Many people still believe that increasing the megapixels results in better photos. This may be true in some cases while in others you may find that even a 8 MP camera can take photos as good as, if not better, a 13 MP one.
These discrepancies are observed because the level of detail captured in an image largely depends on the amount of light in the surroundings. A 13 MP camera may capture minute details in broad daylight but while clicking photos in your college or office parties you will need a camera optimized for taking images in low light.
Firstly, let me clarify a thing behind the megapixels. As you all may know, more the megapixels, greater the image resolution. In day-to-day use this reflects on the quality of photos captured using digital zoom. This means that a 13 MP camera can capture the same amount of details as the 8 MP one even if one is standing further away. This is because you can zoom in on the target without losing much on quality with a 13 MP camera. That's it!!! Nothing great about a 13 MP shooter. This is not even an advantage in cases where you are taking your Facebook party photos because obviously you would be shooting from as close as possible.
But the main issue is that photos are darker than normal. This can be solved by increasing the camera sensor size while keeping the megapixels constant. The camera sensor size is usually 1/4" for common smartphones, 1/3.2" for iPhone 5 and 1/3" for Lumia 925. Usually, larger the sensor size greater is low light image clarity.
This is because a larger sensor can take in more light using a larger pixel size. The pixel sizes are 1.1 µm for common smartphones and 1.4 µm for iPhone 5 and Lumia 925. Higher pixel sizes (ultra pixel) can process more light given megapixels are kept constant. The HTC one has got a high pixel size of 2 µm but it lags because the resolution is limited to just 4 MP.
However, one can increase the light input by keeping the same sensor. This is done by increasing the aperture size to F2.0 from the conventional F2.2 or F2.4. The iphone 5 has an aperture of F2.4 while iPhone 5s has increased it to F2.2. The Nokia Lumia 925 and Xiaomi Mi-2S have the largest aperture of F2.0.
For a detailed pictorial representation you can checkout below link to HTC's website.
http://www.htc.com/www/zoe/ultrapixel-sensor-size
If you want better auto focus and anti-shake features (image stabilization) then it's always better to go for optical image stabilizer. This idea is also best explained on HTC's webpage at http://www.htc.com/www/zoe/stabilization
