Fujifilm X-Mount
Direct Answer
The Fujifilm X-Mount (introduced in 2012 for the X-Pro1) is a dedicated APS-C mirrorless bayonet mount featuring a 17.7 mm Flange Focal Distance, a 40.7 mm throat diameter, and a 10-pin electronic interface. Designed exclusively for APS-C image circles, it allows compact lens designs optimized for Fujifilm's proprietary X-Trans CMOS sensors.
Source: Fujifilm Corporation X-Mount Technical Disclosure / CIPA
Specs
| Flange Focal Distance (FFD) | 17.70 mm (0.697 in) |
| Inner Throat Diameter | 40.70 mm (1.602 in) |
| Back Focus Distance | Minimum rear optical element clearance of 7.5 mm from sensor |
| Bayonet Tabs | 3 locking tabs with counter-clockwise mount twist |
| Electronic Interface | 10 contact pins (opened to third parties in 2020: Tamron, Sigma, Voigtländer, Viltrox) |
| Physical Aperture Rings | Standard mechanical aperture rings with 1/3-stop clicks on XF lenses |
| Camera Systems | Fujifilm X-T5, X-H2, X-H2S, X-T30 II, X-S20, X-Pro3, X-E4 |
Dedicated APS-C Optimization
Because Fujifilm decided not to produce full-frame cameras (jumping straight from APS-C to medium format GFX), the X-mount's $40.7\text{ mm}$ throat was tailored exclusively for $23.5 \times 15.6\text{ mm}$ sensors. XF lenses never carry the dead weight or oversized glass elements required for full-frame coverage.