The Technical Design of a Lens
What Types of Lenses Are There?
A distinction is generally made between entocentric and telecentric lenses:
Entocentric Lenses
Entocentric lenses are among the most commonly used lens types in industrial image processing. Both fixed focal lengths and zoom lenses are based on this optical principle, which is similar to the natural perception of the human eye.
With an entocentric normal lens, the aperture angle remains constant regardless of whether it is aligned with the object or the sensor. In contrast, the aperture angle can vary with entocentric zoom lenses. Normal lenses are lenses with an aperture angle of approximately 50 degrees equivalent to that of the human eye.
Characteristics
With entocentric lenses, objects closer to the camera appear larger, while objects farther away appear smaller. This makes it easy to adjust the image section even with fixed focal lengths. The further away the object is, the larger the image section, the closer the object is, the smaller it becomes.
The working distance can be reduced to the manufacturer’s specified minimum object distance (MOD). By inserting spacer rings, this distance can be further reduced, but this can cause optical image errors. This effect is particularly challenging with wide-angle lenses. While the object in the center of the image is captured vertically, the viewing angle shifts increasingly towards the edges of the image. This effect can be reduced by closing the aperture and increasing the depth of focus.
Applications with Entocentric Lenses
Telecentric Lenses
Telecentric lenses are primarily used in precise measurement applications, especially where the object position within the field of view changes in the X and Y directions, yet distortion-free results are required.
With these optics, the aperture angle is almost zero degrees in a defined range. As a result, the light beams run almost parallel to the optical axis, which prevents perspective distortions. This keeps the image constant across the entire visual field, regardless of whether the object is closer to the camera or further away. Telecentric lenses are particularly suitable for precise length, width and position measurements where a constant imaging scale is crucial.
Unlike entocentric lenses, telecentric systems focus vertically on the object not only in the image center, but also at the edges of the image.
Applications of Telecentric Lenses
Telecentric optics are used in particular for measuring or inspection tasks:
Comparison of structures on different object levels
For example, when measuring holes on the top and bottom of a workpiece, the telecentric optics ensure a uniform imaging scale and enable reliable comparison measurements.Variable object distances with inaccurate feeding
If the distance between the lens and the object cannot be reproduced exactly, for example due to tolerances in the feed or positioning, the image remains true to scale with a telecentric optics.Inspection and measurement of holes and inner contours
With telecentric optics, holes or recesses can be represented without perspective distortion and without showing sloping walls. Contours remain precise, which is particularly critical for precise position measurements, diameter checks or quality controls.
Flat lights are recommended for optimal illumination of applications like these. They ensure homogeneous, shadow-free illumination.
Direct Comparison between Entocentric and Telecentric Lenses
| Entocentric lens | Telecentric lens | |
|---|---|---|
| Aperture angle | approx. 50 °C | Practically 0 °C (parallel) |
| Perspective distortion | Yes | No |
| Image scale | Variable at depth change | Constant |
| Measuring accuracy | Limited | Very high |
| Representation of side walls | Possible (e.g. with fisheye) | Not possible |
| Typical application | Visual inspection, general applications | Accurate measurement technology |
The Key Selection Criteria for the Right Lens
Working Distance
Object Size
Sensor Size
The sensor size specified by the camera has a direct impact on the image size and is decisive for choosing the appropriate optics. Larger sensors usually require higher quality and larger lenses to illuminate the entire sensor surface and ensure high image quality.
Distance from the Optics to the Sensor
The distance between the optics and the sensor is defined by the camera manufacturer and the connection thread used. There are different contact dimensions. The distance can also be adjusted by using spacer rings and focal length doublers.
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Further Decision Criteria
High-quality optics must be matched to the camera, its image sensor and the relevant purpose of use. Depending on the application, such as measurements, color tests, inspections with infrared radiation, robotic applications or presence checks, different requirements for optical quality must be taken into account.
