The industrial camera was fitted with a different lens; the mount screws on, and the focus is working properly.

As soon as the result is displayed:

It’s fine in the middle, but the four corners are black.

This situation is quite common in machine vision projects.

The problem is often not that the camera is broken, but rather:

The lens and the camera’s sensor size are not properly matched.

Let’s first take a look at the common C-mount lenses for industrial cameras:

Just because it can be installed doesn’t mean it can “fit properly.”

Mounting the lens onto the camera simply indicates:

Interface matching.

But the lens must also meet another condition:

The lens’s image circle must cover the camera’s sensor.

The effective image captured by the lens is actually a “circle”; this circle is…Image circleImage Circle.

The camera’s sensor, on the other hand, is rectangular.

As long as the image circle fully encompasses the sensor, the image will be complete. Cognex’s official documentation likewise states that the lens’s image circle must at least cover the corresponding image sensor; otherwise, vignetting will occur.

The diagram is easy to understand: the circle represents the lens’s image circle, and the rectangle inside it is the camera sensor.

When the lens is too small, this kind of image appears.

For example:

The camera uses…2/3" Sensor

But you’ve mounted a lens that’s only suitable for a smaller sensor.

The image circle produced by the lens does not fully cover the sensor; as a result, the four corners of the sensor receive no usable image, leading to this outcome:

This is quite typical:

Mechanical vignetting /Vignetting.

In real-world cases reported by Cognex, when 2/3-inch sensors are paired with M12 or S-mount lenses that have a smaller image circle, similar vignetting at the four corners can occur.

So in the future, when choosing a lens, don’t just look at:

Can it be installed in a USB-C port?

You also need to check the lens parameters:

Maximum supported target plane / Sensor Format / Image Circle.

Sometimes it’s not completely dark—just dimmer around the edges.

Another situation is:

The four corners aren’t completely black, but they’re noticeably darker than the center.

This isn’t necessarily a serious size mismatch; it could also be due to the lens itself.Edge illuminance decreases.

The following diagram is quite intuitive:

Relative brightness may gradually decrease from the center of the image to the edges, and its performance can vary across different apertures. According to Edmund Optics, sensor size, aperture settings, and lens design all influence edge‑related illumination and vignetting characteristics.

So you might find:

It’s very bright in the center and gets darker as you move toward the edges.

This is not quite the same as the earlier case where “the image circle simply doesn’t cover the entire frame.”

How can it be avoided?

It’s actually very simple.

When choosing a lens, pay attention to one more specification:

The lens supports a maximum image circle that is equal to or larger than the camera sensor’s image circle.

For example:

The camera is2/3"

Then the lens must support at least a **2/3"** sensor.

If the camera is1"

Then don’t just support…2/3"To match the shot.

To be on the safe side, you can simply refer to the specifications provided by the lens manufacturer.Image Circle (Diameter of the Image Circle)and the actual diagonal size of the camera sensor.

If you encounter a black corner on site, first check these four areas.

① What is the size of the camera sensor?

First, confirm whether it’s 1/2", 2/3", 1", or larger.

② What is the maximum sensor size supported by the lens?

Don’t just look at the focal length.

③ Have you attached an adapter ring, filter, or protective cap?

The attachment may also block edge light.

④ Stop down the aperture a bit and take another look.

If only the darkening at the edges has improved, it may still be related to lens vignetting and edge illumination, rather than solely to the size of the image circle.

Finally, remember this one sentence.

The fact that the lens can be mounted simply means the mount is compatible.

Whether a lens can be used effectively also depends on whether its image circle can cover the sensor.

If the corners of the screen are noticeably dark:

First, check the “camera’s image sensor size” and the “lens’s supported image sensor size.”

Often, that’s exactly where the problem lies.