The Asus ZenFone 3 Zoom features a 2.3x optical zoom. I did watch the video shot by Digital Trends on YouTube and it's important to note that it's not like there is a smooth continuous magnification (transitional optical zoom) between the first camera 25mm and the second camera with 59mm focal length. It's either the phone capture the image with the first camera or the second camera, and you can switch between the two by pressing a dedicated zoom button near the virtual shutter button in the camera app. It will show 1.0x by default, which obviously means no zoom. If you press this circular button again, it shows 2.3x and now the camera switches to the second camera and shoots at 2.3x zoom with much greater magnification.
It's also important to note is that unlike digital zoom, there is no sacrifice on image quality at all. When you shoot with digital zoom, which is possible using this phone (up to12x digital zoom), there is loss in quality.
Here is a very nice video by Tech Patrol YouTube user. They guy in the videos demonstrates how the 2.3 optical zoom works and what 2.3 optical zoom looks like.
Again, this is nothing spectacular, because the camera just switches between the lenses. Having said that, this allows phone manufacturers to make smartphones with optical zoom that are still slim and stylish, compared to phones like the Asus ZenFone Zoom (12mm thick), Samsung Galaxy S4 zoom (15.4mm) and Samsung Galaxy K zoom (16.6mm thick) that were relatively very thick with a prominent bulge at the back. By using two prime cameras side by side, Asus could offer a 2.3x optical zoom while still keeping the phone very slim, only 8mm thick.
It's worth mentioning that those other optical zoom phones like the Asus Zenfone Zoom XZ551ML that was released in December 2016, had a variable aperture and focal length lens (28-84mm f/2.7-4.8, 35mm equivalent focal length). So when you zoom in from no-zoom (1x zoom) to 3x optical zoom, you continuously changing the focal length of the lens and can zoom not just at 1x and 3x zoom, but also in between (e.g. 2x zoom) and the aperture changes based on the focal length. So at 28mm, the aperture is f/2.7, whether at 84mm, the aperture is at maximum aperture of f/4.8.
The lenses used in the Asus ZenFone 3 Zoom rear cameras are prime lenses, that have a fixed focal length (25mm and 59mm, 35mm equivalent) and fixed F1.7 aperture lens for the wide-angle lens and f/2.8 for the longer lens, which isn't changing. This have some advantages of course, because the two lenses are prime lenses that due to their optical lens structure can produce sharper image. The second advantage is that it's possible to use faster aperture (smaller f-number) which improved the low-light performance of the camera by allowing the lens to transmit more light towards the camera image sensor. This is usually the problem with most zoom, where the aperture at the tele-end is slow (larger f-number), so many zoom lenses therefore has inferior low-light performance at the tele-end.
In fact, some lens manufacturers use that technique, like Leica with its multifocal lenses for interchangeable lens camera. For example, the Leica Tri-Elmar-M 16-18-21mm f/4 ASPH lens features three fixed focal length settings in a single lens, a 16mm, 18mm and 21mm focal lengths. The reason for that is that prime lenses perform optically better than zoom lenses, as well as it's possible to create lenses that are faster yet more compact and still use various focal lengths. It's like having three prime lenses in a single lens. Most lens manufacturers don't use this design, and just keep in mind that those lenses are very very expensive.
One last thing. For those who don't know how the 2.3 number came from. It's the division of the wide-angle focal length (25mm) with the narrow angle focal length (59mm) which results in 2.36, 2.3 for short. Usually optical zoom is described in a number with one decimal precision.
No doubt that this type of camera design has its advantages and I'm sure many people will find the optical zoom very useful and help them capture more creative images, compared to being restricted to a single focal length lens, like most phones.