Guide to fixed focal
lenses: Finding the right lens for your application step by step
Fixed focal length lenses are among the most important components of a machine vision system. They determine which details are visible, how large the image field is, and how reliable the evaluation is. This guide takes you step by step through the most important criteria you should consider when making your selection – from sensor size to focal length and mount to special application conditions.
1. Sensor size – the foundation of every lens
selection
Before you think about focal length or aperture, you need to determine the sensor size of your camera. The lens must project an image circle that fully coversthis sensor– otherwise vignetting or corner fall-off (and potentially reduced resolution at the edges) can occur. In the product filter, you can choose between formats such as 1/3", 1/2", 2/3", 1", 4/3" or full frame under "Sensor Size". Always go by the largest sensor you actually use—or want to use. A lens for 2/3" will also work on a 1/2" camera, but not vice versa.
Practical tip:
It's better to plan for a little reserve. If you use a higher-resolution camera with a larger sensor in the future, the lens will still be compatible.
2. Working distance and image field – calculating
the right focal length
The second step is to define the working distance and image field size (object area to be captured). Together with the sensor size, these determine the focal length you need to capture your object or area in its entirety.
In the product finder, you will find the fields "Focal Length (mm)" and "Working Distance (mm)". As a rule of thumb: the greater the working distance or the smaller the desired image field, the longer the focal length must be – and vice versa.
Example:
If you want to image an area of 200 mm × 150 mm from a distance of 400 mm with a 1" sensor, the ideal focal length is approximately 25 mm. (First-order approximation: f = sensor size × working distance / image field.)
Practical tips:
• Short focal lengths (wide angle) are often advantageous in confined spaces, but increase perspective.
• Longer focal lengths are suitable for larger working distances and reduce perspective; useful for precise measurements.
• If you are unsure between two values, it is better to choose the slightly shorter focal length – this keeps the field of view flexible (you can crop or adjust working distance).
3. Mount type – the interface between camera and lens
The mount determines how the lens and camera are mechanically and optically connected to each other. In the industrial imaging, the C-mount is by far the most common standard. It is suitable for sensors up to approximately 1.1” size format and offers a solid, vibration-resistant connection.
In the product filter, you can filter specifically by "Mount type" – e.g.:
• C-mount: Standard for most industrial cameras (17.526 mm flange focal distance).
• CS mount: compact design, especially for small cameras (note: shorter flange focal distance 12.5mm – C-mount lenses require a 5 mm spacer ring; CS-mount lenses do not focus on C-mount cameras).
• F-mount: For larger sensors; robust bayonet connection with secure locking.
• M42 / T-mount: for line scan cameras or large image circles. ; note thread variants (M42×1 industrial, M42×0.75).
• M58: For very large image circles and high NA, common with line-scan and large area sensors.
• S-mount (M12): for space-critical mini or embedded systems.
Practical tip:
Always choose the mount that matches the camera.
When switching to a larger sensor → consider an F-mount.
4. Aperture and depth of field – making optimal use
of light
The aperture (iris) controls how much light falls on the sensor – and directly affects the depth of field. A low F-number (e.g., F1.4) admits more light but reduces the depth of field. Higher F-numbers (F8 or F11) increase the depth of field but require more illumination and can introduce diffraction softening at small pixels or high magnification.
In the product filter, you can filter by "Aperture."
Practical tip:
Choose a lockable aperture for series systems. For high-speed or low-light applications, a particularly fast lens (e.g., F1.4) is beneficial, provided the available depth of field remains sufficient.
5. Resolution and pixel size – sharpness down to the
last detail
The smaller the pixel size of the sensor, the higher the optical resolving power (MTF) the lens must deliver. What matters is pixel pitch, not megapixel count alone. A 12-megapixel sensor with a pixel size of 3 µm places higher demands from the optics than a 2-megapixel sensor with 5 µm pixels. Check that the lens maintains sufficient MTF at the spatial frequencies implied by your pixel size (vendor specs are given for a stated F-number and working distance).
In the product filter, the "Resolution" or "Min. Pixel Size" filters help you narrow down suitable series. Choose a lens whose minimum pixel size ≤ your camera’s pixel size.. Some margin will help you avoid visible blurring and get the most out of your camera.
Practical tips:
• Verify MTF at your chosen F-number and working distance, especially near the sensor’s Nyquist frequency and toward the field edges.
• Balance DoF vs. diffraction: as pixels get smaller, use faster F-numbers. Remember the effective F-number rises with magnification, so you may need even more light..
• For very small pixels or broad spectral use, prefer apochromatic/high-resolution series to maintain MTF and control chromatic errors.
6. Spectrum and wavelength – not all light is the same
Lenses are usually optimized for the visible range (400–700 nm). In machine vision, however,NIR (near infrared) or SWIR (short-wave infrared) are also common and place different demands on glass, coatings, and chromatic correction.
In the product filter, you can use "Wavelength Range" to filter specifically for lenses for special spectra:
• VIS (Visible): Standard coating for color and monochrome imaging in 400-700nm.
• VIS-NIR: Broadband design/coating, to maintain focus/MTF across VIS into NIR (typically 400–900/1000 nm) common for many industrial lenses.
• SWIR: Dedicated designs with special glass and coatings for 900–1700 nm (InGaAs sensors) – e.g., for material or moisture detection, silicon inspection (>~1100 nm).
• UV: UV-capable materials/coatings (often fused silica) for typically 200-400 nm; used for fluorescence or microscopy applications.
Practical tip:
If you use NIR illumination, always choose a VIS-NIR-compatible lens. Standard VIS lenses can exhibit focus shift and reduced transmission in the NIR. For SWIR or UV cameras, you need series that are specifically designed for this purpose – standard optics will not transmit or deliver sharp images here.
7. Mechanical design and environmental influences
In addition to the optical specs, the mechanical design is also crucial for the long-term stability of the system. Vibration, temperature changes, or dust/moisture can shift focus or iris settings if the lens is not built for such conditions. In the product specifications, parameters such as "Dimension/Weight," "IP Protection Class," or "Shock & Vibration Rating" help you select suitable models.
Recommendations:
• Compact design: when installation space is limited (e.g., in robots or test stations).
• Ruggedized models: bonded lens groups, lockable focus/iris, anti-rotation features – ideal for continuous vibration/shock.
• IP-rated variants: sealed housings/front windows for humid, dusty, or hygienic environments (IP65/67, food-grade options).
• Temperature-compensated series: athermalized designs to keep focus stable despite changing ambient temperatures.
Practical tip:
If the system runs 24/7 or is exposed to vibration, shock, or temperature cycling, never skimp on mechanical build quality. A ruggedized lens will maintain stable focus and calibration for years.
