Alpha is a simple story. It only has two characters of note, and one of them isn't even human. It's about a prehistoric human boy and his attempt to make it home after being left for dead by his tribe. It's a mostly quiet and emotional journey. It may not come across as the sort of film to be given the 3D treatment, but that is exactly what has happened.
Normally, 3D movies are reserved for animated films with eye-popping colors or special effects action blockbusters. Alpha is not either of these things. However, that doesn't mean that it doesn't fit the 3D mold. Because so much of the story of Alpha is told visually, with little to no dialogue, the environment itself is as important a character as the boy and the wolf. For that reason, it's important that the environment is given as much opportunity, and as much dimension, as possible.
Because the environment is essentially a character on screen, it's clear that a great deal of thought and care has been given to every shot that we see. While Alpha was not filmed in 3D, everything about the way the 3D was done here makes it feel like it was designed to be a 3D movie first. Some of the landscape shots feel like they could have been pulled from one of the 3D nature documentaries that used to be the mainstay of your local IMAX theater. Even the subtitles have been layered in as part of the 3D presentation, an utterly unnecessary move that shows how much care was taken with every inch of the presentation.
Most 3D movies these days seem to forgo the ability to set objects in front of the screen or throw them out at the audience, and while Alpha doesn't go for the easy gimmicks (nobody throws a spear at the viewer at any point), the movie never forgets that it has this ability and it makes sure to use it. Animals come so close to the front of the screen that you want to reach out and touch them, and you begin to wonder why you can't smell them. Everything is so close that the barrier of the screen between you and the story sometimes feels like it disappears altogether.
Creating the depth of an image beyond the window is where most 3D movies spend their time these days, but Alpha takes things a step further and perfects it. You really feel like you could walk into the screen and into the background of virtually any image on the screen. Landscapes go as far back as the horizon. The prehistoric world is mostly empty, meaning that there's little obstructing the view between you and as far back as you can look. It's majestic.
Brightness is always tricky as it largely depends on individual projector performance, but I had no particular issues with brightness during Alpha. The necessity of the story to take place largely at night, with no on-screen light source beyond a campfire, forces the movie itself to have a lot of fairly dark scenes, and while the cinematography keeps these scenes from becoming so dark you can't follow the action, the natural screen effect of 3D glasses does dim things slightly. It's not a detriment to the experience in any real way, but it's certainly there.
The glasses off test can seem a little silly, as it only really matters how the movie looks with your 3D glasses on, but it can give you an idea of just how much is going on with 3D and how many layers of information are sitting on top of each other. Alpha is essentially unwatchable without glasses as so many layers are blurring together that you can't really see anything. Even simple close-up shots that don't need depth have been clearly given the full 3D treatment. As mentioned, even the subtitles are layered into the experience, to the point where you can't read them with your glasses off.
While a great deal is done with 3D effects from beginning to end in this film, there's nothing that is likely to cause you any motion-sickness issues. The vast majority of the movie is really just two characters moving through the 3D space. The 3D is designed to let you enjoy the view, not be wowed by it as a spectacle. What action there is is handled with the same care as the rest of the 3D presentation, so it shouldn't cause you problems.