Every Canon EF Lens: Read the Reviews, See the Specification & Check the Price
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The EF lens mount

Technical aspects of the Canon EF lens mount


The EOS system was first developed in 1987, and one of its key features in the electronically connectected lens mount system. This dispensed with mechanical drives, cams and pins found in other lens mounts.

The connection between lens and camera is required for the camera to control lens functions (autofocus, aperture) and for the lens to report back information to the camera body, such as focus settings and status (zoom setting for example).

The EF-S mount was introduced in 2003 for EOS cameras with small sensors, along with a range of (generally cheaper) EF-S lenses.

In 2012 Canon introduced the EF-M mount for their mirrorless camera system. EF and EF-S lenses can be used with Canon 'M' cameras with an adapter.

EFlens.com has links to all Canon EF, EF-S and EF-M mount lenses and continues to build up our collection of reviews and other lens related information.

Mechanical aspects

The EF mount incorporates a number of features that have contributed to its flexibility and popularity.

The camera opening is larger than many other mounts, allowing for more flexibility in optical design (such as the EF50mm f/1.0)

The flange distance is the distance between the front of the lens mount and the sensor (film) plane. In the EF mount it's only 44.0mm.

Since this is less than many other lens systems, it makes for relatively simple mechanical adapters to be constructed. The example below shows a simple M42 to EF adapter, in this instance being used to mount a Zeiss Flektogon M42 screw fit lens on to a Canon 1Ds mk3.

The EF flange distance is more than the 42.0 mm of Canon's previous FD mount system, meaning that without serious modification, you cannot use older Canon lenses.

m42 to canon ef adapter

The diaphragm (aperture or 'F number') is set by the camera, using a stepper motor in the lens to activate the aperture blades. This electrical drive does mean that you need a camera body connected to the lens to stop down the lens, or you need a manually activated lens, such as the Flektogon above.

This makes EF lenses less practical to use with additional camera mount systems, such as bellows, where an electrical connection between camera and lens is required.

Flange distances for various lens types

Camera system Mount type Registration distance (mm)
Arriflex (PL) bayonet 52.00
C-mount 1" x 32tpi thread 17.526 (0.69")
Canon EF bayonet 44.00
Canon EF-S bayonet 44.00
Canon EF-M bayonet 18.00
Canon R/FL/FD breech or bayonet 42.00
Contax/Yashica bayonet 45.50
Fujifilm X-mount bayonet 17.70
Hasselblad/Kiev88 multi start thread 82.10
Hasselblad 500/2000 bayonet 74.90
K-mount bayonet 45.46
Leica M mount bayonet 27.80
M42 screw M42x1mm 45.50
Mamiya ZE bayonet 45.50
Mamiya 645 bayonet 63.30
Mamiya RB bayonet 112.00
Mamiya RZ bayonet 105.00
Micro Four Thirds bayonet 19.25
Minolta AF bayonet 44.50
Minolta MD bayonet 43.50
Nikon F bayonet 46.50
Olympus OM bayonet 46.00
Pentax/Practica M42x1mm 45.50
Samsung NX mount bayonet 25.50
T mount M42x0.75mm 55.00
  • Mechanical drawing of EF lens bayonet mount [DWF file] - from a company specialising in microscope adapters
  • 3D print model for EF lens bayonet [Thing]
  • 3D print model of EF-M lens bayonet [Thing]

Larger adapters can incorporate camera movements, such as this Mamiya 35mm 645 lens (63.3mm flange dist.) mounted on a Canon 1Ds mk3 (44.00 mm flange dist.)

More info at:


The EF-S mount ('S' for short back focus) uses the same mount distance, but is specified for cameras with smaller mirrors and sensors, allowing the back of the lens to protrude into the camera body more than the standard EF mount.

EF lenses can be used on an EF-S body camera, but not vice versa.

