I jsut got an OPO, and I have to agree, camera is really lackluster. To me, the problem is most apparent in medium light and below. Say indoors with daylight outside and one or two lights on and less. In these conditions the poor sensitivity of the sensor is alarming. But even outdoors it is easy to see the issue, just look at the exif data for exposure settings. Even with its wide aperture (f/2.0) it is using higher ISO (more gain) and slow shutter (more light) to make the same exposure.
Look at the attachment, which shows exif from the series of outdoor images: https://www.dropbox.com/sh/jdx1c7dig...c-spabvFa?dl=0
OPO - f/2, 1/913s, iso-100 and sort of a dark shot.
Nexus 5, f/2.4 (half a stop less light - i.e. 25% less light), 1/1021s (10% faster, 10% less light) and iso-101 (same) so dealing with 35% less light and the exposure is roughly the same, or maybe even slightly brighter for the Nexus.
Iphone5 f/2.4, 1/2262 (~twice as fast so ~1/2 the light), iso-50 (half the sensitivity, needs 2x light to saturate it), and if anything the shot is OVER exposed. This is why the iPhone camera is so great, it has great sensitivity.
From this we can tell that the OPO camera hardware or software simply has a sensitivity issue.
I refer you here to see how iso is measured: http://www.dxomark.com/About/In-dept...SO-sensitivity
Look at the definition section. ISO sensitivity = (a constant) / (exposure needed to saturate the sensor in lux-seconds) So at the same light level, better sensitivity (lower iso) is realized if a sensor needs more time to saturate compared to others. The OPO sensor needs very little time to saturate (apparently).