Articles · 8 September 2026 · 7 min read

Flange focal distance: why some lenses adapt and others never do

What flange focal distance is, why a lens only adapts to a body with a shorter one, and a table of every mount's figure, including Leica M at 27.80 mm.

Flange focal distance (FFD) is the gap between the lens mount’s seating face and the sensor. A lens can be adapted to a camera with a plain, glassless adapter only if the lens’s mount has a longer flange distance than the body’s, by enough to fit an adapter into the difference. This site’s compatibility matrix uses 2 mm as a conservative minimum, although a few specialist adapters manage with less. For the search that brings many people here: the Leica M flange focal distance is 27.80 mm.

Everything else about adapting follows from that one subtraction.

What flange focal distance actually measures

Every lens is designed to sit a fixed distance in front of the film or sensor. Mount it at that distance and it focuses from its closest point out to infinity. Move it further away and it focuses closer; the far end of the focus scale no longer reaches infinity. Move it closer and it can never get there at all without being redesigned.

The mount’s flange is the metal face the lens seats against. The flange focal distance is that face’s distance from the image plane, and each mount standard fixes it. It is also called register, registration distance or flange back. All of them mean the same number.

Why the lens mount’s figure has to be the larger one

An adapter is a spacer with a lens mount on the front and a body mount on the back. It can only add distance. It cannot pull a lens into the camera.

So if a lens was designed for 44.00 mm (Canon EF) and the body puts its flange 20.00 mm from the sensor (Canon RF), the adapter has to be exactly 24.00 mm deep, and the lens sits exactly where it was designed to. That is why Canon EF lenses on Canon RF lose nothing optically: nothing optical is in the way.

Reverse the direction and there is no answer. An RF lens on an EF body would need to sit 24 mm inside the camera. The only fix is an adapter with glass in it, which acts as a weak teleconverter to shift focus. That costs sharpness and usually light, which is why this site does not treat it as a real fit.

The 2 mm rule, and why it is only a rule of thumb

A positive difference is necessary but not always sufficient. The adapter needs a bayonet on each face and something holding them together, and that takes depth.

This site’s compatibility matrix uses 2 mm as the minimum difference for a passive adapter. It is a rule of thumb, not a manufacturer specification, and pages that sit within a millimetre of it say so. The borderline case in the data is Sony E lenses on Nikon Z: 18.00 mm against 16.00 mm, exactly 2.00 mm. Adapters for that pairing exist, but it is a precision item, and a cheap one that sits slightly out of true will show it.

The rule is deliberately cautious, and some real adapters beat it. Fujifilm X at 17.70 mm is only 1.70 mm longer than Nikon Z, yet Fotodiox sells a plain X-to-Z ring and Shoten makes an autofocus X-to-Z adapter. Micro Four Thirds at 19.25 mm is 1.25 mm longer than Sony E, and plain manual adapters for that pair exist too, though a Four Thirds-sized image circle will vignette on Sony’s larger sensors. Leica M onto Fujifilm GFX, covered below, clears by just 1.10 mm. Treat a sub-2 mm pairing as “only if a specialist makes it”, not as impossible.

Equal figures fail outright. Canon EF-M and Sony E are both 18.00 mm, so neither adapts to the other.

Every mount on this site, sorted by flange distance

Figures are from this site’s mount data, sourced from the Wikipedia flange focal distance table and checked on 7 September 2026. Throat diameters are given only where a manufacturer has published one.

Read it as a ladder: a lens goes onto a body comfortably further up the table with a plain adapter. Within 2 mm, it depends on whether someone makes a thin enough adapter. A lens near the bottom has the most options, and a body near the top accepts the most lenses.

Leica M: 27.80 mm

The Leica M flange focal distance is 27.80 mm. It is longer than every current mirrorless mount’s, so M lenses adapt with a plain ring onto Nikon Z (11.80 mm to spare), Fujifilm X (10.10 mm), Sony E and Canon EF-M (9.80 mm), Micro Four Thirds (8.55 mm), and Canon RF and L-Mount (7.80 mm). It is only 1.10 mm longer than Fujifilm G, below this site’s 2 mm threshold, yet plain M-to-GFX adapters are sold by Metabones, Novoflex, Fotodiox, Kipon and TTArtisan. Many M lenses, designed for a 36 × 24 mm frame, will not cover the whole GFX sensor.

Going the other way, an M body at 27.80 mm cannot accept a lens from any mount above it in the table.

Because M lenses are fully mechanical, there is nothing electronic to break. See Leica M lenses on Sony E and Leica M lenses on Nikon Z for the optical caveat that does matter: wide-angle M lenses can smear at the corners on digital sensors.

Why mirrorless bodies take nearly everything

An SLR needs room between the mount and the sensor for a mirror to swing up. That box of air is why every SLR mount in the table sits between 38.67 mm and 46.50 mm.

Mirrorless cameras have no mirror, so their designers moved the mount much closer to the sensor: every current mirrorless mount is at 20.00 mm or less, apart from medium-format G. That leaves between roughly 18 and 30 mm of spare depth in front of the body for almost any SLR lens ever made. Nikon F lenses on Nikon Z have 30.50 mm to play with; M42 lenses on Sony E have 27.46 mm.

The practical consequence is one-way traffic. SLR and rangefinder lenses go onto mirrorless bodies. Mirrorless lenses almost never go onto SLRs. They rarely go onto each other either, because the smaller-format mirrorless mounts are bunched between 16.00 and 20.00 mm. Where they do, it takes a thin, precise adapter, and for electronic lenses it also takes one that talks to both sides.

Physical fit is only the first question. Whether autofocus and aperture control work depends on the adapter’s electronics, which this site researches per combination; the adapter guides state what works for each one.

Throat diameter: the second constraint

Flange distance says whether the lens can sit far enough forward. Throat diameter, the inside width of the mount opening, says whether the back of the lens and the light it projects fit through the opening without being cut off.

It matters most when a lens’s rear element sits deep and wide, or when a mount is narrow relative to the format it serves. Among the full-frame mirrorless mounts with published figures, Nikon Z is the widest at 55 mm, Canon RF is 54 mm, L-Mount is 51.6 mm and Sony E is 46.1 mm. A wide throat gives lens designers room for large rear elements, and gives an adapted lens’s light path more room to pass through without clipping.

Many manufacturers have never published a throat figure. Where that is the case, this site leaves the field blank rather than using a measurement from a forum.

Where to go next

• The compatibility matrix applies the 2 mm rule to every pair of mounts at once.
• The mount hubs give each mount’s full specification and third-party lens position.
• For a mount-specific example, read the L-Mount Alliance explained or why Canon EF-M lenses have nowhere to go.