Lenses

Choosing a lens requires at least: sensor size pixel size field of view working distance resolution required mount wavelength permissible distortion depth of field Basler names exactly this combination of sensor, image circle, focal length, working distance, resolution and special requirements as the central selection criteria.

FOV stands for field of view. It describes the area of the object that is visible in the camera image. Example: FOV = 200 × 150 mm The camera then covers an area 200 mm wide and 150 mm high in the defined working plane.

The working distance describes the distance between the object and a defined reference point on the lens. The manufacturer's exact definition should be checked for precise calculations.

Focal length is a fundamental property of a lens and, together with sensor size and working distance, determines the field of view. With the same sensor and working distance, simplified: shorter focal length → larger field of view longer focal length → smaller field of view

For a first rough estimate at normal object distances, the following is often used: f ≈ sensor width × working distance / field of view width Example: sensor width = 8.8 mm working distance = 300 mm FOV = 100 mm f ≈ 26.4 mm This is only an approximation. For short working distances, macro applications and high accuracy requirements, work with real lens data or the manufacturer's calculators.

The f-number describes the ratio of focal length to effective aperture. Small f-number: large aperture more light less depth of field Large f-number: smaller aperture less light more depth of field

Depth of field describes the range around the focal plane that is still rendered with acceptable sharpness. It depends on: aperture magnification focal length working distance the accepted circle of confusion

No. A smaller aperture, or higher f-number, initially increases depth of field, but the image also gets darker. At very small apertures, diffraction increasingly limits the resolution achievable. There is therefore an optical optimum.

MTF stands for modulation transfer function. It describes how well a lens or imaging system transfers contrast at structures of different fineness. A lens can nominally cover many “megapixels” but transfer fine structures with little contrast. Camera and lens should therefore be considered together.

Small sensor pixels only deliver a real resolution advantage if the lens projects correspondingly fine structures onto the sensor with sufficient contrast. A high-resolution camera with unsuitable optics therefore does not use its full sensor resolution.

Because the pixel count is only one factor in the overall optical system. A 20 MP camera can deliver worse results if, for example: the lens does not resolve sufficiently there is too little light the focus is set badly motion blur occurs the SNR is worse More megapixels are therefore not automatically a mark of quality.

Distortion is a geometric aberration of the lens. Straight lines can appear barrel-shaped or pincushion-shaped, for example. For precise measurement tasks, distortion should be as low as possible or accounted for by calibration.

Vignetting describes a fall-off in brightness towards the edge of the image. Possible causes are: an image circle that is too small optical properties of the lens mechanical shadowing unsuitable adapters

Chromatic aberration arises because different wavelengths are refracted to different degrees. This can produce color fringing or different focus positions for different colors.

With spherical aberration, rays from different areas of a spherical lens surface are not focused at exactly the same point. This can reduce image sharpness. Aberrations affect MTF, spot size and image quality, among other things.

A lens only produces a limited usable image circle. The sensor has to lie entirely within that image circle. If the sensor is too large, dark or blurred image corners can result.

C-mount is a very widespread lens mount in industrial machine vision. It has a 1"-32 UN thread and a flange focal distance of roughly 17.526 mm. Whether C-mount is suitable for a camera also depends on the sensor size and the lens image circle.

CS-mount uses the same basic thread as C-mount but has a shorter flange focal distance of roughly 12.5 mm. C- and CS-mount are therefore not readily interchangeable optically.

These are lens mounts with larger thread diameters. They are often needed where larger sensors or large image circles are used. The specific thread pitch and mechanical specification always have to be checked.

F-mount is a bayonet mount with a considerably larger clear diameter than C-mount. It is used for large sensors and large image circles, among other things.

An object-side telecentric lens keeps the magnification largely constant within its telecentric working range, even if the object position changes somewhat in Z. This greatly reduces perspective size errors. That makes telecentricity particularly valuable for precise measurement tasks.

Typical applications: measuring diameters inspecting outer contours measuring bores distance measurement precision metrology parts with varying Z position Telecentricity does not, however, automatically remove all optical errors, nor does it mean infinite depth of field.

A macro lens is optimized for high magnification or small objects at short working distances. It suits very small components, structures or defects.

Typical causes are: wrong working distance focus outside the adjustment range wrong lens too little depth of field motion blur mechanical vibration unsuitable extension ring First distinguish between optical defocus and motion blur.

An extension ring increases the distance between lens and sensor. With many standard lenses this allows focusing at shorter object distances and increases magnification. At the same time the working distance and focus range change; an extension ring is therefore not simply an “extension without optical effect”.

Possible causes: lens unsuitable for the sensor MTF too low at the image edge sensor or lens tilted wrong focus field curvature aperture too large

Frequently: the lens image circle is too small an adapter blocks rays filter or tube mechanically too small unsuitable optics

Check: the actual sensor width the ROI in use the real focal length working distance focus position extension rings distortion

For a rough estimate: f ≈ sensor width × working distance / FOV For a precise design, use real lens data or the manufacturer's calculators.

Check: mount sensor size image circle pixel size/MTF focal length FOV working distance distortion wavelength aperture A lens should not only fit mechanically but also match the sensor optically.

First the sensor size, pixel pitch, desired FOV, working distance and mount have to be settled. Then choose a Fujinon series that: covers the sensor fully matches the pixel resolution offers the focal length range needed achieves the required distortion figure Fujinon states the sensor size and suitable pixel pitches for its machine vision series.

Typical applications: measuring diameters inspecting outer contours measuring bores distance measurement precision metrology parts with varying Z position Telecentricity does not, however, automatically remove all optical errors, nor does it mean infinite depth of field.

Sensor size, pixel size and lens MTF together determine the image information that can actually be used.