I just learnt, reading an Zeiss publication about wide-angle lenses, that smaller formats have less spherical abberations at equal aperture ratio when you simply scale down a lens design linearly to get a design covering the same field of view and he same minimum aperture ratio with a smaller focal length and image circle. The short explanation is that spherical abberation scales Witt he fourth power of aperture size (entrance pupil diameter), so the down-sizing reduces aperture size at a given aperture ratio, and reduces the abberations far more than the reduction in image size.
On the other hand, diffraction is worse at equal aperture ratio in the smaller format case.
This suggests that
- smaller format lenses wiaverted to have their sweet spot of best resolution at a lower f-stop; larger format lenses are more likely to be limited to somewhat higher usable f-stops, even if the minimum f-stop is the same due to using the same optical design
- smaller format lenses are more likely to be usable wide open or close to it
- on the other hand, smaller format lenses are likely to need to be used at lower f-stops, to get enough speed and enough background blur.
This contradicts what has often been said and whatbinbelieved until recently: the idea that lenses for new smaller digital formats have their sweet-spot at a lower f-stop because the designs are more highly corrected, necessitated to compensate by the disadvantages of the smaller format.
This seems counterintuitive at first, but look at it this way: with equal f-stop, the smaller format has a smaller entrance pupil, and so the range of angles between the rays from the same point of the subject that enter the lens is less, and the smaller angular variation reduces spherical abberations. And maybe some other optical abberations too; I will try to read more.
This scaling down might not work so well with DSLR's, particularly ones sharing a lens mount and minimum back-focus distance with a larger format like 35mm, because that simple down-sizing of lenses could make thr back-focus distance too low. But with the new lens mounts of the new mirrorless systems (except Pentax's), this is in play.
It also worked for Leica's 35mm format rangefinder cameras in competition with its medium format predecessors: I wonder if that was part of the key to Leica being able to offer lenses that worked well at f-stops lower than anything offered in larger formats.