Stock Lens originally means a lens that comes from the camera when you buy it, but Arducam uses it as a generic term to refer to those lenses which are not easily interchangeable or focused, just like the ones that come from the official Raspberry Pi camera V1 and V2.
Pros and Cons of a stock lens
Those lenses are cost-effective and compact.
Unlike other interchangeable M12 or CS-Mount lenses, stock lenses are not sold separately. Instead, they are usually packed together with the lens mount and image sensor as a small unit to be connected to a PCB. Therefore, the cost of the whole camera unit will be relatively lower. Besides, the compact size makes it ideal for portable devices or other applications where space is constrained.
For a camera module with a stock lens, it’s hard to adjust the focus, and nearly impossible to replace the lens or remove its IR filters.
There is really little to say about its flexibility. If you are not satisfied with the stock lens, you have to replace the whole lens-sensor unit as a whole, as what we’ve done in our IMX219 drop-in replacement series. This could bring extra cost because you have to replace an image sensor every time you replace a stock lens.
Further Reading: How to replace the stock lens on the Raspberry Pi Camera Module V2
Examples of Arducam camera modules with stock lens