The optical designer doesn’t necessarily aim to achieve the diffraction limit performance. The desired MTF curve is based on design requirements. The lens specifications are usually of the form of an MTF value for specific frequencies. The MTF specification may come, for instance, from the sensor pixel size and the Nyquist theorem, which describes the maximum spatial frequency that the sensor can resolve.

For example, assume a digital sensor with a pixel size of 7.4 μm x 7.4 μm. According to the Nyquist theorem, the highest frequency that can be resolved is 1 cycle/(2 x 7.4 um) = 0.0676 cycles/μm ≈ 68 cycles/mm. Based on the sensor characteristics, a typical performance requirement for the lens could be:

- MTF > 50% at 17 cycles/mm, and
- MTF > 25 % at 68 cycles/mm.

Figure 6 shows the MTF chart from a lens that would meet these requirements for all fields. It is important to know the sensor characteristics during the design process since in many cases the Nyquist frequency of the sensor is only a small fraction of the diffraction cutoff frequency *f*_{c.}