Choosing the right lens is just as important as selecting the sensor in embedded camera systems. One key factor is the F-number, a crucial parameter that directly affects light intake, exposure control, and image sharpness.
In this blog, e-con Systems explores the F-number’s role in low-light performance, depth of field, and power efficiency. You’ll also learn how it influences sensor-lens compatibility and guides lens selection for real-world applications. Gain expert insights to optimise imaging in industrial, medical, and surveillance use cases.
In embedded vision systems, image quality is the result of multiple interconnected components. While sensors and processing units often receive attention, the lens system and its parameters play an equally influential role.
One such metric is the F-number (or F-stop), which assesses the influence of light intake, exposure, and image clarity.
Here, you’ll receive expert insights on F-number, its impact on embedded cameras, and the selection criteria for choosing this parameter.
What is F-number?
The F-number represents how the focal length of a lens relates to the size of its aperture opening. Since it compares two lengths, it is considered a dimensionless quantity. This relationship can be represented mathematically as:
F-number (f/N) = focal length/aperture diameter
A lens with a lower F-number has a larger aperture, while a higher F-number corresponds to a smaller aperture. This directly impacts the amount of light that reaches the sensor. For instance, a lens with an F-number of 1.8 allows a larger volume of light into the system compared to one rated at F4.
The difference can be critical for performance in low-light conditions, where higher exposure helps capture detailed imagery with less noise.
How F-Number impacts embedded camera performance
Low-light imaging
Embedded vision systems used in markets in industrial automation, agriculture, and healthcare operate in lighting environments that are neither uniform nor predictable. A lens with a smaller F-number increases the photon count reaching the sensor, which enhances image brightness.
That way, any dependency on high-gain settings that can introduce unwanted artifacts or signal noise can be greatly reduced.
DoF control
The F-number influences the Depth of Field (DoF) – the part of an image that shows up in sharp focus. A lower F-number leads to a shallower depth of field. It is useful when the application demands isolation of foreground objects, such as in object recognition or defect detection systems.
However, a higher F-number increases the depth of field, which is better suited for logistics or surveillance applications where multiple planes of focus are relevant.
Exposure and frame rate
Exposure time in embedded cameras is affected by the amount of available light. So, a wider aperture, achieved with a lower F-number, shortens the exposure duration. It could happen when a system captures fast-moving subjects or operates at high frame rates. In contrast, a smaller aperture increases the exposure requirement, possibly affecting performance in dynamic scenes.
Thermal and power
Longer exposure times and high gain settings introduce thermal loads on sensors and processing units. The system can reduce processing strain by capturing adequate light through a wider aperture. It can help achieve lower thermal output and power consumption, which dictates hardware design and thermal enclosures in embedded systems.
Sensor compatibility
Lenses and sensors must operate as a cohesive unit. So, a mismatch in parameters can reduce the optical throughput of the system. For example, diffraction can introduce softness in the captured images when a sensor with a small pixel pitch is combined with a lens with a high F-number. Matching the sensor size, pixel pitch, and F-number helps maintain contrast and sharpness across the entire frame.
A large aperture supports better light gathering, especially for sensors with small pixel sizes. But it is also susceptible to issues like spherical aberrations and vignetting if the lens design does not compensate for the optical challenges.
Selection criteria in embedded applications
- High-speed motion capture: a lens with a low F-number minimises motion blur under short exposure
- Bright, static settings: a lens with a higher F-number provides an extended depth of field without performance degradation
- Compact design: system form factor restricts lens diameter, making it necessary to balance F-number with size and thermal considerations
- Outdoor performance: varying lighting conditions require a lens with an adjustable aperture to accommodate sudden changes
Other F-number considerations for embedded cameras
Aberrations
Lenses with low F-numbers may introduce optical aberrations such as chromatic aberrations or flare. Some lenses incorporate advanced coatings to address these challenges, although this can increase cost and integration complexity. Lens vignetting, a condition where the corners of the image appear darker, also becomes more apparent at wider apertures.
Image informality
Maintaining uniform brightness and clarity across the image frame demands attention to aperture size, lens architecture, and sensor alignment. Any mismatch in these areas can affect the consistency of output across the sensor’s active area.
Lens specification
In lens datasheets, the F-number directly correlates with light transmission and aperture width. Some lenses feature manual or automatic iris control, giving room for adaptive exposure in changing environments. Others feature a fixed aperture tuned for specific lighting scenarios.
The F-number must be considered alongside other specifications such as Modulation Transfer Function (MTF), field of view, and distortion metrics. A balanced evaluation of all these factors supports informed lens selection aligned with system demands.
Sensor alignment
Optical calibration must account for how F-number influences dynamic range, brightness, and signal-to-noise characteristics. During prototyping, it is important to explore different F-number settings to evaluate sharpness, exposure consistency, and artifact behaviour across various operating conditions.
