Fujifilm GF 23mm f/4 R WR Aperture Series: Church Mosaic (Fujifilm GFX)
See my Fujifilm GFX wish list.
This scene tests the Fujifilm GF 23mm f/4 on the most demanding target I know of: a finely detailed geometrically planar subject at distance. Such a scene is unforgiving of focus shift or field curvature or any lens symmetry defects. It is particularly difficult for an extreme wide angle lens.
Fujifilm GF 23mm f/4 Aperture Series: Church Mosaic
Fujifilm GF 23mm f/4: Distortion Correction and Micro Contrast Losses
Includes images up to full resolution from f/4 - f/9.
Performance of the Fujifilm GF 23mm f/4 surely ranks among the finest performances available in the 19mm (equivalent) focal length range in terms of total image sharpness and consistency across the field. Accordingly, I have only minor reservations in giving my highest recommendation to the about $2600 Fujifilm GF 23mm f/4—this is the wide angle lens that anchors the system, along with the about $2800 Fujifilm GF 110mm f/2 at the long end.

GFX 50S + Fujifilm GF 23mm f/4 R LM WR @ 19mm equiv (23mm)
[low-res image for bot]