At industrial inspection sites, you’ve surely encountered situations like these: PCB boards being inspected with their sides standing upright, lithium battery electrodes observed at an angle to measure coating thickness, or large-sized workpieces whose sloped surfaces need defect detection... As soon as the camera is tilted, the image immediately becomes “half-bright, half-dark”—the near end appears crisp and sharp, while the far end turns completely blurry, no matter how much you turn the focus ring, it’s all in vain.

This is a classic in the field of machine vision.Depth-of-field dilemmaWhen the object surface is no longer perpendicular to the optical axis, the parallel focal plane of a conventional lens can no longer cover the entire measurement area. Many engineers’ first instinct is to narrow the aperture; however, if the aperture is narrowed too much, not only will the light throughput drop sharply and the image signal-to-noise ratio deteriorate, but diffraction effects will also be introduced, leading to a decline in resolution. In the end, this often leaves engineers in the awkward situation of having an image that’s “sharp yet not entirely sharp.”

The solution to this problem, rooted in the very foundations of geometrical optics, is a special optical design based on a law established a century ago—Scheimpflug Lens.

I. Sham’s Law: The Clear Principle of a Tilted World

The story begins in 1889, when Theodor Scheimpflug, an Austrian army officer, discovered an counterintuitive optical law while conducting aerial photogrammetric research:

When the extension lines of the object plane, the lens principal plane, and the image plane intersect at the same straight line, all points on the tilted object surface can be brought into sharp focus simultaneously.

This is the famous one.Scheimpflug Principle.

In a conventional orthogonal imaging system, the object plane, lens, and sensor are all parallel to each other, and the focal plane is also parallel to the sensor. Once the object plane tilts, the distances from object points at different positions to the lens will vary: points near the front end will be closer to the focal plane, while points at the far end will be farther from it. Naturally, only the line in the middle will remain in sharp focus.

The core breakthrough of the Sham lens lies in...Allow the lens optical axis to be tilted relative to the sensor plane.Through a precision angle-adjusting mechanism, the focal plane is no longer “lying flat” but instead tilts along with the surface of the object being measured, ultimately achieving a situation where the three planes converge into a single line—ensuring that the entire inclined surface is precisely focused within the focal plane.

To put it in a simple analogy: A conventional lens has a depth of field that’s like a stack of pancakes laid flat—allowing it to capture only objects parallel to the surface of the pancakes. In contrast, a Scheimpflug lens slices through that stack of pancakes at an angle, so that the cut surface precisely matches the inclined surface being measured, ensuring that the entire cut surface is sharply and clearly in focus.

II. It’s not “increasing depth of field,” but rather “rotating the focal plane.”

Here, it’s essential to clarify one of the most common misconceptions:The Sharm lens doesn't increase the depth of field; it merely alters the spatial orientation of the depth of field..

The physical formula for depth of field still holds true for Scheimpflug lenses: once you move a certain distance away from the tilted object plane, the image will gradually become blurred as well. The key difference is that while the depth of field of a conventional lens forms a “cylindrical” region along the optical axis, the depth of field of a Scheimpflug lens takes the form of an oblique “wedge-shaped” volume—where the depth of field is slightly greater at the far end and slightly shallower at the near end. Yet, the entire inclined surface being measured happens to fall precisely within this wedge-shaped sharpness zone.

This characteristic determines the irreplaceable nature of the Sham lens:

• Clear full-field view of inclined surfacesFrom proximal to distal, the resolution is uniform and consistent; there is no “half-baked” effect.

• Even with a large aperture, you can achieve a deep depth of field.No need to sacrifice light throughput for depth of field—still suitable for low-light environments and high-speed line-scan applications.

• Avoid worsening perspective distortion.The Sham lens, designed with a telecentric optical path, simultaneously addresses both resolution and magnification consistency.

III. Four Core Application Scenarios: Tackling Pain Points in Industrial Inspection

1. Electronics Manufacturing: PCB and Semiconductor Side Inspection

In PCBA AOI inspection, parameters such as the coating thickness of conformal coatings, the bow height of wire bonding, and pin coplanarity all require observation from an oblique angle. When using a standard lens for oblique imaging, only the central line of the field of view remains sharp, while the edge pads appear blurry and distorted, making it extremely easy to miss defects or trigger false alarms.

