Why using Spacer Rings with C-Mount lenses?

In rare cases, achieving perfect focus with a C-Mount lens may require additional adjustment. This is not a defect, but a result of normal mechanical tolerances in both the camera and the lens.

Background

All C-Mount systems follow a standardized flange focal distance (nominally 17.526 mm). However, in real-world production, both cameras and lenses are manufactured within defined tolerances. In uncommon cases (<1%), these tolerances can accumulate ("tolerance stacking"), which may slightly shift the effective flange focal distance and limit the available focus range.

Camera Design Approach

Our 47G camera series uses a fixed C-Mount front plate designed for high-cost efficiency and robustness. This is a common approach across the industry and ensures:

• Competitive pricing

• Compact and stable mechanical design

• Reliable performance for more than 99% of all applications

Designing the system to mechanically compensate for all possible tolerance combinations across all lenses would require more complex adjustable mechanics or tighter calibration. This would increase the camera cost affecting all customers - even though the issue only occurs in a very small number of cases.

Sensor-Dependent Flange Focal Distance

The effective flange focal distance of the 47G varies slightly depending on the sensor used, due to differences in sensor package thickness:

• IMX900: 17.28 mm

• IMX568: 17.08 mm

• AR0521: 17.275 mm

• IMX676: 17.08 mm

• AR2020: 16.85 mm

These variations are within normal design limits but can contribute to the overall tolerance stack in combination with certain lenses.

Recommended Solution: Spacer Rings

To ensure full compatibility with all C-Mount lenses, we recommend using spacer rings (shims) to fine-tune the flange focal distance when needed.

Spacer rings:

• Provide precise and stable adjustment

• Are easy to install

• Maintain full optical performance

Summary

• Similar issues occur in less than 1% of cases

• It is caused by normal manufacturing tolerances

• Spacer rings provide a simple and effective solution when adjustment is needed

If you need support selecting the correct spacer configuration for your setup, our team will be happy to assist.