Both Canon 650D and Canon 1200D have APS-C sized 18.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the 650D and 1200D sensor size comparison.
Sensor Size and Resolution Comparison image of Canon 650D and Canon 1200D Cameras
Canon 650D and Canon 1200D have the same sensor sizes so they will provide same level of control over the depth of field when used with 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. Canon 650D and Canon 1200D sensors have been tested by DxO and the results show that 1200D has a better overall score of 63, 1 points higher compared to 650D's score of 62.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Canon 650D
62
21.7 bits
11.2 Evs
722 ISO
Canon 1200D
63
21.9 bits
11.3 Evs
724 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
Canon 650D
or the
Canon 1200D ,
including hyperfocal distance.
