The 20-megapixel rear camera on the M9 Plus is, frankly, a disappointment. Despite the high-resolution sensor, the phone only managed to churn out mediocre images that had noticeable noise and compression. The Xperia Z series from Sony, which uses a 20.7-megapixel camera, can capture images that have a lot of detail, but HTC fails to come close to even the Xperia Z1 from 2013. Nighttime photos come out worse than those taken in ample light, and the phone also had issues focusing on close-up objects, making it a poor choice for those that love taking macro shots.
The secondary Duo Camera sensor on the back is used for storing depth information with every photo, which allows you to change the object of focus on a photo and blur the rest. Thanks to the dedicated sensor, this works very well, with a near-perfect blurring out of the things you don't want to focus on. However, this is a function you will probably use very rarely, so it doesn't exactly make up for the shortcomings of the primary camera.
Video recording is unimpressive as well, with the lack of hardware-based optical image stabilization resulting in noticeable shake when the camera panned too quickly, though details and color reproduction were sufficient. The phone can record 4K videos, but you would be better off sticking to 1080p videos as 4K videos mostly serve to increase the file size rather than affect the overall video output.
Thankfully, the UltraPixel camera on the front is pretty good. HTC's UltraPixel technology uses a larger pixel size to capture more light, and this works great for selfies and is something we would like to see on more smartphones. Details are plentiful for a front-facing camera, though it does have the tendency to capture more light than you would need at times.
The camera app provides advanced controls that can be used for customizing various settings, such as exposure, shutter speed and ISO values, so advanced users can get better results than the automatic shooting mode can offer. Using the app is quite simple, and switching between the front and back cameras can be done by simply swiping in from the edges of the display.
All things considered, the imaging experience on the One M9 Plus is severely lacking. Cameras on considerably cheaper devices work much better, and for photography buffs, devices like the Samsung Galaxy S6 and the LG G4 are a much better buy in the same price range.