It's interesting to note that not only is there some serious color changes at the lower resolutions, but we're losing overall sensor sensitivity as well. Is this a bug in the firmware or just an unfortunate characteristic of the sensor? I'm not sure, but it's most likely the latter.
Noticed that as well.
My non-Scientific Observations showed that Resolution does some Weird stuff to the Sensitivity, also that gain Settings are not equal to a linear increase in sensitivity.
Furthermore Framerate also does some weird stuff to sensitivity. I feel like you tend to tickle more the Higher end of framerates, but for me its sometimes the Mid to lower end this camera offers. So i didnt look all too much into the High end, but for the Low end, there is some weird stuff happening, planing to do a new Topic on that at some point, namely exposure Slider at 60fps.
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Now you didnt leave any info what Firmware version you did your test on, so i assume 0.6.0
Now while Noise Output overally vastly improved with the New Firmware Version, Sensitivity took a noticeable hit at moderate Framerates (1k/full Res, didnt check lower res) and a somewhat bigger hit on lower resolutions.
Tested 5.1 vs 5.1 Unstable vs 6.0 when 0.6.0 got released, and the 6.0 was just quite a bit darker. Dont remember clearly anymore but must have been somewhere between 1/3 and 2/3 of a stop darker overall or something on full Res./ 1kfps.
5.1 Unstable is still the most sensitive (and ironically the most stable at stupid low framerates) from my tests, so i keep it on an µSD Card in my camera case to have it with the camera at all times just in case.
So it has to be at least partially be Software.
I dont remember to have any Resolution vs Sensitivity comparisons from what i remember, apart from noticing that Sensitivity on Small Resolutions on 6.0 is generally somewhat worse somehow (never did any actual tests to quantify that, just general feel).