There are two types of MTF chart.
One considers the diffraction quality of light, which is called “Diffraction MTF”, and the other, “Geometrical MTF” does not.
The quality of light appears in the diffracted light, and becomes more distinct as the F value gets bigger, resulting in lower image quality.
Also, diffracted light exists at every aperture, which is why SIGMA has been releasing Diffraction MTF data from the beginning since it is very close to the actual image data.
The advantage of using “Geometric MTF” data is that it is easy to measure and calculate since it does not consider the diffraction quality of light, yet it tends to show higher values in the graph than actual images.
SIGMA used to release “Diffraction MTF” for each new lens release with the edition number up until 013. From 2014 onwards, and for lenses with the edition number from 014, both “Diffraction MTF” and “Geometrical MTF” have been made available.