2.2. Spatial Resolution and System MTF versus Pixel Size
For a fixed sensor die size, decreasing pixel size increases pixel count. This results in higher spatial sampling
and a potential improvement in the system's modulation transfer function (MTF) provided the resolution
is not limited by the imaging optics. For an image sensor, the Nyquist frequency is simply one half of the
reciprocal of the center-to-center pixel spacing. Image frequency components above the Nyquist frequency
cannot be reproduced accurately by the sensor and result in aliasing. The system MTF measures how well
the system reproduces the spatial structure of the input scene below the Nyquist frequency and is defined to
be the ratio of the output modulation to the input modulation as a function of input spatial frequency.7'8
Under certain simplifying assumptions, the system MTF can be expressed as the product of the optical
MTF, geometric MTF, and diffusion MTF.7 Each component causes low pass filtering, which degrades the
response at higher frequencies. In our study, we only account for the optical and geometric MTF. Figure 2(b)
plots system MTF as a function of the input spatial frequency for different pixel sizes. The results are again
for the O.35i process mentioned before. Note that as we decrease pixel size the Nyquist frequency increases
and MTF improves. The reason for the MTF improvement is that reducing pixel size reduces the low pass
filtering due to geometric MTF.
In summary, a small pixel size is desirable because it results in higher spatial resolution and better MTF.
A large pixel size is desirable because it results in better DR and SNR. Therefore, there must exist a pixel size that is "optimum".
This is taken from a Stanford paper. If Canon goes ahead with a 120MP Full Frame camera how many of your existing lenses will benefit from those smaller pixels? How much of that spatial resolution will you achieve? and what will the system MTF be?
Clearly Canon are upgrading their lens line-up and know what they are working on for the future. The EF 16-35mm f4L IS USM, the EF 100-400mm f4.5-5.6L IS ISM II, the EF 35mm f1.4L II are all examples of lenses with optical improvement. But always remember dramatically changing pixel size changes system MTF and therefor the "system" needs to reflect that change.