Deciding whether a lens fits your camera: image circle, sensor diagonal and mount

In short

Work in millimetres, not in format labels: 2/3″ or 1.1″ names a size class, not a measurement. Take the active sensor diagonal, calculated from pixel count and pixel pitch, and subtract it from the image circle the lens states. Our engineering core passes the pair when that margin is zero or positive. A negative margin puts the frame corners outside the circle, so expect vignetting, light falling away toward the frame edges, and soft corners. A positive margin is nominal geometry with the centres aligned; corner quality comes from the lens performance data. Check the mount separately: type, thread pitch or bayonet, the reference surfaces in both drawings, the distance from them to the sensor plane, adapter thickness and rear-element clearance.

Why the format label does not settle the question

No arithmetic on the fraction in a format label returns a length, so our sensor reference publishes two numbers per sensor: the class diagonal stored for the label, and the active sensor area, the width and height the output pixels occupy, calculated from resolution and pixel pitch, whose diagonal is the hypotenuse of those two sides. Use the active figures, and the class diagonal only when resolution and pitch are not at hand.

The two right-hand columns need not agree, and the 1.1″ class holds two different frames: IMX304 at 17.516087 mm of active diagonal and GMAX0505 at 18.101934 mm (calculated), both stored against the same 17.6 mm class figure. Compare the active diagonal with the image circle of the lens you are actually choosing, because a lens chosen on the class label alone is chosen against the wrong number.

The coverage check and what it settles

The check is one subtraction: the coverage margin is the stated image circle diameter minus the active sensor diagonal. Our engineering core passes the pair when that margin is zero or positive, exact equality included, and fails it with a vignetting warning once it goes negative. Basler and Edmund Optics give the optical reason: a circle failing to illuminate the sensor diagonal leaves edge shading and reduced corner sharpness (vendor documentation).

Both figures are diameters, so halve the margin for the spare circle at a corner: 0.2 mm of margin is 0.1 mm radially. That radial figure is not a permitted decentring; how far the optical axis may sit off the sensor centre comes from the camera and lens documentation.

A negative margin is firm on one question and silent on the next: the corners lie outside the circle the lens is specified for, so the pair fails, while no percentage of darkened area, usable crop or corner contrast follows from a diameter. A positive margin is bounded in the same way, being nominal geometry with circle and frame sharing an axis and their centres aligned. It confirms neither the tolerance on the stated diameter nor the alignment of the assembled pair, and relative illumination, the brightness at a field position compared with the brightest point of the image, is read from the lens document or from an image of your own part.

What a larger image circle changes and what it does not

Field of view comes from the sensor axis, the focal length and the working distance, and the image circle is not a term in that relation; the camera and lens calculator carries the derivation. Swapping in a lens built for a larger format therefore leaves the modelled field the same size, provided the actual focal length is the same and the optical working distance, measured from the lens principal plane, is unchanged; the same camera and the same mechanical distance do not by themselves secure either condition.

The extra circle buys room for the tolerance on the stated diameter and the option of moving the same optical family onto a larger sensor later. It does not buy corner quality, which the lens document reports separately as relative illumination, distortion and the modulation transfer function, the contrast a lens passes at a given level of detail and field position. The chief-ray angle, at which the ray through the centre of the aperture stop reaches the sensor, belongs there too and governs how the pixel microlenses collect light.

In our records, larger circles arrive with other interfaces: four C-mount lenses of one 25 mm focal length state Ø9 mm, Φ11.2 mm, Ø17.6 mm and Ø23 mm, while LF2528M-F states Φ43.2 mm with an F-mount and MVL-LF3040M-005-M72 states Φ58 mm with M72 × 0.75 (datasheet). Read that as what those products state rather than a ceiling built into the C thread, since manufacturers publish C-mount designs whose stated circle reaches 24 mm, past the 22.5 mm our table holds for the 4/3″ class (vendor documentation).

The mount is the second check, and it has an order

Coverage says nothing mechanical, and the mechanics have an order: mount type, then thread pitch or bayonet designation, then the reference surfaces the two drawings name, then the distance from those surfaces to the sensor plane, and last adapter thickness and rear-element clearance, the space the rear of the lens needs behind the mount.

Flange focal distance is the distance from a defined mount seating surface to the image plane, and it decides whether a lens reaches focus at all. C-mount is a 1″-32 UN thread of 25.4 mm nominal diameter at 17.526 mm, CS-mount the same thread at 12.526 mm (vendor documentation), so a 5 mm adapter lets a C lens work on a CS camera while the reverse pairing has no 5 mm to give back. F-mount is a bayonet at 46.5 mm (vendor documentation), a figure of that bayonet and not of threaded M mounts.

M42, M58 and M72 name a diameter and nothing more until the pitch stands beside it, which is why the records read M42 × 1.0, M58 × 0.75 and M72 × 0.75. The seating distance is published per product, and the records do not even use one name for it.

