Here's how Sony's Xperia Z5 phone with 4K screen can have two day battery life

That amazing 4K phone display has a special trick: It won't actually show everything at a super high resolution; just 4K content itself, which can save battery life.

When Sony introduced the Xperia Z5 Premium and its 4K display, the first question that came to my mind is: How long will the battery last since the handset has to push so many pixels?

Sony's claim is two-day battery life, which didn't make sense. Now it does because the company has explained how the 4K display actually works.

Sony issued the following statement to PhoneArena on Tuesday:

"Xperia Z5 Premium features a 4K display with a resolution of 3840x2160 pixels based on SID Standard and enables all video and image content to be enjoyed in 4K resolution. All other content is displayed at 1080P or lower resolution in order to optimise the performance and battery stamina for this device, ensuring you can enjoy the 4K resolution when you need it most."

That's clever, although I wish Sony was more forthcoming when it actually introduced the phone.

Instead, it waited for the world to "ooh" and "aah" over the first 4K smartphone display for about two weeks.

So as I understand it then, the only time you'll get a true 4K experience is when viewing appropriate video content or images that can take advantage of it.

The rest of the time -- which should be most of the time -- the Xperia Z5 Premium will display a 1080p or less resolution for app, the Android interface, web browsing, etc...

By doing so, the phone won't have to work as hard to display content and therefore will use less battery than if it had to upscale every visual object.

I'm still not sold on the need for a small 4K display, but at least Sony appears to be going about it in a smart way that will maximize battery life.