Sony A7S III has a
12.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz XR processor. On the other hand, Sony A9 has a
24.0MP Full frame (35.6 x 23.8 mm ) sized Stacked BSI-CMOS sensor and features BIONZ X processor.
Sony A9's sensor provides 12MP more than Sony A7S III'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.
Below you can see the A7S III and A9 sensor size comparison.
Sensor Size and Resolution Comparison image of Sony A7S III and Sony A9 Cameras
Sony A7S III and Sony A9 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 III has 99% larger pixel area compared to Sony A9, 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 III and Sony A9 sensors have been tested by DxO and the results show that A9 has a better overall score of 92, 7 points higher compared to A7S III's score of 85.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Sony A7S III
85
24.2 bits
13.4 Evs
3900 ISO
Sony A9
92
24.9 bits
13.3 Evs
3517 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 III
or the
Sony A9 ,
including hyperfocal distance.
