Of course Sony's 4K phone looks great — but beyond the technological milestone it represents, there remains a lot to learn about the Z5 Premium.

It had to happen eventually. Even as "Ultra HD" TVs remain out of reach to most buyers, the blistering pace of mobile hardware advancement has launched us into the era of 4K smartphones. Announced alongside the vanilla Z5 and Z5 Compact at IFA today, the Xperia Z5 Premium brings Sony's high-end smartphone experience to a 5.5-inch screen size, while cranking up the display density all the way to an insane 806 pixels per inch. We've seen it with our own eyes, and can offer some first impressions after the break.

Let's start by stating the obvious: The Xperia Z5 Premium's display looks absolutely bonkers. It's easy to recall the oft-repeated Steve Jobs quote about 300 pixels per inch being the point at which, at normal holding distance, the human eye isn't able to distinguish between pixels. A 4K smartphone screen takes that and blows it out of the water. You can position your eye a couple of inches from the display and not see pixels. We'd be surprised if you were even able to get close enough to focus on the 8.3 million microscopic dots making up each image on the Z5 Premium's screen.

In fact, we'll just call this right now: This is as much much resolution as you're ever going to need in a gadget you're supposed to hold with one hand. And as you might expect, trying to convey exactly how sharp it is in pictures or video is futile. It looks good. The colors are punchy. It's absurdly sharp.

Xperia Z5 Premium

This is as much resolution as you're ever going to need in something you're supposed to hold with one hand.

There's certainly a discussion to be had about whether a 4K display on a smartphone is a waste of battery power or graphics power, but the history of mobile tech tells us that this is inevitably the shape of things to come. The real question for the here and now is how that ridiculous display impacts other areas of the device. Can the 3,430mAh internal battery really deliver "up to two days" of use, as Sony promises? Will the Snapdragon 810 processor — a chip we've seen struggle in 1080p phones — competently handle an Ultra HD screen without issue? We'll need to wait until the Z5 Compact's November release date to conclusively answer those questions.

As for the rest of the experience, it's a repeat performance of the regular Z5, only in a larger form factor. You've got a glass-backed, aluminum-framed design with waterproof credentials, fingerprint-base security features thanks to the scanner contained in the side-mounted power key, and a Snapdragon 810 processor running the show. We weren't able to see exactly how all that stuff comes together, as the units on show at IFA today were behind glass, and didn't appear to be running usable software. Essentially what we have is the promise of a super-high-end interpretation of what looks like a solid phone.

If Sony's doing its job right, you shouldn't even notice that the phone's pushing all those extra pixels. But that's the biggest question right now. As we pass the 4K mark, smartphone displays start to step beyond the point of diminishing returns. And are the bragging rights of owning such a spectacular piece of technology going to be worth the real-world, day-to-day compromises when you're pushing 8.3 million pixels using the mobile hardware of 2015?

Sony deserves praise for getting to market first with a 4K phone, but it remains to be seen how the final product will come together. We'll be watching with interest.