OnePlus 5 2x tele camera uses 1.6x optical in combination with digital zoom
(Digital Photography Review)

When we shot our sample images with the brand new OnePlus 5 we noticed that the dual-camera`s 2x tele-module did not quite deliver the pixel-level image quality you would expect from the 20MP Sony IMX350 sensor. Images showed low levels of fine detail and looked as if they had been upscaled which would point towards some form of digital zoom implementation. This has now been confirmed by OnePlus co-founder Carl Pei in a tweet. He clarified that the second lens on the back uses a 1.6x optical zoom and that digital zoom is used to reach the claimed 2x zoom factor. The cropped image is then upscaled to achieve the specified 20MP image size. The company says it is using its SmartCapture multi-frame technology to make the zoom `lossless` but arguably not everybody would agree with this term. Exif viewers show the focal length of the wide-angle and tele lenses to be 24mm and 36mm equivalent respectively which would mean a 1.5x zoom factor. However, there is a chance Exif isn`t taking the SmartCapture portion of the zoom into account. Some other dual-cam implementations we have seen, for example on the iPhone 7 Plus are using a 2x optical zoom with a smaller sensor than the main camera. It appears OnePlus opted for the same 1/2.8` sensor size in both cameras. An optical 2x lens would probably have required a thicker body or noticeable camera bump.