Yeah, I have also been puzzled by the DXO results measuring a lens to 3-4-5 MP on a 16 MP 4/3 sensor...
Here is my explanation:
You know the sampling theorem maybe:
f < 0,5 * fs
To sample audio up to 20 kHz you need a sampling frequency of more than 2 x 20 kHz = 40 kHz.
Its the same for images which can be described as sampled when its dvided up into pixels.
The sensor is the sampler and the sampling frequency is the pixel raster, i.e. 5184 x 3888 pixels (20 MP sensor).
Before the sampler/sensor there has been an optical low pass filter in basically all modern digital cameras up until recently.
Removing the optical low pass filter results in moiré and aliasing because of the sampling theorem limitations.
The bayer filter which is usually used to get color from a single sensor compunds the problem further. This gives you color moiré as well.
[Given the example above the low pass filter should soften the image down to 2592 x 1944 pixels, effectively give you a 5 MP image. :-) ]
[The nyquist freqency for the image sampler seems to be dictated by the number of pixels at each mm on the sensor:
I don't know the details of the actuall level of optical low pass filters (anti alising filters) aka softening used in a system camera , but you can see an example here so the effect is quite dramatic:
On cameras which has removed the optical low pass filter you have two possibilities:
1. Reduce the offending moiré and aliasing manually in post processing.
2. Use an DSP which analyzes the image and removes/soften problem areas directly in camera. At least the new GH5 does this, probably the other M43 cameras from Panasonic as well.
Here are some more info on the AA-filter: http://www.outdoorph...go-no-low-pass/
"In high-quality digital maging systems, optical low-pass filters (OLPF) are used to eliminate color Moire fringes. An OLPF cuts off the lens MTF above the sampling frequency of the imager resulting an overall MTF curve that approximates a step function in spatial domain." http://www.optics-online.com/lpf.asp
The short summary here is that the lens+sensor resolution will never be the same as the pixel count on the sensor...