TFL mount vs C-mount and F-mount for large sensor cameras

Industrial camera sensors have outgrown the 1.1-inch format that C-mount optics cover comfortably, while pixel sizes have shrunk below 3 µm. At that point the lens mount — the threaded or clamped seat between lens and camera — decides whether corners stay sharp or turn dark and soft. The whole comparison reduces to three numbers: thread diameter, flange distance and image circle. Work through those numbers for C-mount, TFL and F-mount, and the right mount for a 4/3-type, APS-C or larger sensor follows from the sensor diagonal alone.

What a lens mount actually fixes

A mount is not a shape; it is three specifications. The thread or bayonet defines the mechanical seat: C-mount is a 25.4 mm thread with 32 threads per inch, TFL is an M35x0.75 thread — 35 mm across, 0.75 mm pitch — and F-mount is a bayonet clamp, not a thread at all. The C-mount thread and its dimensions long predate machine vision (Wikipedia: C mount).

The flange distance is the fixed gap between the lens seat and the sensor: 17.526 mm for both C-mount and TFL, 12.526 mm for CS-mount, 46.5 mm for F-mount. Miss it by a few tens of micrometres and a high-resolution lens never snaps into focus — the same geometry the C-mount versus CS-mount explainer and the adapter fit and flange-distance guide work through in detail.

The image circle is the disc of light the lens projects onto the sensor. It must exceed the sensor's diagonal with margin, and that single comparison drives the mount decision. Sensor diagonals by format — 11 mm for 2/3 inch, 16 mm for 1 inch, 17.6 mm for 1.1 inch — are tabulated in the sensor format guide.

Why C-mount tops out near 1.1 inch

The 25.4 mm opening is not what limits C-mount. Behind the thread, a lens barrel needs its own mechanics — the focus barrel, its bearings, a retainer to hold the glass — and those stepped parts cut the last optical element down to roughly 17 mm of clear diameter. A 1.1-inch sensor's image circle is 17.6 mm across, an 8.8 mm radius, so the corners of the format sit right at the edge of what the rear optic can physically fill with light. Anything wider cannot be lit without steeper ray angles, and steep rays cost brightness as the fourth power of the angle:

Here d is the sensor diagonal, f the focal length, and the estimate assumes infinity focus. Worked through:

A well-corrected design widens the exit pupil toward the edges and recovers part of the natural loss, but the steeper chief ray angle also strains the sensor's microlenses, and the two effects stack. Below the 1.1-inch ceiling none of this bites hard, C-mount stays the right mount, and it is where lens choice runs deepest. Two lenses built for the formats C-mount still owns:

TFL: the same seat, a wider mouth

TFL keeps the geometry that makes C-mount easy to live with and enlarges only the part that limits optics. The flange stays at 17.526 mm, the thread grows from 25.4 mm to 35 mm at a 0.75 mm pitch, and both figures are published as a standard through the Japan Industrial Imaging Association, so camera and lens makers design to one shared set of numbers. The useful mental model: a C-mount with the thread diameter enlarged and the seat-to-sensor distance unchanged.

The wider thread leaves room for rear optics in the mid-20s of millimetres of clear diameter — call it a 13 mm radius. That covers a 4/3-type half-diagonal of 10.8 mm with clearance and reaches the 14.0 mm half-diagonal of APS-C at the design limit, which is exactly the range the standard was written for.

The formats it targets fill a real gap. A current 24.5 MP, 4/3-type sensor with 2.74 µm pixels spans a 19.3 mm diagonal; a 31.4 MP APS-C part spans 27.9 mm. Both are far past what a C-mount barrel can illuminate, yet both are well short of the 43.3 mm diagonal that F-mount glass is built around. The catalog mirrors it: 1,432 of its 1,765 lenses carry C-mount and 55 carry F-mount. The sensor-format filter already lists 72 lenses for 4/3-inch sensors, while M35x0.75 is the smallest mount group at 3 models — TFL lens supply is the youngest of the three.

Flange distance: 17.526 mm versus 46.5 mm at short focal lengths

F-mount's 46.5 mm flange exists because 35 mm SLR bodies needed clearance for a swinging mirror. A 16 mm lens focused at infinity forms its image roughly 16 mm behind the optics; the mount demands 46.5 mm of clear space instead. The designer has to go reverse-telephoto: 46.5 / 16 = 2.9, so the back focal length runs nearly three times the focal length, bought with extra elements and paid for in resolution and distortion.

