QUOTE(Maxieos @ Sep 5 2014, 06:57 PM)
Anyone ?
Still dont get it , what's 1/2 compare to f2.0 ? compare to high end note 4 1/2.6-inch 16MP BSI CMOS unit with F2.2 maximum aperture ?
Those things aren't really related. There are several ways to increase low light performance.
a) Bigger pixels. Bigger sensor, lower resolution, newer tech that uses more of the available space for light gathering rather than for example transferring the pixels to the imaging processor. The light sensitivity is measured in ISO. The higher the value, the more sensitive it is, however depending on the sensor it can be extremely noisy. A compact camera may go to ISO 3200 or 6400 (but it's unusable). The steps are for example 50, 100, 200, 400, 800, 1600, 3200, 6400, ... each step means 2x the light sensitivity. A decent DSLR will go to ISO 51200 perhaps, the current low light champion Sony A7S does something like 409600 or so. Those maximum values are usually barely usable.
b) Faster lens (wider open aperture). f1.8 beats f2.0 beats f2.2, ... to be honest the difference between f2.0 and f2.2 is not big, the steps are 1 → 1,4 → 2 → 2,8 → 4 → 5,6 → 8 → 11 → 16 → 22. Each step represents 1/2 as much light hitting the sensor. For ISO it is 2x the ISO = 2x the sensitivity. i.e. ISO 200 with f2 = ISO 400 with f2.8. Both will result in a photo of the same brightness, however the higher the ISO the more noise there is in the photo. The Mi3 goes to ISO 3200, but at those values it's a mess.
c) Longer exposure. However you can't hold the camera steady for long, especially if it is something like a phone, which is very light and is only held by your hands (a DSLR touches your face, which stabilizes it a lot, and it has a lot more weight). A solution that works to a certain degree is an optical image stabilizer, which will try to balance out your shaking. Again, 2x the exposure time = 2x as much light being collected. The rule of thumb is 1/focal length (converted to 35mm equivalent). So with a standard wide angle lens of 28mm you should be able to hold 1/30th, if it is longer than that, you need a stabilizer. However with smartphones that is IMHO pretty hard to hold already.
So that means: 1/50th at f2 at ISO 100 = 1/25th at f2.8 at ISO 100 or 1/50th at f2.8 at ISO 200. They will all result in the same exposure, though the last case will be noisier, while the second one may be shaken. The first one will be optimal in the case of a smartphone, because everything is in focus anyway thanks to the small sensor.
Going back to the initial question: if 1/2 means the size of the sensor, i.e. 1/2", that's an advantage over a 1/2.6" sensor, though it also depends on the MP. (Also, if you reduce the resolution you can get rid of some noise, i.e. a 10 MP photo with a 1/2.3" sensor downscaled to 5 MP should be similar-ish to a photo taken with a 5 MP 1/2.3" sensor, all other things being equal (I'd say the photo with the 5 MP sensor is probably better, but not by as much as it used to)). Then f2 beats f2.2, too.