Those three values are datasheet fields of those three cameras. Back focal length is conventionally measured from the last optical surface to the image plane, a flange figure from a mount seating surface, so the reference surfaces come from the product drawing; read it before comparing one of these numbers with another or subtracting an adapter thickness. A family table listing several flange values across several mounts assigns none of them to a particular variant.

Worked example: the same 25 mm focal length in both directions

One focal length against two camera frames gives both directions. MV-CH120-10UM carries a Sony IMX304, 4096 × 3000 pixels at 3.45 µm in the 1.1″ class, and MV-CA050-12UM a Sony IMX264, 2448 × 2048 pixels at the same pitch in the 2/3″ class with a C-mount (datasheet); their active diagonals are 17.516087 mm and 11.011397 mm, the 2/3″ frame measuring 8.4456 mm across (calculated). The lenses are three C-mount records of 25 mm focal length stating Φ11.2 mm marked 2/3″, Ø17.6 mm and Ø23 mm (datasheet).

Every margin there is calculated from the datasheet circle and the calculated diagonal. The first row is the failing direction: the 2/3″ lens falls 6.316087 mm short of the 1.1″ frame, the core warns of vignetting, and how large the dark region is comes from an image of your own part. The second row passes by 0.188603 mm of diameter, which is 0.094302 mm of spare circle at a corner (calculated); arithmetic stops there, and the tolerance on the stated circle and the relative illumination at the corner of this format come from the lens document.

The field does not follow the circle. At an optical working distance of 500 mm, an assumption of this example measured from the lens principal plane rather than from the camera housing, the thin-lens relation puts 160.4664 mm of object width across the 8.4456 mm sensor width at 25 mm (calculated). The model returns that figure for all three covering lenses, because its only inputs are sensor width, focal length and distance. Three real lenses will not match it to the fourth decimal, since actual focal length carries a tolerance, each design places its principal plane differently and distortion varies by model. What survives is the structural point, that the image circle is not an input to the field of view.

Calculate for your case

Open the sensor reference for the sensor your camera uses: it gives the active width, height and diagonal beside the class figure. Take the image circle from the lens record, subtract that diagonal and read the sign, halving the result for the spare circle at a corner.

Use the camera and lens calculator for what coverage never touches: enter your camera, the field you must see on a named axis and your working distance, and it returns the focal length and the field a standard lens gives there. Neither page takes an image circle or a mount, so that comparison stays with the records of the exact pair.

Equipment that fits this example

MVL-MF2528M-10MPE, MVL-KF2528M-12MP and SA2520M-10MP share the 25 mm focal length and the C-mount thread and differ in stated circle, and the same focal length exists at Ø9 mm as MVL-HF2528M-6MPE in the 1/1.8″ family. Aperture, minimum object distance and corner performance differ inside each family, so send us the part number and we confirm the variant against its record.

Common mistakes

• The inch fraction is read as a length, or two matching labels are taken as proof, and the undersized pair announces itself only as dark, soft corners in the first frames. Compare the stated circle with the active diagonal in millimetres before ordering, since the same 1.1″ class covers 17.516087 mm and 18.101934 mm here (calculated).
• A larger image circle is expected to widen the field, so the lens is swapped when the focal length or the distance is what has to change. At the same actual focal length of 25 mm and the same optical working distance of 500 mm, the thin-lens model returns the same 160.4664 mm across an 8.4456 mm sensor width whatever the stated circle (calculated).
• M42, M58 and M72 are treated as one interface, or a seating figure is copied from another camera. The pitch is part of the designation, and the 12 mm, 11.48 mm and 19.55 mm above belong to those three cameras and to their own field names.

When this rule breaks

The margin is a geometric screen rather than a forecast of the picture. A pair passing by a hundredth of a millimetre and one passing by ten millimetres get the same verdict from the subtraction, and the size of the margin does not promise that the two will differ on the part: the tolerance on the stated circle, the alignment of the assembled pair, relative illumination and contrast transfer are settled by the documentation of the concrete pair and by an image of your own part. The model assumes aligned centres as well, so anything that moves the sensor off the optical axis falls outside its scope.

Passing coverage proves nothing mechanical either: thread pitch, flange distance, adapter thickness and rear-element clearance stay open, and only the drawings of the specific camera and lens settle them.

Next step

Send us the camera model, or its resolution and pixel pitch, together with the lens part number you are considering. We compare the stated image circle with the active diagonal, check the mount, the pitch and the seating dimensions on both records, and tell you plainly whether the pair fits as it is, fits with an adapter, or does not fit.

Related

• Choosing a lens when the working distance is fixed

How this material was prepared

Prepared with MVisionPro AI agents from stated sources and the calculation core; MVisionPro retains editorial responsibility. A physical test or human engineering review is claimed only when explicitly stated. Read the editorial method.

Editorial status: verified. Content updated 2026-09-21.