since the the newer camera came out, everybody saying that the sensor is more demanding and that you should have better lens on that camera to get "everything the sensor can give you".
I no quite I understand why. If you have a certain lens that perform "ok" on a 16 Mp sensor, it won't get worse because you put it on a 36 MP camera or does it?
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).
wouldn't all lense would be affected by this? I mean some lens are better than others in corner/border resolution but I don't know any lens that resolve equally all over the image circle... even Zeiss lens, which I think would be one of the best lens, drop in significantly in corner resolution vs center when wide open.
You are right, the sensor does not influence at all how the lens performs. The projected image stays the same, just the sample rate goes up. You will not notice a difference between an image taken with a 12mp D3 and a 36mp D800 (with the same lens) printed at the same size, Unless the size you print in is that big and the viewing distance is that small that the limiting factor starts to be the sensor resolution.
From this follows, that one can print bigger, and have smaller viewing distances, with the 36mp D800 before the resolution of the sensor starts to become a limiting factor.
You can also understand that if it is the lens that is limiting resolution, at the bigger print size and the smaller view distances it will be more apparent.
The D3 has a horizontal resolution of 4256 pixels. The D800 has a resolution of 7360 pixels. The D800 then has a resolution advantage of 72.9%. You can print about 73% bigger before the sensor resolution starts to be a limiting factor, if the viewing distance remains the same. Of course, the bigger you print, the bigger the viewing distance will be...
So If I understand well, since there more resolution, when we put the image a 100% on our screen, we're seeing "closer". So from there we can see the "decays" from closer. So they are more apparent because you have a better resolution on those "decays".
So basically, even with a not so good lens, you'll get a better resolution on your image if you shoot with higher Mp camera.