TFL asks for 17.526 / 16 = 1.1 times the focal length. Near-symmetric layouts stay possible, element counts stay lower, and the whole optical path stays shorter — which is also why TFL cameras and lenses come out smaller for the same job.

Watch one F-mount trap: lenses that dodge the retrofocus penalty by protruding deep behind the flange. In a machine-vision camera that protrusion runs into the sensor package or its cover glass, and the versions that do fit shrink their rear diameters — steepening corner rays again and undoing the advantage of the big mount.

Threaded versus bayonet: tilt, clamp force and vibration

A bayonet swaps lenses in a second — a photography benefit that buys nothing on a machine that never changes its lens — and it adds what industrial cameras do not want: a spring-loaded clamp with unpublished tolerances, inside which the lens can rock. A thread pulls two machined flanges into flush, metal-to-metal contact, so clamp force is higher and repeats exactly under vibration and gravity.

The reason the play matters is arithmetic. At f/2.8 with 2.74 µm pixels, the half depth of focus is N × c = 2.8 × 2.74 µm = 7.7 µm. Across a 4/3-type half-diagonal of 10.8 mm that tolerates a tilt of atan(0.0077 / 10.8) = 0.04°, and across an APS-C half-diagonal of 14.0 mm it tightens to 0.03°. Rock the lens by a few hundredths of a degree and one edge of the sensor is in focus while the other is blurred.

This is why cameras built for these formats align the sensor actively during assembly, correcting centre position, rotation and tilt to micrometres. Pixels of 2.74 µm also need lens resolving power to match — the Nyquist limit and camera resolution explainer covers that side — and a mount that preserves the alignment is wasted on a lens that cannot resolve the pixels.

Picking the mount from the sensor diagonal

One rule covers nearly every order: the stated image circle must exceed the sensor diagonal, and 5-10% of margin absorbs focus travel and manufacturing tolerance. A 4/3-type diagonal of 21.6 mm wants a circle above about 23.8 mm, so the 22-30 mm class clears it with room to spare. From there the mount follows:

Changing sensor size changes the focal length you need, not the mount's role. The same 16 mm lens frames 1.23 times wider on a 4/3-type sensor than on 1.1 inch, because 21.6 / 17.6 = 1.23 — the HFOV-on-another-sensor explainer shows the pattern, and the calculator below redoes it for your own field of view:

Checks before you order

• Image circle first: the lens's stated circle must beat the sensor diagonal, with the 5-10% margin, whatever thread it carries.
• Mount named on both sides. If you adapt, compute the spacer from the flange difference — a C-mount lens on a CS-mount camera needs 17.526 − 12.526 = 5 mm of spacing.
• Rear clearance: check the lens's rear protrusion against the sensor package and cover glass, especially on F-mount designs.
• Resolution class matched to pixel count. A 2.74 µm pixel needs a lens sold for the format's megapixel class; the thread tells you nothing about that.
• Threaded adapters over loose clamps where alignment matters, then re-check focus after the first thermal cycle of the line.

When the usual answer is wrong

Very small sensors with very wide fields invert the ladder. Board-level sensors sit on CS-mount's 12.526 mm flange or bare M12 lenses, and the M16 board-camera adapter guide covers that end of the scale.

No TFL lens at your focal length is the more common gap, since the M35x0.75 group is small today. Long-working-distance work on APS-C often lands in the large-format M58 and M72 groups instead — 34 and 19 models, with image circles from 45 to 60 mm that swallow an APS-C diagonal several times over. The LS-FFL-1676 is a 60 mm, 65 MP lens on M58 with a 214-1536 mm working distance.

The large-format M58 and M72 guide works through that class in detail, and the catalog's large-format fixed-focal-length series (LS-FFL-S105) holds 40 of these models. M58 and M72 flange distances vary between designs, so match the adapter to the printed flange distance of the specific lens and camera pair.

Where to start

The decision in one line: C-mount from 2/3 inch through 1.1 inch, TFL for 4/3-type and APS-C, F-mount or M72 at full frame, M58 and M72 above that. Start from the camera's sensor diagonal and your field of view, then let the machine vision lenses category, the lens finder and the lens configurator narrow the list by mount and image circle. If the choice is still open after that, ask the applications team with the sensor part number, and browse the rest of the series on the lenses topic page.