SAN FRANCISCO—When I first used prototypes of HoloLens, Microsoft's hologram-based augmented reality headset, I was blown away. The hardware seamlessly integrated virtual objects into the real world, and it was the first device I've used in many years that felt truly like something from the future.

At Microsoft's Build conference this week, I got to use HoloLens for the second time. This time around, the experience was quite different. Back in January, we were using clunky prototype units. They included a chest pack to hold various processors, a heavy headset with exposed chips and circuit boards, and an umbilical cord to keep the whole thing powered.

This time it was production-style hardware. I don't know how close to production it really is, but it at least had the look of something sellable. It's self-contained and battery powered. Above your left ear are brightness buttons; above your right are volume buttons. The back of the headband includes a power button and some small LEDs to indicate its charge level.

Once again, no cameras or recording devices were permitted. Microsoft isn't talking about its tech specs, though I did manage to glean a few details before our handlers noticed what I was doing: it has an x86 processor of some kind, 2GB RAM, 802.11ac Wi-Fi, and a 60Hz refresh rate (though the resolution, and the screen technology in general, remains an unknown). The prototypes were running a cut down build of Windows 10 that's a couple of weeks old.

The unit was a lot more comfortable than the early prototype. It fit over my glasses without a problem and could be adjusted and tightened sufficiently to make it stable without any pinching or digging in. The weight is noticeable, and I wonder how I would feel in an extended usage session, but for the short time we were using it, it never felt heavy, it just felt like it was there.

The demo I did involved writing a simple app using the Unity 3D engine. To a basic 3D scene we added physics, spatial mapping positional sound effects, and gesture and voice triggers. The canned demo was straightforward, and developing for HoloLens I think is going to be easy for anyone with any 3D development experience. The HoloLens is just a camera in the 3D scene; it has a few tricks of its own (since it's attached to your head, it moves around on its own, and so the program has to figure out what it's looking at), but the model seems straightforward enough.

So first, the high points: the positional audio is hands down the best two-speaker positional audio I've ever heard. It convincingly nailed the illusion of creating a sound source behind me and provided clear audio even though there are no headphones as such on the HoloLens.

The spatial mapping, using Kinect-style infrared lasers to map out the world around you, is surprisingly fast and accurate, and the way it was visualized in Unity (overlaying a triangle mesh on the objects detected in the real world) looked cool.

The motion tracking was impressive. Although the units suffered a few glitches, wherein the scene would jump to some other location before settling to where it should be, once it was settled it was firmly anchored in place. There was no wobble or shifting as I moved my head and walked around, and so the illusion of looking at something that was physically there was convincing.

The picture quality and 3D effects remain stunning. The imagery (we still don't really know if it's true holograms, and I still assume it's just stereoscopic imagery instead) is bright, the resolution seems sufficient, and the opacity of the image meant that it could substantially occlude things behind it.

Everything about the HoloLens experience is nailed. Except for one thing. The field of view was narrow. Very narrow. In both the horizontal and the vertical directions. You have this glorious 3D augmented reality experience... but only with your eyes looking straight ahead.

And it's not just me; I talked to other journalists who'd been at the January preview, and they had the same experience. The January prototypes didn't fill your entire field of view. The edges of the "screen" were visible. But they weren't this tight. I could look around a bit and still see the holograms. This time around, I couldn't.

I don't know why. It's possible that there are trade-offs being made to ensure performance is acceptable or that there's enough peripheral vision even if the entire screen is obscured. It's possible that my experience with the old device gave a wider field of view than it should have, due to the poor fit of the device; it was pressed close to my glasses, so probably seemed a little larger than it should have.

Whatever the cause, it profoundly changed the experience. The unit I used improved every aspect of the HoloLens experience—the fit, the sound, the weight, the appearance—except for the part tying it all together. The holograms. They were simply too damn small.

The HoloLens is still exciting. I still can't wait to get my own and see what developers can do with it, and I want to fit it into my life. And I wouldn't go so far as to say the narrower field of view kills it. It still does impressive, even spectacular things. But it's a lot less magical than it was in January, and that is very disappointing.

I'm hearing some quiet noises that the field of view isn't finalized, and so it may go up in the future. I very much hope that this is the case. I want other people to gape, open-mouthed, in awe of what they're seeing the way I did in January. I want them to experience that magic first-hand. I know that hardware can be built that gives that magic; I've used it. It's real. But the devices we used yesterday let a little too much reality creep in.