As a huge camera geek as well as smartphone geek, the amount of misinformation about the "Ultrapixel" sensor is really annoying me. I just wanted to post to clear somethings up.
Assertion 1: Megapixels don't matter
This is true to a point. For the vast majority of users, 4 MP is enough for a smartphone. You're probably going to downscale the image to post online anyhow. Even a 1:1 pixel image on a relatively big monitor is 1920x1200. That's about 2MP. Unless you're going to print these on paper at a big size, the MP don't matter much. That is, unless you plan to crop your images a lot. Then you may notice it.
Assertion 2: Bigger pixels (Ultrapixels) collect more light, hence they have less noise
While it is true that a bigger pixel will collect more light and have less noise than a similar small pixel, this is noise at the PIXEL-LEVEL. So if you view your images enlarged to a 1:1 ratio on your monitor, the 4 MP of the HTC One camera will look cleaner than the center 4 MP out of the 13 MP of a Sony Experia Z. However, we don't view pictures like this. We view images as a whole. If you have an image sensor the same overall size as the HTC One's with the same everything but split into more pixels, the resulting image should look very close to the HTC One's once you scale the image down to 4MP. It is not a fair comparison to compare at the pixel level without scaling because then you're comparing only the part of the image that fell on a fraction of the higher MP sensor. (Note, higher MP counts DO make a difference when scaled, but not by much. It also depends on how the imaging portion of the sensor scales with non-imaging circuitry.)
Assertion 3: The image quality of the HTC One will be superior to the current smartphones
If you don't need images of more than 4MP, this is true. And it's not really because of how big those pixels are. It's all about good Signal to Noise ratio. and the HTC One has many things in its advantage. First, the sensor is overall bigger. It is 1/3" sized instead of 1/3.2" like the iPhone 5 or Samsung Galaxy S 3. A bigger sensor means you can collect more light. Secondly, the lens is a F/2.0 lens versus the F/2.4 of the iPhone or F/2.6 of the GS3. That means half a stop more light entering due to a larger aperture. (A one stop increase is equivalent to double the light.) Thirdly, HTC is touting an image processor with low read noise. That means they can turn up the ISO sensitivity with less noise. And finally, the HTC One sports optical image stabilization. That means it should be able to use a longer exposure time while remaining sharp. Of course, that's for relative static subjects. All four of these contribute to a higher S/N ratio, so less noisy pictures.
In conclusion, I'm very excited about the possibilities with the HTC One camera. But not because of the "ultrapixels." It's because of the sensor size, fast lens, low read noise, and image stabilizer. Actualy, it shares a lot of specs with the Nokia Lumia 920. 1/3" sensor, F/2.0 lens, and Optical Image Stabilization. I'm hoping quality is at least on par with that.
The HTC system is smaller but less effective.
Really wish they had kept a similar design but:
