The ZenFone 3 Zoom front-facing camera uses the Sony IMX214 sensor, the same sensor in the Huawei Ascend Mate 7's rear facing camera. It's a 1/3.06" Exmor RS Stacked CMOS sensor with 1.12-micron pixels and a f/2.0 aperture lens. This is a good sensor, but it's getting old and I do question why Asus has decided to use this sensor for the front camera of the ZenFone 3 Zoom. The pixels are obviously small, so the low-light performance could be better, but in daylight, it's capable of delivering very sharp and detailed images as we seen in the Mate7.
Although I didn't expect this phone to come up with this type of camera, after all, Asus invested a lot in the rear camera, and it's great to see that Asus hasn't compromise on the front camera capabilities. The ZenFone 3 Zoom is of course a very photography-friendly camera and most of the focus on this phone is on the camera capabilities. The dual lens camera is obviously the main selling point, but obviously in the selfie generation, many consumers pay a great deal of attention also on the front-camera (sometimes referred to as the Selfie camera) and not just the main camera.
An f/2.0 is a fast aperture and allows a lot of light to pass through the lens to the sensor, and it does help of course, but the low-light performance isn't great and there is a lot of noise and details are murky. It's still produces descent shows for sharing in low resolution on social networks, but don't expect anything spectacular, because the sensor is limited by its very small pixels, and obviously there is not OIS for the front camera.
I think that users will benefit more from a lower resolution camera but one that is capable of better low-light performance. There isn't any front flash either, only screen flash. So although the Sony IMX214 is used good as a rear-facing camera, but you need to understand that people don't use the front camera to take photos of their surrounding, they use the rear camera for that. The front camera has a very distinctive purpose of taking selfie pictures, group photos and for video chatting. Video chatting works fine for almost all front camera, it's really not the issue here. The real use for the front camera is for taking selfies, and that's where a better low-light performance would be more beneficial.
This is why I categorized this opinion under the disadvantages category, regardless of its very good performance in daylight. If you find yourself shooting most of your slefies in daylight, it won't be an issue, but again, a selfie camera should be versatile and allow people to shoot selfies with good quality image in both daylight and at night. This is also quite an old sensor, it was used in the Nexus 6 and the OnePlus One. Two years ago, I would probably not complain, but we are in 2017, so I expect more.