Both Olympus E-M10 II and Sony A57 have 16.0 MP resolution sensors but Olympus E-M10 II's sensor is Four Thirds (17.3 x 13 mm ) and Sony A57's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A57's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Olympus E-M10 II.
Below you can see the E-M10 II and A57 sensor size comparison.
Sensor Size and Resolution Comparison image of Olympus E-M10 II and Sony A57 Cameras
As seen above, Sony A57 has a 1.6x Larger sensor area than Olympus E-M10 II. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Olympus E-M10 II and Sony A57 sensors have been tested by DxO and the results show that A57 has a better overall score of 75, 2 points higher compared to E-M10 II's score of 73.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Olympus E-M10 II
73
23.1 bits
12.5 Evs
842 ISO
Sony A57
75
23.4 bits
13.0 Evs
785 ISO
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Olympus E-M10 II
or the
Sony A57 ,
including hyperfocal distance.
One thing to watch when comparing these two: they have different sensor sizes
(Four Thirds versus APS-C),
so compare them at the same framing rather than the same focal length.
Put the same focal length on the smaller sensor and it will show less depth of
field, not more, because it also crops tighter. The calculator shows the
full frame equivalent focal length for each body so you can match the framing.