The most important setting parameters and decision criteria when selecting a lens are:
Sensor and Pixel Size Resolution
For optimal image quality, the alignment between sensor and lens is crucial. The lens should have a high optical quality to accurately capture details on the image sensor. The pixel size plays a central role here: The smaller the pixels, the higher the resolution and imaging power of the lens must be to achieve the full sensor performance.
Modern manufacturing technologies enable increasingly compact sensors with finer pixel structures. This trend towards smaller sensors and pixels increases integration density and enables more efficient material usage. At the same time, advances in light sensitivity and noise reduction improve image quality even with small pixels.
The following always applies: Larger sensors with larger pixels often provide higher light output and better image quality, but require more space and are usually associated with higher costs.
Impact of Pixel Size and Lens Resolution on Image Quality
The associated graphic shows the relationship between the pixel size of a camera sensor and the optical resolution of the lens used. In both examples, the sensor size remains constant while the pixel size decreases and the overall resolution increases accordingly.
For sensors with low resolution, i.e. larger pixels, there are usually no significant differences in the image result between different lenses. However, as the pixel size decreases, the requirements for the imaging performance of the lens increase. In this case, the optical quality of the lens can make a significant contribution to image sharpness and detail.
If the sensor resolution exceeds the optical performance of the lens, the image quality can be visibly impaired. If, on the other hand, the pixel density is within the cutoff frequency that can be transmitted by the lens, the sensor can be used efficiently.
The overall image quality of a camera system is therefore not determined solely by the sensor, but by the interaction of all optical and electronic components. A coordinated system design is therefore crucial for the reliable capture of fine structures in challenging image processing applications.
Image Circle Diameter
Lenses with the same connection standard, such as C mount, may differ in image circle diameter. The image circle refers to the area that a lens illuminates evenly without any significant edge shadowing or brightness drop.
For complete and homogeneous illumination of the sensor surface, the image circle diameter of the lens must correspond to at least the sensor size.
Lenses with larger image circle diameters can also be used. They change the image angle and are typically associated with larger dimensions and higher costs without improving the image quality, while maintaining the same sensor size.
Aperture Setting
The aperture is the variable aperture in the lens through which light reaches the sensor. Their size is controlled by an arrangement of overlapping slats and the function corresponds to the pupil of the human eye. A smaller aperture (higher number of apertures) reduces the amount of light, extends the exposure time and increases the depth of focus. A larger aperture (lower number of apertures) allows more light and shorter exposure times, but reduces depth of focus.
The aperture directly influences:
the camera’s exposure time
the depth of focus in the image
Depth of Focus
Depth of focus refers to the area along the optical axis where objects in the image are sufficiently sharp. It depends on the aperture, focal length and image scale. For inspection and measurement tasks, it is crucial that all relevant features are within this range. In fixed camera systems, the depth of focus is primarily controlled via the aperture: A smaller aperture (higher number of apertures) increases the depth of focus, but reduces the amount of light and requires longer exposure times. An image is considered sharp as long as the blur remains smaller than the pixel size of the sensor.
Minimum Object Distance (MOD)
MOD refers to the shortest distance between the front of a lens and the object where sharp focusing is still possible. It is determined by the optical design, focal length and mechanics of the lens.
Wide-angle lenses usually have a lower MOD. Reduction is also possible by dimming or using spacer rings, which increase the object distance to the sensor and thus reduce the closest focusing distance.
Spacer Rings
Spacer rings are mounted between the lens and the camera and increase the distance between the sensor and lens group (image width). This can reduce the lens’s closest focusing distance. The object can be brought closer to the optics and the shown object field becomes smaller. This means that even very small image sections, e.g. in the range of 9 mm × 6 mm, can be captured sharply with standard lenses. Spacer rings offer an efficient way to take close-ups and can be an alternative to macro lenses.
Vignetting
Vignetting refers to the edge darkening of an image that can distort measurement results and make object detection difficult, especially with binary image-based algorithms.
To reduce this effect, it is recommended to use either a smaller aperture or powerful, even illumination. This minimizes flat marginal rays, which are typically responsible for darkening.
