Shortly after the launch of its latest flagship smartphone, the One, HTC published a white paper explaining the thinking behind the 4MP 'ultrapixel' camera in its latest device. This covers some key factors affecting image quality, including the lens, sensor and processor, in a fairly non-technical way. The document is light on numbers though, and worryingly high in quotes from journalists so scientists and engineers may find it unsatisfying.

The core of HTC's argument is based around the idea that using fewer, larger pixels on a sensor that's the same size as the 13MP unit found in competitors such as the Sony Xperia Z should offer better image quality. This of course runs counter to conventional marketing, which has always contended precisely the opposite, i.e. that more pixels are better. We're a little dubious about these claims - there’s a pretty strong argument that what really determines image quality when looking at the image as a whole is the total amount of light captured by the sensor, not by each individual pixel.

However, the HTC One's advantages are not all about pixels - HTC is also talking about the (pretty unarguable) low-light advantages offered by the optically-stabilised F2.0 lens, along with the improvements to video footage that image stabilisation brings. The fast sensor read-out means its electronic shutter completes the exposure more quickly than previous sensors - 1/48sec vs 1/30sec - which should reduce the 'rolling shutter' effect in movie recording.

And there is an argument for using fewer pixels on a smartphone, since most images are shared at much lower resolution than 4MP, it's difficult to imagine the main advantage of high resolution sensors, i.e. higher image detail, being especially important to smartphone users. We have now finally had a chance to test HTC's ultrapixel-claims first hand. Andy Westlake at our London outpost got hold of a HTC One review sample and has shot a range of comparison scenes in different light situations with his the new HTC flagship and his iPhone 4s.

The current weather in the UK (only very brief moments of sunshine) meant we could not take as many shots as we would have liked to but the images below should give you a good idea of what to expect from the HTC One. We will follow up with a more detailed test under controlled lighting once we receive a test unit here at headquarters in Seattle.

Daylight, low ISO

The 4MP backside-illuminated CMOS sensor in the HTC One is made by ST Microelectronics and also being used in a number of video devices. This explains the, for a still camera, unusual aspect ratio of 16:9. Switching to 3:2 or 4:3 aspect ratios crops the image sideways. The latter for example reduces the overall pixel count to 3MP.

In the well-illuminated scene below the Apple sticks to its base ISO setting of 50 while the HTC reports a slightly unusual sensitivity of ISO 112. The HTC is doing a decent job in these conditions albeit with a much warmer color response than the iPhone, but the latter is clearly better. In good light more pixels simply means more detail and this is clearly visible in the samples below. Whether this persists in typical social media use is a totally different question though.

 HTC One, ISO 116, ISO 112
 100% crop
 Apple iPhone 4s, ISO 50
 100% crop

Here we've got another shot in good light but more overcast conditions. Both phones struggle rendering the distant low-contrast areas but again the Apple's 8MP sensor captures more detail overall. On the flip-side the HTC produces a cleaner image with visible less shadow noise.

 HTC One, ISO 102
 100% crop
 Apple iPhone 4s, ISO 50
 100% crop

Digital Zoom

Digital zoom is generally awful on smartphones apart from the Nokia 808 with its PureView technology and both the HTC One and iPhone 4s are no exception. That said, with its comparatively low 4MP pixel count the HTC's image quality is deteriorating faster than the digital zoom of higher resolution smartphone cameras as you can see in the samples below.

 HTC One - 4x digital zoom, ISO 116
 100% crop
 Apple iPhone 4s - zoom, ISO 50
 100% crop

Low light, high ISO

In low light the HTC One is clearly better than the iPhone, producing a much cleaner, more noise-free image and giving brighter, more colourful images. It's difficult to isolate the effect the effect of the ultrapixels in this context as both the IS and the fast F2 lens playe a role too but in a way that’s academic, it's the end result that counts.

These images have been shot in Auto ISO mode but the HTC allows you to select ISO manually. However, a lack of information about shutter speed renders this feature pretty much useless.

HTC One, ISO 395, 1/15 sec
 100% crop
Apple iPhone 4s, ISO 800, 1/15 sec
 100% crop

When taking into account that many editing and sharing apps for smartphones reduce the size of images it also makes sense to compare the low-light samples above at roughly the same size. For this purpose we have reduced the iPhone 4s image to the same width as the HTC image (1520 pixels). Due to the Apple's 4:3 aspect ratio you end up with an image just over 3MP but the size of objects in the image is approximately identical in both photos.

As you can see in the 100% crops below the low-light advantage of the HTC is somewhat reduced when looking at images of the same size but the HTC One's image is still visibly cleaner with less luminance and chroma noise. So, when shooting in low light the HTC offers a real advantage over higher resolution sensors, even when the image size is reduced for sharing, social media or editing.

 HTC One, ISO 395, 100% crop
 Apple iPhone 4s, ISO 800, reduced image size

Soft image areas

In quite a few, but not all of our HTC One sample shots we noticed extreme softness on the right side of the frame. We haven't been able to determine the exact reasons for this but we are suspecting the effect could be caused by the image stabilization system decentering one of the lens elements. Once we get a review unit here in Seattle we will check if this is only a problem with the particular copy of the phone that Andy had in London and invesitage further. The problem is very visible in the samples which were taken within a few images. Exposure data for the two images is identical but if you click through to the full version of the image on the left you can see that are large area on the right side of the frame is extremely soft, making the image pretty much unusable.

 This picture shows extreme softness on the right side of the frame.
 This one, taken only minutes later at the same settings, does not exhibit any signs of softness.

First image quality impressions

Overall, from what we can see from these first sample shots, the HTC One with its 4 "ultrapixels" CMOS sensor performs pretty much as we would have expected. In low light and at higher sensitivities the HTC delivers cleaner images than the higher-resolution iPhone 4s. However, this is as much due to the optical image stabilization and fast F2 lens as the sensor. The HTC's advantage decreases if you compare images at the same size but is still clearly visible.

It doesn't come as a surprise that in good light the HTC trails noticeably behind its higher resolution peers in terms of detail capture. In good light more pixels simply mean more detail. The question is by how many users this will be viewed as a disadvantage, given a large proportion of smartphone images is being edited and shared at lower resolutions anyway.

We should also mention the HTC's strong tendency to produce purple flare with light sources that are located just outside the frame, a very similar effect to the one found on the iPhone 5. This is very obvious in the first image of our sample gallery below.

We will analyze the purple flare effect and image quality of the phone in general in much more detail once we receive a reviewable unit at our studio in Seattle. In the meantime we have included a few additional sample images in the preview gallery below for you to examine.

Sample Gallery

There are 12 images including a couple of panoramas in our HTC One preview samples gallery. Please do not reproduce any of these images on a website or any newsletter / magazine without prior permission (see our copyright page). We make the originals available for private users to download to their own machines for personal examination or printing (in conjunction with this review), we do so in good faith, please don't abuse it.

Unless otherwise noted images taken with no particular settings at full resolution.