Both Nikon D700 and Nikon D300S have 12.0 MP resolution sensors but Nikon D700's sensor is Full frame (36 x 24 mm ) and Nikon D300S's sensor is APS-C (23.6 x 15.8 mm ).
Since Nikon D700'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 Nikon D300S.
Below you can see the D700 and D300S sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D700 and Nikon D300S Cameras
As seen above, Nikon D700 has a 2.3x Larger sensor area than Nikon D300S. 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 Nikon D700 and Nikon D300S 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, D700 has scored 80, 10 points higher than D300S.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Nikon D700
80
23.5 bits
12.2 Evs
2303 ISO
Nikon D300S
70
22.5 bits
12.2 Evs
787 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
Nikon D700
or the
Nikon D300S ,
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.
