elements. All this tends to result in 50mm lenses being small and cheap, both factors that work in its favor when it comes to picking a default lens. All things being equal, smaller and cheaper wins over bigger and more expensive.
Yet the power of photography owes at least in part to its ability to show us the world in a way we don't normally see it. To give us a new perspective on something, ordinary or extra-ordinary. Personally, I find that I shoot more often at the ends of the focal length scale and less so in the middle. I really love a good ultra-wide angle. A medium focal length 50mm lens may provide us the default perspective of the human eye, but if a different focal length will provide a more compelling perspective, my vote would be for something other than the middle. The middle is too predictable. Too safe.
This same line of reasoning could also be extended to where we go to shoot images. The default would be to visit famous landmarks, national parks and the like. That's where everyone says we should go. The easy answer would be shoot where the scenic overlook sign tells us we should shoot. They often even have a camera icon on such signs. But while there are amazing images to be made when standing next to the visitor center parking lot, it's all the more probable that such images will come out looking pretty much the same as those of everyone else who has ever or will ever stand at that same spot with their camera. Mind you, it's hard to take a bad image when surrounded by such beauty, but it's also not easy to rise above being "average." For one thing, there's just too much competition unless you're willing to get off the beaten path, whether that be further down the trail in the same national park, in a city park or other small pocket of nature closer to home, or in your own backyard where no one but you is likely to shoot.
It may not always be easy working without defaults, but it's there that you will discover what you are truly capable as a photographer. And who really wants to be an "average" photographer in the middle of the pack anyway?