What is the f-number, on which camera models can it be adjusted, and how does adjusting it impact tracking?

F-number (also referred to as f-stop or focal ratio) is the ratio of the lens's focal length to the diameter of the entrance pupil. It is an important variable in optical motion capture, because it controls both image brightness (which impacts camera-to-marker range) and depth of field (which impacts image sharpness throughout the camera-to-marker range).

Flex Series, Prime 13, and Prime 13W cameras have fixed-diaphragm lenses, meaning their f-number cannot be adjusted. Prime 41 and Prime 17W cameras have adjustable-diaphragm lenses, meaning their f-number can be adjusted to fine-tune image brightness or depth of field.

Notes to consider when adjusting a lens’ f-number:

• Image brightness increases as the f-number decreases. This means that lowering the f-number can improve camera-to-marker range in large setup applications where increasing exposure, illumination, or gain is insufficient for obtaining desired range.

• Depth of field decreases as the f-number decreases. This means that lowering the f-number will reduce image sharpness more quickly as a marker moves away from the in-focus point.

• Practically, the above two principles combine to produce two types of optical tracking scenarios:

  • Better data quality preferred over capture area size: A higher f-number will enable markers to better remain in-focus throughout the capture volume (producing more stable data), but the reduced image brightness will limit the size of the capture volume.

  • Larger capture area size preferred over data quality: A lower f-number will increase the potential setup area size, but because depth of field will be reduced, markers will begin to blur in the cameras throughout a larger portion of the capture volume. Applications with higher marker counts, or markers in close proximity, will likely produce less stable reconstructions in this scenario.