Few individuals tasked with developing or maintaining a vision system possess expertise in photography, making lens selection seem like a complex process. Two common areas of confusion are focal length and f-number. Let’s clarify what these terms mean.

The focal length indicates the angle of view the lens provides. A shorter focal length offers a wide-angle perspective, while a longer one gives a narrower field of view. For instance, compare a fisheye lens to a telescope: the former captures an extremely wide area but compresses distant objects significantly in size, whereas the latter has a narrow viewing angle and is ideal for observing faraway subjects.

In machine vision applications, short focal length lenses (e.g., 8 or 12mm) are suitable when the camera is close to the target. As you move the camera farther away, the subject appears smaller. Switching to a longer focal length lens (like 25 or 50mm) maintains image size despite increasing working distance.

The f-number measures light transmission through the lens; it’s an inverse relationship meaning larger numbers allow less light and smaller ones more light. A lens’s aperture adjustment changes its f-number, with wider openings yielding lower values. Comparing C-mount and F-mount lenses reveals that the latter typically has a lower f-number at full opening due to its larger diameter.

This matters because adequate lighting is crucial in machine vision for minimizing exposure time. However, wide apertures reduce depth of field, making focus challenging across multiple planes. Solutions include increasing illumination or choosing lenses with smaller f-numbers. Careful consideration might lead one to prefer large format lenses over standard C-mounts depending on specific needs.

To summarize: focal length defines the lens’s angle of view while f-number inversely indicates light transmission—lower is better for brightness and shallower depth-of-field control.

Last Updated: 2025-09-04 21:39:15