Lenses basics

The lens is the "optical system" of any industrial vision application. It controls how light from the object is projected onto the camera sensor and has a decisive influence on image quality, sharpness, and geometric accuracy. An industrial lens consists of multiple optical elements (glass lenses), a mechanical housing, and often an iris mechanism. The most important technical characteristics of a lens include:

• Focal length: Determines the field of view and magnification. Short focal length → wide field of view (wide angle) Long focal length → narrow field of view, higher magnification

• Image circle: Defines the size of the image projected by the lens. It must be at least as large as the camera sensor to avoid vignetting (dark corners).

• Aperture (F-stop): Controls the amount of light passing through the lens. Large aperture → more light, shallow depth of field Small aperture → less light, greater depth of field

• Depth of field: The range in front of and behind the focus point that appears sharp. It depends on aperture, focal length, and working distance and is especially important for 3D objects.

• Working distance: The distance between the lens and the object. It affects field of view, resolution, and mechanical integration.

• Resolution / MTF: The ability of the lens to reproduce fine details and contrast. It must match the sensor resolution to fully utilize camera performance.

• Distortion: Geometric deviations in the image (e.g. curved lines). Low distortion is essential for measurement applications.

• Spectral range: The wavelength range the lens is optimized for (visible, NIR, SWIR). Important for specialized lighting conditions and applications.

• Lens types:
o Standard lenses: For general machine vision applications
o Telecentric lenses: For precise measurements without perspective errors
o Wide-angle lenses: For large fields of view in limited space
o SWIR lenses: For imaging beyond the visible spectrum
o Line scan lenses: For continuous inspection tasks
o Ruggedized lenses: For harsh environments with vibration, dust, or water
o Radiation-resistant lenses: For nuclear, aerospace, or research applications

• Mount: The mechanical interface to the camera (e.g. C-mount, F-mount). It defines compatibility and flange distance.

Summary: The lens is a key factor in the performance of a vision system. It affects not only image quality, but also measurement accuracy, field of view, and system integration. Careful selection ensures optimal performance between camera and lens and meets the requirements of the application.

Aperture

The aperture is the opening inside a lens that controls how much light reaches the camera sensor. A larger aperture lets in more light, which is helpful in low-light environments. A smaller aperture lets in less light but increases depth of field. In machine vision, aperture also affects image sharpness and contrast. It is usually expressed as an f-number (e.g. F2.8).

Back focus

Back focus describes the distance from the vertex of the rearmost glass element of the lens to the point where the image is in focus. This distance must match the camera and mount type to achieve sharp images. If back focus is incorrect, the image cannot be focused properly. Some lenses allow back focus adjustment. It is especially important when using different sensors or adapters.

Broadband coating

Broadband coating is a multi-layer optical coating applied to lens surfaces to reduce reflections over a wide range of wavelengths. The benefits: it improves light transmission and image contrast. Broadband coatings are especially useful when a system operates across multiple spectral ranges, such as visible and near-infrared (NIR) illumination. It also helps reduce ghosting and flare.

MTF (Modulation Transfer Function)

MTF describes how well a lens can reproduce fine details and contrast from the object to the image. It indicates how accurately different spatial frequencies (levels of detail) are transferred by the optical system. High MTF values mean that the lens can maintain strong contrast even for very small structures. MTF performance typically decreases toward higher spatial frequencies and toward the edges of the image. MTF is an important parameter for evaluating lens quality and ensuring that the lens can match the resolution of the camera sensor.

Field of view

Field of view describes the physical area of the object that is visible to the camera. It is determined by the sensor size, lens focal length, and working distance. A wider field of view captures more area but with less detail. A narrower field of view shows more detail but less area. Choosing the right field of view is critical for accurate inspection.

Focus shift

Focus shift is a change in focus when lighting conditions or wavelengths change. It commonly occurs when switching between visible and infrared illumination. If a lens is not optically corrected for multiple wavelengths, the image may become blurry. Focus shift can reduce measurement accuracy. IR-corrected lenses help minimize this effect.

IR-correction

IR-correction means the lens is designed to minimize focus shift between visible and infrared wavelengths. Without IR-correction, images may appear out of focus when switching between visible and IR illumination. IR-corrected lenses ensure consistent image sharpness. They are commonly used in machine vision and surveillance.

Flange back

Flange back is the fixed distance between the camera's lens mount flange and the image sensor. Each mount type (such as C-mount or CS-mount) has a defined flange back distance. The lens must be designed for this distance so that the image forms correctly on the sensor. If the flange back distance does not match, the lens cannot focus properly.

Iris control

Iris control regulates the size of the aperture, controlling how much light reaches the image sensor. Automatic systems adjust dynamically to changing lighting conditions, ensuring consistent exposure, while manual systems are typically used in stable lighting environments. Proper iris control helps maintain stable image brightness, improves image quality, and ensures reliable and repeatable inspection results.

Working distance

Working distance is the distance between the lens and the object being imaged. It affects magnification and field of view. Short working distances allow high detail but limit space. Longer working distances are useful for large objects. Lenses are often specified for a certain working distance range.

Focal length

Focal length is a fundamental property of a lens that influences field of view and image scale. Short focal lengths produce a wide field of view, while long focal lengths result in a narrower field of view and greater image magnification at the same object distance. Focal length is specified in millimeters and is a key parameter when selecting a lens.

Focus range

Focus range describes the distances over which a lens can be adjusted to achieve sharp focus. It usually extends from a minimum object distance to infinity. A wider focus range offers greater flexibility for different working distances. In fixed imaging setups, the working distance must lie within the lens’s focus range.

