Updated 27/3/13: We received an email from German website Techstage which seems to explain the mysterious behavior of the HTC One at ISO 800. According to them, even though we manually set the ISO to 1600, the camera might still take pictures at lower ISO settings in good lighting conditions. But it'll still save the EXIF data in the JPEG as ISO 1600, instead of the actual value.
This seems to bear out when we checked our test images and their shutter speeds. In our lab test with indoor lighting, the HTC One progressively reduced shutter speeds as ISO increased; the more sensitive the sensor was to light, the less time it needed to open up its shutter. At ISO 800, the One recorded a shutter speed of 1/120th of a second. At ISO 1600 however, the shutter speed jumped back down to 1/30th of a second, which is identical to the shutter speed required at ISO 200. Indeed, looking at the shots taken at ISO 200 and ISO 1600 now, they look remarkably similar.
In our extreme low-light test however, the shutter speeds remained constant at 1/20th of a second throughout all the ISO ranges.
In short, it seems that the HTC One has two bugs - one; it won't always listen to your command to set the ISO speed, secondly; it will report in the EXIF data that it did even though it didn't. Techstage says that the HTC product manager they are in contact with said there would be a firmware update to fix the issue, and that the issue has been confirmed directly by HTC Taiwan.
We have reached out to HTC Singapore for a comment but they were unable to issue one at press time. Until we have confirmed the issue with HTC ourselves we're letting the ISO 800 test images stand, but they should be considered suspect. Techstage's original article about the issue can be found here (in German).
Updated 26/3/13: HTC has clarified that the second unit which we received "has the final commercial firmware, compared to the original unit which was pre-production and had an isolated hardware issue, which resulted in differences between the two sets of photos taken with both devices." According to our understanding, all the commercial sets which will launch in April will come already updated with this firmware. As such, there will be no second camera update for the One, unlike what we previously wrote below.
HTC deserves recognition for taking a bold, unconventional step towards improving image quality with the 4MP HTC One. However, the proof of the pudding is in the eating, and HTC's implementation of 'ultrapixels' doesn't convey any significant advantage to image quality over the better camera phones of today. There's more to image quality than just the size of a photosite, just as there's more to it than just megapixel count. There's also the matter of optics, and how the image engine processes the images coming out of a sensor.
It's not that the HTC One's camera is entirely bad, if you're using the pictures just for social media, we doubt that most will notice the loss in image detail when viewing on their desktop browser or on another mobile phone. AF is snappy and the shutter is generally fast to respond. But there's also no special need to abandon the 8MP camera phones of today for the 4MP on HTC One. You'd sacrifice image detail for no noticeable improvements in low-light.
There's also the archival question; years from now your 8MP photos will still look decent on a future, more pixel-dense monitor. Your 4MP photo will not. This reviewer still has 3MP photos taken way back when with his first Fujifilm digital camera, and they look woefully small today on his PC monitor. If you print however, the HTC One's largest image resolution, 2688 x 1520 will give you a maximum print size of 8.96 x 5.06 inches at 300ppi, nearly A5 size or somewhere in-between 5R and 6R, which is a decent size.
If you do already have a HTC One and it's shooting images with soft corners, watch out for the camera firmware update which HTC tells us is coming in April for the general public - you'll see a noticeable difference in image quality. With both the previous and new HTC One phones though, we observed the same weirdness happening at ISO 800 - pictures shot at that setting appear softer and noisier, even noisier than ISO 1600.
About the other smartphones - the Nokia 808 Pureview is the undisputed best camera phone you can buy. However, since it's running on Symbian and the auto-focus is decidedly laggy, it's also the camera phone that nobody should buy. Still, it's groundbreaking technology and we're surprised that one year after its announcement, we still haven't seen any other Nokia phone use the same 41MP sensor.
We'd have loved to have the iPhone 5 show up more in the tests, but without a way to control the ISO, we can't conduct a proper 1:1 comparison yet. All we can say is that it looks like the iPhone has the second-best lens in this selection behind only the Nokia 808 PureView, with consistently sharp corner to corner performance. But it does also have a well-documented purple lens flare issue.
So, discounting the iPhone 5, from our small comparison it looks like the Nokia Lumia 920 is the best bang for your buck when it comes to image performance. It's not perfect - images are a little soft - but it delivers the best balance between image detail capture and image noise. It also has optical image stabilization, which is not available on the iPhone (but is available on the HTC One).