The shift-and-tilt lens for the Sham camera enables angle adjustment within a range of 0° to 45°, ensuring that the entire side edge of the PCB remains in the focal plane. As a result, details such as gold wire patterns, adhesive layer boundaries, and pin solder joints are all rendered sharply, significantly improving detection accuracy and yield.

2. 3D-line scanning and laser triangulation

3The D-line scanning camera is based on the laser triangulation principle. The camera and laser are arranged at a fixed angle, causing the imaging plane to be naturally tilted. If a conventional lens is used, only the central region of the laser line will be in sharp focus, severely limiting the measurement range along the Z-axis.

After introducing the Shack-Hartmann imaging design, the lens’s focal plane precisely aligns with the laser plane, ensuring that the entire laser line remains sharply focused across the sensor’s entire field of view. As a result, the effective measurement depth range can be increased several times over. This is also why Shack-Hartmann optics have become nearly standard equipment for high-end 3D line-scan cameras.

3. Lithium Batteries and Coated Materials: Inspection of Wide-Format Oblique-Coating Processes

Production lines for lithium battery electrode coating, separator coating, and printed films often require cameras to be mounted at an angle due to limitations in installation space or the curvature of rollers. At meter-scale widths, ordinary lenses suffer from a severe degradation in image sharpness at both ends.

When paired with a large-format industrial camera, the Sham lens ensures consistent imaging across the entire roll of material—from left to right—under conditions of long working distances and wide fields of view. It leaves no defects—including thickness variations, missing coatings, and pinholes—unnoticed.

4. Structured Light and Stereo Vision

In structured-light 3D reconstruction systems, both the projector and the camera have tilt angles. If the projection grating is out of focus, it will directly lead to phase calculation errors. The Sham lens can perform focal-plane tilt correction separately at both the camera end and the projector end, ensuring that the projected pattern maintains high contrast throughout its entire trajectory on the surface of the object being measured, thereby simultaneously enhancing reconstruction accuracy and extending the effective depth range.

IV. Selection and Deployment: Three Realistic Issues That Cannot Be Overlooked

Although the Sham lens is powerful, it’s not a one-size-fits-all solution that simply “works as soon as you plug it in.” In actual engineering implementation, there are three key issues that must be considered in advance:

1. Space and Interface Constraints

The standard C-mount flange distance is only 17.526 mm, leaving very limited space for the tilt mechanism. Traditionally, Shim lenses typically offer an adjustable angle of only ±5°, making it necessary to resort to custom designs for scenes requiring larger angles. Currently, the mainstream solution is to add a Shim adapter component between the camera and the lens, thereby balancing both versatility and adjustment range.

2. Distortion and Measurement Accuracy

Tilting the lens introduces additional perspective distortion, causing variations in optical magnification across different parts of the image. For simple defect detection, this effect is still acceptable; however, if the system is used for precise dimensional measurements, it must be paired with a telecentric optical path design and undergo specialized calibration and correction. Otherwise, measurement errors will increase significantly.

3. Adjustment accuracy and locking reliability

The precision of fine-tuning the Sham angle directly determines the imaging quality; even a tiny deviation can lead to edge blurring. In industrial environments subject to vibration, the reliability of the adjustment mechanism's locking function is crucial. High-quality Sham lenses typically come equipped with scale markings and locking devices, enabling quantitative adjustment of the angle and ensuring its stable maintenance.

V. Conclusion: From “Insufficient Depth of Field” to “Focal Plane Alignment”

Looking back at the longstanding depth-of-field dilemma in machine vision, our habitual thinking often remains fixated on “how to increase depth of field”—by using larger aperture numbers, shorter focal lengths, and greater working distances... Yet the Sham lens offers an entirely different approach:

Rather than pursuing a wide depth of field that covers everything, we focus on ensuring that the limited depth of field is precisely centered on the plane where it’s needed.

This is both the elegance of geometrical optics and a testament to the wisdom of engineering thinking—when faced with a problem, it’s not always necessary to simply throw more resources at it head-on. Instead, by shifting our perspective and precisely aligning the system with the requirements, we can often achieve better results at a lower cost.

From 19th-century aerial photogrammetry to today’s Industry 4.0 production lines, the century-old optical law known as the Scheimpflug principle continues to thrive in the new frontier of machine vision. Next time you encounter the challenge of blurry images taken at an angle, consider this: Perhaps it’s not a matter of insufficient depth of field—maybe the focal plane just hasn’t been aligned correctly yet.