Anyone working in machine vision has probably encountered this situation: you’ve chosen the right lens and adjusted the magnification, but you still can’t capture the image you want. The problem is likely not with the lens itself, but rather that you’ve overlooked a key component—the lens barrel.

1. What is a ‘mirror tube’?

A lens extension tube is, essentially, a set of ring-shaped attachments fitted between the camera and the lens.

Its core function is straightforward: to push the lens forwards, placing it further away from the camera’s sensor. Do not underestimate this displacement—ranging from a few millimetres to several tens of millimetres—as it can tangibly alter your focal length and working distance.

Extension tubes typically consist of multiple ring-shaped components of varying lengths, which users can use individually or stack together according to their specific needs, offering a high degree of flexibility. In ZOOM LENS systems, extension tubes are not merely ‘spacers’; they are a crucial component for adjusting optical parameters.

II. The Working Principle of the Mirror Tube

The principle behind a lens barrel is not complicated, but to understand it, we need to return to a fundamental concept of optics: the focal length.

When a lens forms an image, the light rays pass through the lens elements and must converge on the sensor to produce a sharp image. The distance from this point of convergence to the rear-most lens element is the focal length.

The function of the lens barrel is simply to artificially extend the focal length.

To be more specific:

Once the extension tube is inserted, the physical distance between the lens and the sensor increases, resulting in a longer image distance. According to the lens imaging formula, an increase in image distance → an increase in effective focal length → an increase in magnification. At the same time, as the lens is pushed further away, the working distance (WD) also increases; in other words, the distance from the front of the lens to the object being observed becomes greater.

To summarise in a single sentence: the lens barrel trades space for magnification and distance for sharpness.

This is also why, in ZOOM LENS systems, the lens barrel and zoom ring often need to be used in conjunction—the zoom ring adjusts the magnification, whilst the lens barrel adjusts the working distance; together, they determine the final imaging result.

III. Applications of Mirror Tubes in Visual Inspection

Scenario 1: Inspection of large-sized workpieces

When inspecting large-sized products such as PCBs, display screens and metal sheets, placing the lens too close to the object can easily result in obstruction or interference. By adding a lens tube, the working distance is increased, allowing the lens to capture images from a safer position and avoiding the risk of collision.

Scenario 2: High Magnification Inspection Requirements

Some defect inspections require very high magnification, but the native magnification range of ZOOM LENSES is limited. By increasing the focal length with a lens tube, the system’s magnification can be further enhanced without changing the lens, thereby reducing hardware costs.

Scenario 3: Side-by-Side Multi-Camera Layout

On production lines where multiple cameras perform simultaneous inspection, the installation space between cameras is limited. The extension tube allows for flexible adjustment of the working distance for each camera, enabling multiple lenses to find their optimal focus positions within a compact space and avoid mutual interference.

Scenario 4: Use with Telecentric Lenses

In precision measurement scenarios, telecentric lenses are highly sensitive to object distance. The extension tube serves as a fine-tuning tool, allowing for precise adjustment of the object distance without changing the lens, thereby ensuring measurement accuracy.