The built-in lens is a 3.3 to 12mm vari-focal lens, which will allow you to adjust the horizontal field of view between approx. 22 and 72°.
The supplied lens fits snugly inside the front of the camera housing with the ring-shaped pcb carrying the IR LEDs fitting closely around it.
If you need to achieve a field of view which this camera cannot provide then you'd need to use a box camera, such as the ACTi ACM-5611. Of course you would also need to purchase a suitable megapixel lens, camera housing and possibly lighting.