Sony A7R III has a 42.0MP Full frame
(35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Canon 1Ds MII has a 17.0MP Full frame
(36 x 24 mm ) sized CMOS sensor .
Sony A7R III's sensor provides 25MP more than Canon 1Ds MII'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.
Another difference between these two cameras is that Sony A7R III's sensor lacks 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 A7R III and 1Ds MII sensor size comparison.
Sony A7R III and Canon 1Ds MII have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
DxOMark Sensor Scores
Both Sony A7R III and Canon 1Ds MII 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, A7R III has scored 100, 26 points higher than 1Ds MII.
Sony A7R III
Canon 1Ds MII