This aperture series explores close range performance which includes the quality of the background bokeh. Color correction is examined as well as overall imaging performance.
It shows f/1.8 through f/9, but it also includes a 2-frame focus stack at f/9. Set against the single-frame f/9, it shows the dramatic power of focus stacking to do what stopping down cannot do.
Sigma 14mm f/1.8 DG HSM Aperture Series: Precocious Cluster of Yellow Aspen Leaves
Includes images up to full camera resolution, and crops.
Also new (started) are some examples with the Sigma 14m f/1.8 DG HSM Art:
Sigma 14mm f/1.8 DG HSM Art Examples: Eastern Sierra
Precocious Cluster of Yellow Aspen Leaves
f1.8 @ 0.8 sec, ISO 64; 2017-09-22 19:13:43[low-res image for bot]
NIKON D810 + Sigma 14mm f/1.8 DG HSM Art