Mount

The mount is the mechanical interface between the lens and the camera. It defines how the lens is attached and aligned with the camera sensor. Common mount types include C-mount, CS-mount, F-mount, and M42. Each mount standard specifies parameters such as thread type and flange focal distance, which determine compatibility between lenses and cameras.

IP-rating (67, 69)

IP rating describes the protection of a device against dust and water ingress. For example, IP67 means the device is dust-tight and protected against temporary immersion in water. IP69K indicates protection against high-pressure, high-temperature water jets used in washdown environments. Such ratings are important in harsh industrial environments.

Chief ray angle

Chief ray angle (CRA) describes the angle between the chief ray of a light bundle and the normal (perpendicular) to the image sensor. If this angle becomes too large, it can cause shading, reduced sensitivity, or color shifts on the sensor. This effect is especially important for modern sensors with small pixels and microlens structures.

Ruggedized

Ruggedized refers to equipment designed for harsh environments. This includes resistance to vibration, shock, temperature, and humidity. Operating shock describes sudden mechanical stress and is measured in G-forces. Rugged systems ensure reliable and stable operation under demanding conditions.

Pixel size (µm)

Measured in micrometers (µm), pixel size is the physical size of one pixel on an image sensor. Larger pixels collect more light and generally provide better sensitivity and low-light performance. Smaller pixels allow more pixels to fit on a sensor of the same size, enabling higher resolution. Pixel size also influences lens requirements and tolerance to chief ray angle.

Image circle (mm)

Image circle is the diameter of the usable image projected by a lens onto the sensor plane. It must be large enough to fully cover the camera sensor. If the image circle is too small, vignetting or dark corners will appear in the image. Larger sensors require lenses with larger image circles. This specification is important to ensure compatibility between the lens and the sensor.

Minimum Illumination

Minimum illumination is the lowest light level at which a camera can produce a usable image. It is measured in lux. Lower values mean better low-light performance. This depends on sensor sensitivity, lens aperture, and noise. It is important for night or low-light applications.

Resolution

Resolution describes the level of detail that an imaging system can capture. It is often specified by the number of pixels in the image (e.g., 1920 × 1080). Higher resolution allows finer details to be detected. The effective resolution of a system depends on both the sensor and the optical performance of the lens. A high-resolution sensor requires a lens with sufficient optical performance to fully utilize the available pixels.

Distortion

Distortion describes geometric deformation in an image caused by the lens. Straight lines in the scene may appear curved, especially toward the edges of the image. Low distortion is important for measurement and inspection tasks where geometric accuracy is required. Distortion is usually expressed as a percentage relative to the image height. Industrial lenses are often designed to minimize distortion.

Vignetting

Vignetting is the darkening of the image corners compared to the center. It occurs when light intensity decreases toward the edges of the image, either due to optical limitations of the lens or partial obstruction of light rays. Vignetting can reduce image uniformity and may affect measurement accuracy in some applications. Proper matching of lens and sensor size helps minimize vignetting.

Wavelength range

Wavelength range defines the spectral range of light for which a lens is designed to perform optimally. This can include visible light, near-infrared (NIR), or short-wave infrared (SWIR). Outside the designed wavelength range, transmission and image quality may decrease due to optical aberrations and coating limitations. Matching the wavelength range of the lens with the illumination and sensor is important for specialized imaging applications.

SWIR

SWIR stands for Short-Wave Infrared and typically covers wavelengths from about 900 to 1700 nm for most imaging sensors. Light in this range interacts differently with materials than visible light, allowing certain features to become visible that cannot be seen in the visible spectrum. SWIR imaging can improve visibility in haze, smoke, or dust because longer wavelengths scatter less.

Telecentric

A telecentric lens produces images with nearly constant magnification over a defined depth range. This means that small changes in object distance do not significantly change the image size. Telecentric lenses reduce perspective errors because the chief rays are parallel to the optical axis. This property is important for precise measurements and dimensional inspection. Telecentric lenses are widely used in metrology and machine vision applications.

Radiation resistance

Radiation resistance describes the ability to withstand exposure to radioactive radiation. Radiation can cause glass to darken and degrade image quality. It is measured in Gray (Gy). Testing is performed under controlled irradiation conditions. Such optics are used in nuclear applications and research environments.

Relative illumination

Indicates how evenly brightness is distributed from the image center to the edges. High relative illumination ensures uniform brightness without dark corners and is important for reliable image analysis.

Chromatic aberration

Color errors caused by different wavelengths of light being refracted differently. They appear as color fringes at edges and are reduced through special lens designs to ensure sharp and accurate color reproduction.

Depth of field

Refers to the range in front of and behind the focused object that appears sharp. It is influenced by aperture, focal length, and working distance, and is important when imaging objects with depth.

Magnification

Defines the ratio between the object size and its image on the sensor. It determines how large details appear in the image and is especially important for precise measurement and inspection tasks.

Transmission

Indicates the light transmission of a lens in percent. Higher transmission results in brighter images and improves the signal-to-noise ratio, especially in low-light conditions.

Filter thread

Standardized thread size at the front of the lens for mounting filters, such as polarizing filters or protective glass. It allows easy adaptation to different lighting and protection requirements.

Aspherical elements

Use of aspherical lens elements to reduce optical aberrations, especially spherical aberration. They improve image quality while enabling a more compact lens design.

Entrance pupil

Refers to the apparent position and size of the aperture as seen from the object side. It influences how light enters the lens and is important for lighting design and accurate system calibration.

Focus and iris locking

Refers to mechanical locking mechanisms for focus and aperture. They prevent unintentional adjustments caused by vibration or movement and ensure stable settings during operation.

Lens construction

Refers to the number and arrangement of individual lens elements within the lens. It has a major influence on image quality, sharpness, and the correction of optical aberrations. More complex designs generally enable better optical performance.