Flange distance
Simulator
- 1Sensor, 24 mm tall on full frame
- 2Shutter, a few millimetres in front of the sensor
- 3Main mirror, 45 degrees, half silvered at the centre
- 4Sub-mirror behind it, sending light down
- 5Autofocus sensor in the floor of the mirror box
- 6Focusing screen, as far from the lens as the sensor is
- 7Pentaprism, which turns the image upright
- 8Eyepiece
- 9Mount face: the flange distance is measured to here
- 10Rear screen
- 11Viewfinder panel, a small screen fed by the sensor
- 12Processor, which also runs the autofocus
What filled the 24 millimetres
A DSLR shows you the scene through the lens by bouncing it up into the finder. Everything that does that work sits between the lens and the sensor, and the mount has to be far enough forward to leave room for it.
The mirror and its swing
A mirror the size of the frame sits at 45 degrees in front of the sensor. When you press the button it swings up against the focusing screen, the shutter opens, and it drops back. The swing sweeps through roughly 40 mm of space in front of a full frame sensor, and no glass may be in it: the rear of the lens stops at the mount. Canon's EF flange is 44.0 mm, Nikon's F 46.5 mm, Pentax K 45.46 mm. The figure is set by the mirror, not by the lens.
The sub-mirror and the autofocus sensor
The centre of the main mirror is half silvered. A smaller mirror hinged behind it catches the light that comes through and sends it down to the autofocus sensor in the floor of the mirror box. So a DSLR measures focus with one sensor and takes the picture with another, and the two must sit at exactly the same optical distance. When they do not, the camera focuses in front of or behind the subject, and the owner calibrates each lens against the body. The sub-mirror only sees the middle of the frame, which is why DSLR focus points cluster there.
The focusing screen and the prism
The mirror throws the image up onto a ground glass screen that is as far from the lens as the sensor is, so what looks sharp there is sharp in the picture. Above it, a glass pentaprism (or a set of mirrors on cheaper bodies) turns the image the right way up and the right way round and sends it out to the eyepiece. It is the prism that gives a DSLR its hump, and a good part of its weight.
What is left in a mirrorless body
The sensor does all three jobs. It makes the picture, it measures focus with pixels set aside for it across the whole frame, and it feeds the viewfinder, which is a small screen behind an eyepiece. In front of it there is only the shutter, a couple of millimetres thick, and the filter glass on the sensor itself. The mount can come in as far as that allows: 20.0 mm for Canon RF and the L mount, 18.0 mm for Sony E, 16.0 mm for Nikon Z.
Why 20 and not less
Canon chose 20 mm with a 54 mm throat, the same opening as EF. A shorter flange would help lens design a little more, but light that reaches the corners of the sensor at a very steep angle is harder for the sensor to collect, and some room is wanted for the shutter and for a drop-in filter adapter. Every full frame mirrorless mount settled between 16 and 20 mm.
Why a shorter flange makes better lenses
The gain is not in the body. It is that the designer of a lens may now put glass where the picture needs it, close to the sensor, instead of keeping everything 44 mm away for a mirror that is no longer there.
Less colour fringing
Lateral chromatic aberration is the red and blue edges on things near the corners of the frame. It comes from asymmetry: the colours a strong front group spreads are not gathered again. Wide lenses designed close to the sensor can be built nearer to symmetric, so they start with less of it, and what remains is corrected with fewer special elements. The simulator on chromatic aberration shows what the fringes look like.
Less distortion, and better corners
Barrel distortion and coma cancel in a symmetric design for the same reason. A rear group near the image plane can also flatten the field, so the corners focus on the same plane as the centre instead of in front of it. Some of the remaining distortion is now corrected in the camera from a profile, which is a choice the designer makes knowingly: a little barrel left in the glass buys a smaller, sharper lens.
Smaller and lighter, where the glass allows
Wide angle and moderate lenses lose the negative front group and the elements that corrected it, so they shrink. Pancake lenses of 16 to 28 mm became possible on full frame. Fast standard lenses and long telephotos do not shrink: their size is set by the aperture and the focal length, and Canon's RF 50 mm f/1.2 is larger than its EF forebear because it is corrected harder. The table below gives a few pairs.
The one cost
Light that arrives at the sensor corners at a steep angle is harder to collect: the microlenses over the pixels have to be offset towards the centre, and the filter glass over the sensor kept thin, or the corners darken and shift in colour. This is why the full frame mounts stopped at 16 to 20 mm, and why some rangefinder wide angles, built for film with their rear element very near the frame, show magenta corners on a digital sensor.
| EF lens | Length | Weight | RF or mirrorless lens | Length | Weight |
|---|---|---|---|---|---|
| EF 14 mm f/2.8L II | 94 mm | 645 g | RF 16 mm f/2.8 | 40 mm | 165 g |
| EF 24 mm f/2.8 IS | 56 mm | 280 g | Sony FE 24 mm f/2.8 G | 45 mm | 162 g |
| EF 24-105 mm f/4L II | 118 mm | 795 g | RF 24-105 mm f/4L | 107 mm | 700 g |
| EF 50 mm f/1.8 STM | 39 mm | 160 g | RF 50 mm f/1.8 STM | 41 mm | 160 g |
| EF 50 mm f/1.2L | 66 mm | 580 g | RF 50 mm f/1.2L | 108 mm | 950 g |
Makers' quoted figures, rounded. The 14 and 16 mm are different classes of lens and are listed to show the shape of the change, not as equals. The 50 mm f/1.8 pair shows that a lens which did not need the mirror clearance gained nothing in size; the f/1.2 pair shows a designer spending the freedom on correction instead.
