[frank]

As far as I understand it, for the same size sensor and the same lens, with a higher pixel density sensor (higher sensor resolution) you will see clearer the resolution problem of the lens: image resolution decays quicker toward the image boundary. This comes from the formula: 1/image resolution = 1/sensor resolution + 1/lens resolution + 1/filter resolution. The higher the sensor resolution is, the closer the image resolution is to the lens resolution (let us ignore the filter resolution effect at the moment). In a sense the sensor has an effect to make the image resolution uniform from center to boudary and this effect decreases with increasing sensor resolution (according to the above formula). I think this is the sense of the term "more demanding" that people use here. This is the reason why some lenses behaves good on D700 but not good on D3x: on D3x you can see the image resolution decays quicker toward image bounderies/corners than on D700, although technically even on the image boundaries/corners the image from D3X can have higher resolutions than the image from D700 (if only sensor resolution differences are considered).

 

Frank