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

In short

Format labels such as 2/3″ are size classes, not measurements, so compare millimetres. Calculate the active sensor diagonal from pixel count and pitch, and subtract it from the lens image circle. A negative margin fails: the corners lie outside the stated image circle, so expect vignetting or reduced corner sharpness. Zero or more passes, assuming aligned centres, but corner quality still comes from the lens data. Check the mount separately: type, thread pitch or bayonet, reference surfaces, distance to the sensor, adapter and rear clearance.

Why the format label is not enough

A format label is a historical size class, and no arithmetic on the fraction gives millimetres. So our sensor reference gives two numbers per sensor. One is the class diagonal stored for the label. The other is the active area, the width and height the output pixels cover, calculated from resolution and pixel pitch. Its diagonal is the hypotenuse of those two sides. Use the class diagonal only when you lack resolution and pitch.

The two right-hand columns need not agree. The 1.1″ class holds two frames here, 17.52 mm and 18.10 mm, both filed under 17.6 mm.

The coverage check and what it tells you

The image circle is the round area a lens is specified to project at its intended quality. The sensor's corners sit farthest from the centre, so its diagonal must fit inside.

The coverage margin is the stated image circle diameter minus the active sensor diagonal. Our calculator passes the pair at zero or more. Below zero, it fails the pair and warns of possible vignetting, or reduced illumination toward the image edges. The corners then lie outside the stated circle and can show shading or lose sharpness (vendor documentation).

Both numbers are diameters, so halve the margin for the spare circle at each corner. That half is not an allowed off-centre shift. The allowed shift comes from the camera and lens documentation.

A negative margin is a firm fail. It does not say how much of the frame darkens, what crop is usable or what corner contrast remains. Only an image of your own part shows that.

What a larger image circle does and does not change

Field of view depends on the sensor size along one axis, the focal length and the working distance. The image circle is not one of them (derivation in the camera and lens calculator). So a larger-format lens gives the same modelled field on your camera, at the same actual focal length and optical working distance. That distance is measured from the lens principal plane. Keeping the same camera at the same mechanical distance guarantees neither.

The extra circle buys room for the tolerance on the stated diameter. It also lets the same lens family move to a larger sensor later. It does not buy corner quality, which the lens document reports separately. Relative illumination is the brightness at a field point compared with the brightest point, usually the centre. MTF, the modulation transfer function, is the contrast the lens passes at a given detail size and field position. Distortion is listed too. So is the chief-ray angle, the angle at which the ray through the aperture-stop centre meets the sensor. It governs how well the pixel microlenses collect light.

In our records, the larger circles come with other mounts. LF2528M-F states Φ43.2 mm on an F-mount, and MVL-LF3040M-005-M72 Φ58 mm on M72 × 0.75 (datasheet). The C-mount lenses in this article stop at Ø23 mm, but that is not a limit of the C thread. Other manufacturers publish C-mount lenses with a stated 24 mm circle (vendor documentation). That is past the 22.5 mm our format table gives for 4/3″.

The mount is a second, separate check

A lens can cover the sensor and still not fit or focus. Check the mechanics in this order:

1. Check the mount type.
2. Match the thread pitch or the bayonet designation.
3. Find the reference surfaces named in both drawings.
4. Compare the distance from those surfaces to the sensor plane.
5. Check adapter thickness and rear-element clearance, the space the back of the lens needs.

Flange focal distance runs from a defined seating surface of the mount to the image plane. It decides whether the lens can reach focus at all. C-mount uses a 1″-32 UN thread of 25.4 mm nominal diameter and a flange focal distance of 17.526 mm. CS-mount uses the same thread with a 12.526 mm flange focal distance (vendor documentation). A 5 mm spacer allows a C-mount lens on a CS-mount camera. A CS-mount lens cannot reach its designed flange distance on a C-mount camera with a simple spacer. It would sit 5 mm too far from the sensor, and a spacer only adds distance.

F-mount is a bayonet with a 46.5 mm flange focal distance (vendor documentation). That figure does not apply to threaded M mounts. M42, M58 and M72 name only a diameter, and the pitch completes the name. The seating distance is published per product, and the records do not even agree on its name.

Back focal length usually runs from the last optical surface to the image plane, not from a seating surface. So take the reference surfaces from the product drawing before comparing numbers or subtracting an adapter. A family table listing several flange values for several mounts assigns none of them to a particular variant.

Worked example: one 25 mm focal length, both directions

MV-CH120-10UM carries a Sony IMX304: 4096 × 3000 pixels at 3.45 µm, 1.1″ class. MV-CA050-12UM carries a Sony IMX264: 2448 × 2048 pixels at the same pitch, 2/3″ class, C-mount (datasheet). The 2/3″ frame is 8.45 mm wide (calculated). All three lenses are 25 mm C-mount lenses (datasheet).

The first row fails: the 2/3″ lens falls 6.32 mm short of the 1.1″ frame.

The second row passes. The calculated margin is about 0.19 mm in diameter, or 0.094 mm radially at each corner, using unrounded geometry. Circle tolerance and relative illumination at the corner come from the lens document.

Assume an optical working distance of 500 mm, from the principal plane (assumption of this example). The thin-lens model then puts 160.5 mm of object width across the 8.45 mm sensor width at 25 mm (calculated). It gives that figure for all three covering lenses, since the circle is not an input. Real lenses will not match it exactly: focal length has a tolerance, principal planes sit differently, and distortion varies by model.

Calculate for your case

Open the sensor reference for your camera's sensor and take the active diagonal. Subtract it from the image circle in the lens record and read the sign.

Use the camera and lens calculator for the rest. Enter your camera, the field you must see on a named axis and your working distance. It returns the focal length and the field a standard lens gives there. Neither page takes an image circle or a mount, so compare those by hand from the exact pair's records.

Equipment that fits this example

Besides the three lenses in the example, MVL-HF2528M-6MPE in the 1/1.8″ family is a 25 mm C-mount lens with a Ø9 mm circle (datasheet). Aperture, minimum object distance and corner performance differ within each family. Send us the part number, and we will confirm the variant against its record.

Common mistakes

• The inch fraction is read as a length, or matching labels are taken as proof. The undersized pair then shows up as dark or soft corners in the first frames. Compare the stated circle with the active diagonal in millimetres before you order.
• A larger image circle is expected to widen the field, so the lens is swapped. In the example, all three covering lenses give the same modelled 160.5 mm (calculated). To change the field, change the focal length or the working distance.
• M42, M58 and M72 are treated as one interface, or a seating figure is copied from another camera. The pitch is part of the name, and each seating figure above belongs to its own camera. Take pitch and seating distance from the drawings of your camera and lens.

When this rule breaks

The margin is a quick geometric check, not a forecast of the picture. A pair passing by 0.01 mm gets the same verdict as one passing by 10 mm. The check also assumes aligned centres, so anything that shifts the sensor off the optical axis falls outside it. Circle tolerance, alignment of the assembled pair, relative illumination and contrast transfer need the pair's documents and an image of your part.

Next step

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

Related

• How to calculate lens focal length and field of view when the working distance is fixed
• Deciding whether a measurement needs a telecentric lens, and choosing one

How this material was prepared

Prepared by MVisionPro from the stated sources and the MVisionPro calculation engine. A physical test is claimed only when the material says so explicitly. Read the editorial method.

Editorial status: verified. Content updated 2026-09-23.