Permanently attached to the front of the Fujifilm X100F is a Fujinon f/2 23mm Super EBC Aspherical lens. It’s an excellent 35mm (equivalent) prime, but, like all lenses, it’s not perfect.
I’ve owned many different lenses by different brands over the years. One thing that I have learned is that every lens has its sweet spot. There is an aperture where the lens is at its peak performance.
Ever since my X100F arrived two months ago, I’ve been trying to figure out just where the lens is at its best and worst. I wanted to know its strength and weakness so that I can maximize the one and minimize the other. I discovered pretty quickly where the “worst” apertures are, and that’s f/2 and f/16.
Not that f/2 is necessarily a bad aperture, but it is indeed noticeably softer than the others. Not just at the corners, but center sharpness, too. It lacks the crispness at f/2 that one expects from a Fujinon lens. But I’ve used plenty of other lenses, primes and zooms, that were just as soft if not softer. So just because it’s a soft aperture for this lens doesn’t mean that it’s an aperture that should be avoided. In fact, Fujifilm actually calls this softness a “feature” in the manual, so perhaps it is something you might find useful for some images. You can also find some chromatic aberrations at f/2.
Things get better as you stop down. There’s significant improvements by f/2.8, although there is still some noticeable corner softness. By f/4 the lens is crisp all over.
I noticed diffraction when the aperture is smaller than f/8. It’s not really pronounced until smaller than f/11. The smallest aperture is f/16, and I would avoid it because the diffraction is pretty obvious.
My opinion is that the sweet spot on the Fujifilm X100F lens is at f/4, including the intermediate stop on either side (f/3.6 and f/4.5). At this aperture the lens is corner-to-corner sharp, and at its sharpest in the center. It produces lovely images, and bokeh, while not heavily pronounced, is still very nice.
I would also say that image quality is excellent between f/2.8 and f/11, and I freely use these apertures without thinking twice about it. I don’t go higher than f/11. I do sometimes go lower than f/2.8, albeit not often.
The lens on the X100F is excellent. If it were an interchangeable-lens camera and the lens could detach, I imagine that you’d proudly use it often. Like all lenses, it has its strengths and weaknesses, and you get the most out of it if you understand what those are. With this lens, the strengths are many and the weaknesses few. Still, if the situation allows, keep it around f/4 and you’ll see the best that it can do.