The EF lens specifications include a requirement for the image circle of the lens to cover at least 43.2mm, so as to cover the full size 36x24mm sensor. With EF-S this is reduced to cover Canon's version of APS-C (~22.5mmx15mm) or a 27.3mm image circle.

Note that a 100m mm EF-S lens is still a 100mm focal length lens. The smaller sensor gives a 1.6x crop factor to the field of view (FOVCF), so the picture is the same field of view as you'd get with a 160mm lens on a full frame sensor. If you like using long lenses, then this is great - if you like wide lenses, then it's not so good...


The short 18.00 mm flange distance allows for smaller lenses. These cameras (currently with APS-C size sensors) have no reflex mirror.

EF-M mount design details

EF-M mount design details above are from a May 2017 US Patent

ef-m lens from camera end

The mount is also smaller than EF/EF-S

compare ef and ef-m mount sizes

Note the nine electrical connection pins. These have different electrical functions to those in EF.

EF and EF-S lenses can be used with an appropriate adapter [Canon].

Ef-M to EF lens adapter

The adapter above shows the EF mount pins.

EF 70-200 lens on an EOS-M

With the short 18mm distance, expect adapters for many more lens types to appear over time.

Instruction manual for Canon mount adapter EF-EOS M (PDF)

Electrical aspects

There are a number of electrical contacts on the mount of any EF mount lens. These supply power to the lens and handle communication between the camera and lens.

Name Function Notes
VBat +6 volts to power internal lens focus motors Present on all EOS bodies and lenses.

Note that power is only supplied to the lens when the camera needs it to do something

P-Gnd Power ground (large contact - counts as two)
VDD +5.5 volts Digital logic power
DCL Data from camera to the lens (MOSI)
DLC Data from the lens to the camera (MISO)
LCLK Camera body generated clock signal (SCLK, CPOL=1)
D-GND Digital logic ground
COM1 Teleconverter common Only on longer focal length and some macro lenses
EXT0 Short to COM1 for 'Life Size Converter' and x1.4 teleconverter
EXT1 Short to COM1 for x2 and x1.4 teleconverter

electrical contacts on a Canon EF lens


The EF-M mount changes some aspects of the design, with the following pinouts

ef-m mount pin functions

The lens and camera communication are shown in this diagram.

ef-m mount electrical communications

The DTEF line determines 'lens type' and bases this on the voltage of the DTEF line measured by an A/D converter (DTEF_IN), suggesting that this is currently based on a resisitor value in the lens. The MIF pin is used to tell the camera that a lens is correctly attached. (info from the US patent).

Data transfer - EF / EF-S mount

EF Data is transferred using a very basic clocked serial interface. One connection is for data going to the lens and one is for data returning to the camera. The data is shifted using a camera supplied clock signal (active low data). The communication protocol between the camera and lens is mentioned as 8 data bits, 1 stop bit SPI (mode 3) - however attempts at reverse engineering suggest that the lens generates a busy signal on the clock line.

Reverse engineering Canon's protocols is not a trivial matter - if you're going to do it, have a read of this POTN thread for some good pointers.

Another good helper is the article at Pick and Place, where this info is included.

Command (hex) Lens response Description
$97$01 (or $CA) 7 bytes Camera asking lens info: Lens ID; Min/Max zoom; Protocol version; Brand
$B0 4 bytes Min/Max aperture
$A0 2 bytes Current zoom value
$07$13+$xx 1 byte ($11) Close/Open diaphragm to $xx value (xx stops from lens full open value)
$13$80 1 byte ($11) Re-open the diaphragm (to fully open)
$0A 1 byte Bust poll? (Lens answer $AA)
$C0 2 bytes USM motor counter position (relative)
$05 Turn focus ring to AF min
$06 Turn focus ring to AF max
$44+$xx+$xx Turn focus ring $xxxx steps (pos or neg value from current value, 1's complements)

Like all our technical pages - we'll update this one as we find new information.

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