I am looking at X-Trans X series sensors. I am not looking at the Fuji medium format sensors.
Increased acuity reveals more texture and finer detail. It is most noticeable when making large prints, or with heavy cropping.
It is least put to the test in portraits, because the elements are so large in the frame that the eye makes up detail easily. It is most put to the test in subjects with fine detail such as in landscapes.
So what is the difference in perceived sharpness and fine detail rendering between a 40MP sensor (like in the Fuji X-H2 or X-T5 or X-T50) and a 24MP or 26MP sensor (like in the X-T3, X-T4, X-S10, or X-S20)?
Sensor size definitely affects image quality and allows for more cropping. But what about the number of pixels? Is a higher MP (megapixel) count better?
A 24MP X-Trans III sensor is 6,000 x 4,000 pixels. An X-Trans IV sensor is 6240 x 4160 pixels. An X-Trans V sensor is 7728 x 5152 pixels.
Compared to the two smaller megapixel sensors, the X-Trans V sensor has 50% more area. But it is not area that determines extra sharpness. It is linear resolution that determines acuity. In other words, how much longer the longest side is, all other things being equal. And the 40MP sensor translates to approximately 33% more linear resolution.
In photography, acuity refers to the sharpness and clarity of an image, particularly in the details and fine edges. Acutance, a closely related term, describes the edge contrast and the perception of sharpness in an image.
In film photography, some chemical developers increase micro-contrast on edges and give the viewer a perception of increased sharpness. Sharpness tools in applications like Photoshop do a similar thing.
Anyone show has played with the sharpness sliders in Photoshop or Lightroom or any of the other tools knows that is possible to introduce a white halo around edges. If that is done carefully it can increase the apparent contrast between dark and light edges.
Either way, acuity and acutance are related to the ability of the human eye to see and compare the sharpness of different images. After all, if the human eye can’t see the difference in acuity between a lower megapixel image and a higher megapixel sensor then there is no difference in practical terms.
So to repeat, increased acuity is most noticeable when making large prints, or with heavy cropping.
Oh yes, and then there’s the fact that lenses need to be able to resolve that detail. For lenses that can resolve a Fuji 40Mp sensor, see this article: Fuji X-Mount Lens Release Dates: A Complete List.