The adapters: a ring makes up the difference
A lens is built to focus at infinity with its mount a fixed distance from the sensor. On a body whose mount sits nearer the sensor it is too close, and a ring exactly as thick as the difference puts it back where it belongs. Nothing optical happens in the ring; it carries the electrical contacts through. Pick a lens mount and a body mount to see the ring.
Lens made for
Camera body
fits the chosen body with a plain ringcannot be adapted without glassthe chosen body
It only works one way
Every mount above the body's line on the ladder fits it with a ring, which is why one mirrorless body takes EF, F, K, M42, Leica M and PL lenses, and why RED put the RF mount on Komodo and V-Raptor: a cinema camera with 20 mm of flange takes PL, EF and RF glass through rings. A lens built for 20 mm cannot go the other way. Moved farther from the sensor than it was designed for, it focuses only on near things and never reaches infinity, so RF and E lenses fit nothing older.
Canon's three rings
The plain EF-EOS R adapter is 24 mm of metal with the contacts passed through; EF and EF-S lenses focus, stabilise and report as they did, and an EF-S lens, which a full frame DSLR refused because its rear intruded into the mirror box, mounts and the camera crops. The control ring adapter adds the RF lenses' ring for aperture or ISO. The drop-in filter adapter uses the space for a slot: a polariser or a variable ND behind the lens, which also gives filters to the EF 11-24 mm and other lenses whose bulging front takes none.
The other makers
Nikon's FTZ is 30.5 mm (F to Z); Sony's LA-EA adapters are 26.5 mm (A to E); Sigma's MC-11 is 26 mm (EF to E) and MC-21 24 mm (EF to L). Rangefinder lenses need thinner rings: Leica M to Sony E is 9.8 mm, to Nikon Z 11.8 mm. A ring under about 3 mm cannot hold a proper bayonet on each face, so a pair that close, such as Nikon F to Canon EF at 2.5 mm, gets a thin adapter and nothing electrical.
Adapters with glass
Some adapters hold a lens group. A focal reducer (a speed booster) shrinks the image circle by about 0.71, so a full frame lens on a Super 35 or APS-C sensor gets its angle of view back and gains about a stop. Others add a weak element so that a lens can be moved farther from the sensor than it was built for. Both cost some sharpness at the edges; the plain ring costs nothing.
What else the mirror took with it
Focus is measured where the picture is made
With no separate autofocus sensor there is nothing to calibrate against the image sensor, so front focus and back focus are gone, and the focus points can cover the whole frame rather than the middle the sub-mirror could see. The camera reads the image itself, which is how it finds eyes and follows them.
You see the picture you will get
The finder shows the exposure, the white balance and the depth of field as the sensor will record them, and can brighten a dark scene. An optical finder shows the scene as the eye sees it, which some photographers prefer, and it needs no battery and has no lag.
No mirror slap
The mirror's swing shook the camera a little at slow shutter speeds and made the noise a DSLR makes. Without it the camera can be silent, with an electronic shutter, and can shoot at high frame rates without the finder going dark between frames.
A thinner body, up to a point
The sensor can sit 20 mm behind the mount instead of 44, so the body in front of the sensor is thinner. Behind it, the battery, the screen and a grip that fits a hand set the depth, which is why a full frame mirrorless camera is lighter than a DSLR but not as thin as the flange alone would allow.
Where the figures come from
Flange distances
The makers' published figures, to the hundredth of a millimetre where they publish them (Pentax K and M42 share 45.46 mm; Panavision is 57.15 mm, which is two and a quarter inches). Throat diameters are the nominal inner diameter of the bayonet: 54 mm for EF, RF and PL, 55 mm for Nikon Z, 46.1 mm for Sony E, 51.6 mm for the L mount, 44 mm for Nikon F.
The drawings
The sensor and the flange distances are to scale. The mirror, prism, finder and lens designs are schematic: the right parts in the right order at about the right size, not a particular camera. The space a full frame mirror needs to swing is an estimate of roughly 40 mm; it differs by a few millimetres between bodies.
Lens sizes
Length and weight are the makers' quoted figures, rounded to the millimetre and to 5 g. Element counts and designs are not quoted, because the lenses in the figures are drawn to show a type of design, not a product.