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 manually 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 our ISO 800 test images stand, but they should be considered suspect. Techstage's original article about the issue can be found here (in German).
First, let's take a look at the camera's image clarity and image noise performance, using 100% crops of our indoor test scene. Right away, we can see that the HTC One captures the least detail, not a large surprise considering its low megapixel count. And the HTC One does something odd at ISO 800, image quality takes a sharp dip, with smudged details and high noise - strangely enough, ISO 1600 looks better than ISO 800. Perhaps another firmware update to fix this anomaly?
The one thing we can say about the HTC One is that it's fairly consistent from ISO 200 to ISO 1600, barring ISO 800, there isn't a dramatic rise in image noise nor drop in image clarity.
The HTC One XL's max usable ISO is at ISO 400, at ISO 800 details start falling apart. The Nokia 808 PureView captures the most detail, also not surprising with its 33MP capture. It manages to retain image detail and suppress noise even up to ISO 1600, which is impressive for a smartphone camera. The Nokia Lumia 920 does quite well, keeping a good balance between noise and detail, but its highest ISO setting (manually, at least) is one stop less than the HTC One's. Since we couldn't set the iPhone 5's ISO manually, the most we can say about it is that at ISO 250, it shoots a good photo in comparison with the rest of the phones.
Note 1: To fairly compare the HTC One, its competitors' images were down-sampled to 4MP before cropping, using Adobe Photoshop CS4 and a Save for Web setting on Very High quality. Not down-sampling the images would have been akin to asking you to watch a DVD on a small screen and a Blu-ray disc on a large screen and then trying to see the difference - you can only see a fair difference by watching them on a similarly-sized screen. Clicking on the thumbnails however, will bring you to the original full-resolution images.
Note 2: Due to bandwidth concerns, we won't be offering the Nokia 808's original monster 33MP files for download.
Note 3: Because the HTC One captures more pixels in 16:9 than 4:3, we've used that aspect ratio on all the other smartphones, except for the iPhone 5 which can only capture in 4:3.
Note 4: Because we couldn't mount lock the smartphones on a tripod, the images will vary somewhat in viewing angles and composition.
Note 5: The iPhone 5 only makes an occasional appearance because it was the only smartphone which we couldn't manually set the ISO.