Sony A7S II has a
12.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Sony A9 III has a
24.6MP Full frame (35.6 x 23.8 mm ) sized Global Shutter sensor and features BIONZ XR processor.
Sony A9 III's sensor provides 12.6MP more than Sony A7S II's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
One other difference between these two cameras that is worth mentioning is that Sony A9 III'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 A7S II and A9 Mark III sensor size comparison.
Sensor Size and Resolution Comparison image of Sony A7S II and Sony A9 III Cameras
Sony A7S II and Sony A9 III 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. On the other hand, since Sony A7S II has 99% larger pixel area compared to Sony A9 III, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
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. Sony A7S II and Sony A9 III sensors have been tested by DxO and the results show that A9 Mark III has a better overall score of 87, 2 points higher compared to A7S II's score of 85.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Sony A7S II
85
23.6 bits
13.3 Evs
2993 ISO
Sony A9 III
87
24.1 bits
13.0 Evs
3224 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 A7S II
or the
Sony A9 III ,
including hyperfocal distance.
