Lens speed
1 2 3 4 5 6 7 8 9 15

Lens speed

Aside from their native focal lengths, another key differentiator between these two cameras is the speed of their lenses, expressed in terms of their maximum apertures. The Fujifilm X100F offers an F2 maximum aperture (equivalent to ~F3 in full-frame terms) while the GR III's lens is slower, at F2.8 (~F4.2 equiv).

The difference in maximum aperture has a couple of important effects. For one, you'll have more ability to blur backgrounds and isolate your subject from the faster lens of the X100F, aided also by its longer focal length. You'll also be able to shoot at lower ISOs in lower light with the Fujifilm without resorting to a tripod. On the other hand, the wider lens of the GR III, and its stabilized sensor (more on this in a minute) mean that the low-light disadvantage is somewhat mitigated (assuming static subjects) and you may not need a tripod except in very dark conditions.

Verdict: Tie. Too many variables.