It was Charles Dickens who best summed up the state of photographic optics:
IT WAS the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way—in short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only.
In my case, however, it is not a comparison between two, but among three.
In the days of film, the comparison of optics largely came down to how fast and how sharp. Low dispersion glass made sharper images, but the choice of film could easily negate any advantages brought by the glass. To be sure, Leica lenses gave images a different feel, but again—printing any image onto Kodak Polycontrast paper was the great equalizer. Without a good match between gear, film, paper, and presentation, the differences in optical quality were easily lost to the eye.
Similar problems exist with digital images: what I edit on my 30 inch, color corrected display is not what is likely to be seen on a Dell notebook display, or on a smartphone. And it will look different when it's printed on paper.
We had everything before us
As the man said, it was the best of times, it was the worst of times, and the more things change, the more they stay the same.
Issues of handling speed have always been, and remain, issues of preference based on the individual. While I can focus a Leica rangefinder quickly, I can focus a Nikon SLR film camera faster. Either of those manual options is faster than selecting an autofocus point on the Fuji or the Canon, and then recomposing. Depending on what tool I need, I use them all.
Issues of sharpness are also generally about preference. Some photographers like to have depth of field, and some don't. So the objective ability of the lens to deliver a certain level of resolution or a certain style of bokeh may be a selling point, or may be irrelevant. (Was Dickens a photographer?!)
But this is February, and therefore the winter of our despair. The path out of unmeasurable preference is objective data, and it was in the vein of seeking the season of light that I did a quick, "real world" comparison to three 50mm lenses that I have: a Nikkor f/1.8, a Leica Summicron f/2, and the Fujifilm TCL-X100 for the X100s. I shot this series of images at f/2 on all of the cameras. (Because the Summicron is 47 years old, the Leica M240 guesses at the aperture, so the EXIF data is never accurate.) The Nikon was loaded with bwxx, and the negative scanned with an Epson V600.
Nikkor.
Leica.
Fujifilm.
Regardless of film versus digital versus your display versus mine, what's objectively noticeable is the bokeh of the lenses. Regardless of the subjective reaction to the quality of each lens's bokeh, what's clear is that the APS-C sensor of the Fuji creates more depth of field at the same aperture than does the full-frame Leica or the film Nikon—resulting in a different (read: lesser) level of blur to the background.
The main real world difference between Nikon and Leica is the difference between digital and film. Film is softer, and digital is sharper.
We had nothing before us
The insidious (and confusing) part is the illusion that the Fuji doesn't provide the same level of bokeh. It doesn't, but not because of anything to do with the sensor size. The confusion stems from the aperture setting itself.
Any f/2 is not the same as any other f/2 when it comes to how you visualize with different sensor sizes because just as a smaller sensor requires one to recalculate its full-frame equivalent (e.g., the X100s has a 23mm lens that gives a 35mm full-frame angle of view), one must also recalculate the f-stop to anticipate its visual effect. As it turns out, the f/2 on the Fuji is the equivalent of f/3 on the Leica or Nikon. At those "equivalent" apertures, the bokeh would actually look quite similar.
This being the epoch of incredulity, one is tempted to do another test to prove the point. But other people already have, and in the end it's quite irrelevant. What is relevant is that if one shoots at f/2 expecting the results to be the same across all cameras, then disappointment will follow.
Unnecessary disappointment, sadly. At the end of the day, what any photographer wants, what any artist wants, what any human wants, is predictability. It's true that f-stop = focal length / diameter of lens opening, and thus there is no lie in Fuji putting "f/2" on a lens. But there's a wide gulf between that technical truth and the application of that technical truth to the creation of an image. Despite all the differences in cameras and lenses that used film, the application of shutter speed and aperture resulted in known effects. That predictability allowed photographers to accumulate experience over time, and then apply that visual experience to the next creation. Unless one is using all full-frame equipment, that experience is at best muddled, and often lost in the confusion.
Received in the superlative degree of comparison only
Digital sensors will continue to come in a variety of sizes and that's a good thing. What's not a good thing is to recycle the technical habits of the past. Digital sensors are different than film, and a new lexicon should be used to give photographers a non-confusing way to understand the benefits and detriments. Call it a d/stop.
If you've invested time and experience in translating full-frame apertures into visual effects, here's a bit of mental math which can help cope with smaller digital sensors: just remember to divide that APS-C sized f/stop by 1.5 to get its full frame visual equivalent. It's distracting and unnecessary math, but it might make it a bit easier to anticipate how your final images will look. Which, by the way, is the whole point of translating camera settings in the making of images. (Looking at you, camera makers!)
With luck, this age of foolishness will pass and apertures will return to their rightful place as an element of artistic choice. Until then, enjoy the number crunching!