As a committed leader in optical design and technology, the Sigma Corporation has always released MTF (Modulation Transfer Function) charts as one of the measurements to evaluate a lens performance. It can be used to interpret how a lens reproduces detail from the subject to the actual image produced by the lens. However, the specific data does not have an official way to display or calculate, and it is known that the value will change drastically depending on whether or not it considers diffraction quality of light. Since we have always strived to communicate accurately the optical performance of Sigma lenses we have decided to include the “Geometrical MTF” chart in addition to the “Diffraction MTF” chart that have traditionally been provided for all future product releases.
The “Diffraction MTF” chart considers the diffraction quality of light. It 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 traditionally released Diffraction MTF data 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 will continue to provide other important charts such as Distortion and Vignetting data which can also be used as measurements for a lens’ performance.