Sigma’s recent release of the 28-70mm f/2.8 Contemporary lens a few days ago presents a rare – maybe unique – opportunity to see what improved coatings alone can do for image quality. Sigma stated that the new lens employs an identical optical formula for the new lens as the 2019 mirrorless-native 24-70mm f/2.8. They just shrunk it a little bit and narrowed the zoom range. But they also added some improved coatings, and this is where things get interesting.
When the lens launched, Camnostic found the MTF charts (calculated curves showing what the lens design should provide for sharpness and contrast) on the Sigma website, but the one for the new 28-70mm f/2.8 was plainly just a copy of the older 24-70 f2.8 MTF chart. This morning, a Sigma manager presenting to the CP + 2021 conference showed what appears to be the actual new MTF chart for the 28-70mm lens, and it’s a bit different.
The new lens appears to have improved image quality toward the edge of the frame (represented by the right-hand side of the graphs above), with an unnoticeable reduction in quality on the inner parts. So far, the new coatings are the only new element mentioned by Sigma that would be in the optical path, affecting image quality. It is possible that there are other, unmentioned, changes, such as a change in glass quality, but if those changes were to affect the image quality, it likely wouldn’t be considered the same design.
Coatings have been the tool for much of the image quality improvements seen in lenses since 2012, and particularly in the areas of light transmission, ghosting, and image quality at the edges of the frame. The frame edge is particularly relevant to coatings because the mechanism of excluding certain rays of light with coating particles often uses the angle of light as the factor determining what gets through. It is at the edge of the frame where the light is at its lowest incident angle.