Sony Xperia Z5 Premium is the first phone to deliver a 4K screen but is it worth buying?

4K screens are taking over lounge rooms, and are now creeping into smartphones. But do you really need Ultra High-Definition on the screen in your pocket?

Is the first 4K phone worth buying?

SONY’S latest phone is the first in the world to add a 4K screen but it may not even be this phone’s best feature.

The Xperia Z5 Premium delivers even more features other top models cannot match to create a standout flagship phone.

It is dust and water-resistant despite two openings, delivers an impressive 23-megapixel camera with the fastest focus in its class and a dedicated shutter button, and it retains a memory card slot to store its high-resolution photos.

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It’s also the first Sony phone with a fingerprint scanner, runs Google Android on a powerful octa-core chip, and comes in a premium metal and glass form with space for a lanyard.

There are a handful of factors that detract from the experience but Sony’s new smartphone shows real innovation is possible outside the current smartphone duopoly.

Shiny finish ... Sony's Xperia Z5 Premium smartphone delivers a 23-megapixel camera in a waterproof body.

The 5.5-inch 4K screen on this smartphone is certainly one of its top selling points, with 806 pixels per inch, but it only really shines as brightly as possible when you play 4K video captured by its own camera. At other times, the screen merely serves up full high-definition resolution.

Side by side with rival phone playing the same UHD video file, it’s obvious which has the better screen. You can quite literally count the hairs on the subject’s head when viewing 4K video on Sony’s phone.

And, of course, some credit for this video experience is due to this phone's camera. Sony supplies image sensors to other phone makers but says it keeps the best for itself.

The 23-megapixel sensor inside this handset is inarguably impressive, and its autofocus is perhaps even more so. It locks on to subjects in .03 of a second and, in our tests, was more capable of holding focus and accurately tracking subjects. This is particularly important when shooting video, where its rivals can blur subjects mid-shoot.

Added security ... Sony's Xperia Z5 Premium is the first Sony phone to add a fingerprint scanner to the On button on its side.

Sony’s camera also excels in exposure, regularly delivering even lightning and handling backlight and highlights better than its rivals. Its white balance is more hit-and-miss than you’d expect, however, and a warmer, darker treatment of some photos would deliver better results.

Those who want to tweak these settings can do so in the camera’s manual mode, where you can also kick this camera up from the default 8-megapixel setting to the full 23.

It also helps that this phone, like Sony models before it, comes with a dedicated shutter button to start the camera and take photos, and a wider angle at 24mm.

The other standout feature of this phone has to be its water resistance. If your phone is dirty, you can hold it under a running tap to clean it, even with its charging and headphone ports exposed.

You can even dunk it in 1.5m of water for 30 minutes.

With so many phone makers backing away from waterproof bodies, it’s refreshing to see a premium phone deliver this technology.

It’s also worth noting this phone is one of the only premium Android handsets to let users expand its 32GB built-in memory with a card, delivers a reliable fingerprint scanner on its first outing, features NFC technology for wireless payments, and packs all of this into a body that is surprisingly narrow and comfortable to use for a 5.5-inch phone.

New family ... with the launch of the Z5 Premium, Sony’s new Xperia family is complete. Other models include the Z5 Compact and Z5 phone.

There are downsides to Sony’s flagship phone, however, and they may be deal-breakers.

Its mirrored back panel — in gold, chrome or black — attracts fingerprints and provides a dangerously slippery surface for a phone.

Its promise of a two-day battery life does not hold up to real-world testing, with that beautiful 4K screen chewing through power, and the phone gets so warm it threatens to shut down if you use its camera for an extended period.

The phone also omits wireless charging, its volume controls are awkwardly situated down its right side, and it won’t give you the latest Android software just yet.

Despite these issues, smartphone users who prioritise their pocket cameras are bound to be impressed by this phone — those who regularly capture videos even more so. Its 4K screen delivers high-resolution video better than any other, and future-proofs this device for an Ultra High-Definition era.