Both Sony A7 III and Pentax K-3 II have 24.0 MP resolution sensors but Sony A7 III's sensor is Full frame (35.8 x 23.8 mm ) and Pentax K-3 II's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A7 III'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 Pentax K-3 II.
One other difference between these two cameras that is worth mentioning is that Pentax K-3 II's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the A7 III and K-3 II sensor size comparison.
Sensor Size and Resolution Comparison image of Sony A7 III and Pentax K-3 II Cameras
As seen above, Sony A7 III has a 2.3x Larger sensor area than Pentax K-3 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
Both Sony A7 III and Pentax K-3 II sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, A7 III has scored 96, 16 points higher than K-3 II.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Sony A7 III
96
25.0 bits
14.7 Evs
3730 ISO
Pentax K-3 II
80
23.6 bits
13.6 Evs
1106 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
Sony A7 III
or the
Pentax K-3 II ,
including hyperfocal distance.
One thing to watch when comparing these two: they have different sensor sizes
(Full frame 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.
