What Each of Them Threw Away to Get Closer to the Sensor

02.2 §1The Geometry Behind the Decision

Mirrorless cameras work because they remove the mirror box. That box occupied roughly 38–47 mm between the lens mount and the film or sensor plane in a DSLR, and it was there by necessity: the reflex mirror needed room to flip. Eliminate the mirror and you can push the mount physically closer to the sensor, reducing the flange-to-sensor distance. The gains are real — shorter flange distances allow larger exit pupils closer to the sensor, wider maximum apertures, and more aggressive optical designs. Every major manufacturer made this trade. All of them also made a second calculation: whether the existing lens ecosystem was worth preserving in native form, or whether an adapter was enough.

The four mirrorless mounts that emerged — Canon RF, Nikon Z, Sony E, and L-mount — each chose a different flange distance, and each inherited or severed a different legacy.

02.2 §2Canon, Nikon, Sony: Three Departures

Canon's EF mount, introduced in 1987, carried a flange distance of 44 mm and was the backbone of the EOS system for over three decades. The RF mount, announced in 2018, brought that distance down to 20 mm. Canon offered the EF-EOS R adapter from day one and has been explicit that native EF production continues to be maintained — but no new EF lenses have been developed since the RF transition accelerated. The adapter is technically competent; the message it carries is transitional.

Nikon's situation was sharper. The F-mount had run since 1959 with a flange distance of 46.5 mm, arguably the longest continuous interchangeable-lens mount in camera history. The Z mount, launched in 2018 alongside the Z6 and Z7, dropped that to 16 mm — the shortest of the four major mirrorless flanges — and simultaneously opened the widest throat diameter of the group at 55 mm. Nikon offered the FTZ adapter and has since refined it. What the adapter does not do is save the F-mount from production wind-down; Nikon has released no new F-mount glass since the Z system launched.

The four mirrorless mounts that emerged — Canon RF, Nikon Z, Sony E, and L-mount — each chose a different flange distance, and each inherited or severed a different legacy.

Sony is the outlier in chronology. The E mount arrived in 2010 with the NEX-3 and NEX-5, establishing an 18 mm flange distance before any of the others had committed to mirrorless interchangeable systems at scale. Sony's A-mount — the Minolta-derived screw-drive system it had carried since acquiring Konica Minolta's camera assets — was bridged by the LA-EA series of adapters. By the early 2020s the A-mount body line had effectively ended, though CIPA shipment data illustrates how entirely the industry's volume shifted toward mirrorless during that period.

The L-Mount Alliance, announced by Panasonic, Leica, and Sigma in 2018, is structurally different: it is a shared standard rather than a single maker's pivot. Leica's prior M-mount and R-mount systems were not directly discontinued by the alliance — Leica still produces M-mount bodies — but for Panasonic it represented a decisive break from Micro Four Thirds as a full-frame solution, and for Sigma it meant exiting the SA-mount it had developed independently. The L-mount's flange distance of 20 mm matches Canon RF; its diameter, at 51.6 mm, is slightly smaller than the RF mount's 54 mm.

02.2 §3What the Adapter Strategy Admits

An adapter is an engineering statement. It says the old glass works well enough to be worth carrying forward, and simultaneously that the old mount is not worth producing new bodies for. Each of these four manufacturers shipped a first-party adapter alongside or shortly after their mirrorless body — a commercial acknowledgment that the installed lens base represented real customer value, and a strategic signal that the native mount's future lay in the new system.

Pentax's K-mount, with its 45.46 mm flange distance maintained since 1975, was never submitted to this arithmetic. Ricoh kept the mirror and kept the mount, which means Pentax shooters face no adapter question — and no rupture. Whether that is conservation or delay depends on which direction the market moves next.