00LENS MOUNTS, EXPLAINED

The gap that decides
which lens fits.

Two hands holding a plain, unmarked machined metal ring up to a window, with soft light passing through its center — AI-generated image, not a photograph of a real person or place.

Every lens is built to sit a set distance from the sensor.

That distance, the flange focal distance, is fixed for each mount. Taking the mirror out made it much shorter, which is why so many older lenses can now be adapted with a plain spacer ring.

16–20 mmmirror gone, gap shrinksUpdated October 22, 2026

Check an adapter

01Principles

Short body, long lens: easy
Long body, short lens: hard
Electronics may not follow
Distance first, then contacts

02THE SHORT ANSWER

Add distance easily,
never take it away.

A lens focuses at infinity only at its own distance.
An adapter can add space, not remove it.

DISTANCE

Flange focal distance

The gap from the mounting flange to the sensor. It must be made to hundredths of a millimeter, or lenses will not focus accurately.

Check an adapter

ADAPTERS

A tube, or extra glass

A lens from a longer-flange system needs only a spacer on a shorter-flange body. Going the other way needs an optical adapter, or infinity is lost.

Before you buy

ELECTRONICS

Contacts do not translate

Modern mounts pass aperture and focus signals through contacts. Cornell's mount list notes most conversions lose autofocus, auto exposure or both.

See the numbers

03A QUICK ADAPTER CHECK

Which way
are you adapting?

Pick a pairing to see whether a simple adapter can keep infinity focus. The bar shows how hard the job is.

Mount distances and throat sizes vary: check the figures for your own lens and body before buying.

Adapter check

Choose a pairing35 mm SLR lens on a full-frame mirrorless bodyTwo mounts less than a millimeter apartLens with autofocus and electronic apertureMirrorless lens on an SLR body

35 mm SLR lens on a full-frame mirrorless bodySimple tube

SLR mounts sit about 42 to 47 mm from the sensor, mirrorless ones about 16 to 20 mm, so a plain spacer of the difference keeps infinity focus.

Two mounts less than a millimeter apartMaybe

Cornell's list gives a 0.5 mm gap as possibly too thin for a usable adapter. The lens thread or bayonet must also fit inside the body's mount.

Lens with autofocus and electronic apertureFeatures may drop

Active adapters carry electronic connections, but many conversions sacrifice autofocus, automatic exposure or both, and sometimes metering.

Mirrorless lens on an SLR bodyNeeds glass

The lens would have to sit closer than the body allows. An adapter needs a corrective lens, which costs light and adds a crop; without it, distant focus is lost.

Sources: Wikipedia, Flange focal distance and Lens mount; Cornell University Program of Computer Graphics, Camera Mounts Sorted by Register.

04TWO NUMBERS WORTH KNOWING

Taking out the mirror
halved the gap.

Typical distances for full-frame mounts,
from the published table of mount registers.

16–20 mm

Flange to sensor, current full-frame mounts

Wikipedia's flange focal distance table puts current full-frame mirrorless mounts between 16 mm and 20 mm. With no reflex mirror to swing, the lens can sit much closer.

MIRRORLESS · FULL FRAME

42–47 mm

Flange to sensor, common SLR mounts

Most 35 mm SLR mounts in the same table sit between 42 mm and 47 mm, leaving room for the mirror. The shared M42 screw thread, used since 1949, is 45.46 mm.

SLR · 35 MM FILM AND DIGITAL

Removing the mirror box was a design aim: the Micro Four Thirds standard cut its flange back to about half that of the earlier Four Thirds SLR system. The first mirrorless camera went on sale in Japan in October 2008.

05FOUR CHECKS BEFORE YOU BUY

Measure twice,
adapt once.

An adapter is a precise spacer. These checks come straight from how flange distance works.

Compare the two distances
Look up the flange focal distance of the lens mount and the body. The lens's figure should be larger, by more than a thin sliver.
Check the throat fits
The lens thread or bayonet must fit inside the body's mount. Close distances can still fail on diameter.
Decide what you need to keep
Plain adapters pass no signals. If you rely on autofocus or auto exposure, look for an active adapter and expect limits.
Avoid glass if you can
Optical adapters reduce the light reaching the sensor and add a crop. Plain non-optical adapters are preferred where they work.

Hands seen from above twisting a plain black lens with a smooth barrel onto a plain metal adapter ring on a wooden table — AI-generated image, not a photograph of a real person or place.

06WHY PRECISION MATTERS

Hundredths of
a millimeter.

Extreme close-up of small gold-colored contact pins on a plain black circular mount, mostly out of focus — AI-generated image, not a photograph of a real person or place.

Flange focal distance is a fixed mechanical specification. Even small deviations can stop a lens reaching accurate focus across its range, so both the body and any adapter must be made to tight tolerances.

That is also why close pairs of mounts are awkward. Cornell's mount list gives the example of a 0.5 mm difference, which might be too thin to make a usable adapter, and notes the lens thread or bayonet must fit entirely inside the body's mount.

07Questions

Questions people ask

What is flange focal distance?

The distance from the mounting flange, the interlocking metal rings on the body and the back of the lens, to the film or sensor plane. Each lens mount has its own fixed figure.

Why can mirrorless cameras adapt so many old lenses?

Without a reflex mirror their flange distances are short, about 16 to 20 mm for full-frame mounts. Lenses made for longer distances can be held further out with a simple spacer and still focus to infinity.

Can I put a mirrorless lens on an SLR?

Not with a simple adapter. The lens would need to sit closer to the sensor than the SLR mount allows, so an adapter needs a corrective lens, which loses light and adds a crop, or it will not focus far away.

Will autofocus work through an adapter?

Sometimes, with an active adapter that carries electronic connections. Cornell's mount list notes most conversions sacrifice autofocus, automatic exposure or both, and some lose metering.

Does a tiny difference in distance matter?

Yes. The specification is set to hundredths of a millimeter, and Cornell's list suggests a 0.5 mm gap between two mounts may be too thin for a usable adapter.

08Sources

The gap that decides
which lens fits.