If you’ve been searching for a small-camera lens, you’ve likely encountered terms like M12, S-Mount, Board-Mount, Miniature Lens, C-Mount, CS-Mount, and more. With so many names floating around, it’s easy to get confused. So, why does it seem like there are so many options? And how do you know which one is right for your project? In an effort to answer these questions, we thought we’d explore what these terms mean, where they come from, and how they relate to your lens selection.
In the beginning (at least in this story), there was C-mount.
Going back decades, the standard for interchangeable industrial lenses (as opposed to most consumer photographic cameras) was the C-mount. The C-Mount lens, still a staple in industrial machine vision, some security camera circles, and university labs, was one of the first solutions to standardize the lens mount format. It paved the way even before the days of CCD and early CMOS cameras.
The thread of a C-Mount is 1-32; more specifically it’s 1” in diameter with 32 TPI (threads per inch), or: M25.4 x 0.794mm. The C-mount has a standardized back focal length (BFL) of 0.69” (17.526mm), meaning that all these lenses were designed with the same flange BFL. In practice, the lens was screwed all the way down to the mount until tight and then a focusing mechanism allowed fine focus depending on the object distance. This of course made for some long TTL lenses, especially those with a long EFL. To help address the length issue, the CS-Mount lens format was introduced. While it uses the same thread and mounting strategy as a C-mount, it has a shorter fixed BFL of 0.4931” (12.526mm).
Although the C/CS-Mount makes for very straight-forward interchangeability, there are several drawbacks to their formats. First, the standardization of the FBFL is somewhat arbitrary in terms of optimizing optical performance and actually imposes a design constraint. Second, since the FBFL is fixed, the lens must have a secondary focus mechanism, and since the standard 32 TPI thread is not fine enough to focus 10s of microns of DOF (depth of focus), the lens must incorporate a relatively complex mechanical means of achieving fine focus. Third, the fixed FBFL alone means the TTL of the lens will be at least 12.5mm long (more for C-Mount) before even considering the physical length of the lens. Fourth, the C/CS mount is typically (but not always) integrated into the housing or chassis of the camera with the sensor-board mounted separately. This means there is no direct mechanical reference or interface between lens and sensor, which you’ll know could be a potential from our AA article (Sunex Knowledge Center: What Is Active Alignment?). Lastly, but admittedly not exclusive to C/CS lenses, there is a tendency to add more features since they are interchangeable. While these features may be ideal in applications where flexibility is needed, it is less desirable for fixed, high-volume circumstances. Often, these features also come at the cost of well, cost, in addition to reliability, design and performance tradeoffs. Despite these drawbacks and the fact that C/CS-Mounts aren’t technically Board-Mounts, they still have plenty of utility. It’s important to recognize how these formats helped establish standards for lens mounting and continue to serve many applications today.
Now, the “Board-Mount”.
“Board-Mount” or “S-Mount” lenses address the C/CS-Mount issues in a few ways. Board-mount lenses have no dependency on a fixed FBFL/BFL and no need for a separate focus mechanism. They are designed to thread into a threaded mount directly attached to the sensor PCB. The thread doubles as the focusing mechanism because it typically has a 0.5mm or 0.35mm pitch making it fine enough to focus a lens (see our article Sunex Knowledge Center: Basic Thread Considerations). It also eliminates many (but not all) sources of alignment error between lens and sensor by placing the lens directly on the sensor board. Of course, this means that the BFL, FBFL and MBFL are coupled to the focal position of the lens. This means the focal position changes slightly from camera to camera, but the differences are on the order of 10’s of microns, so it is generally not a problem.
A natural result of this board-mount approach is the proliferation of optimized, design-for-purpose lenses. “Board-Mount” is simply a general, all-encompassing term for lenses that are mounted and focused in this way. Within this broad category, M12 lenses, also referred to as S-mount, are the most common. Both terms refer to an M12x0.5mm lens, that is, a 12mm diameter lens with 0.5mm thread pitch.
In fact, “M12” has become almost synonymous with Board-Mount but in truth, while all M12’s are Board Mounts, not all Board-Mount lenses are M12 lenses. Other popular sizes of board-mount lens include M14, M10, M8, M7 and even smaller. Thread pitch tends to scale roughly with diameter and M8x0.35mm are fairly common, but in theory any size thread can be used with any diameter lens. For example, M12 “fine-focus” (M12x0.35) or even larger diameters may be specified in critical higher-megapixel applications, to gain a bit more focus control.
M12 and other Board-Mount lenses are also ideally suited to active alignment because there is not a fixed BFL and therefore no secondary focus requirement. In Active Alignment, an M12 lens can have its thread removed and can be focused and fixed directly over the sensor in one step without impacting the rest of the design. For example, you could prototype with a threaded M12 lens and mount and then go straight to mass-production with a threadless version of the same lens and mount.
