ON TRIAL SIGMA
Even with the extra freedoms derived from the mirrorlss camera configuration and the availability of in-camera correction processing, there are optical designs that remain challenging. One of them is combining an ultra-wide focal length with an ultra-fast maximum aperture… say 14mm and f/1.4. There's already a 14mm f/1.8 ultra-wide prime from Sigma which was originally designed for DSLRs, but is now also available in the FE and L mirrorless mounts. The Sigma 14mm f/1.4 is a mirrorless era creation so it's two-thirds of a stop faster and takes over the crown as the fastest ultra-wide prime for full frame sensors. At f/1.4 there's 1.6x more light entering the lens than at f/1.8.
To add to the design challenges, Sigma has optimised the 14mm f/1.4’s optics for astrophotography which requires additional optical correction, primarily for comatic aberrations – also known as coma distortion or coma flare (although it's not actually flare) – which otherwise renders points of light as more tear-shaped near the edges and corners of the frame. In a nutshell, it's caused by the curvature of the lens elements – either concave or convex – which causes differences in the amount of refraction depending on where the light rays pass through them with the more oblique angles resulting in differences in magnification. So the ‘flare’ is actually a series of ever-smaller off-axis renderings of the light point which looks a bit like the short tail of a comet… hence the name of this type of lens aberration. Another important requirement for a lens designed for astrophotography is the correction
