I thought a bit more about your issue. I hadn't fully considered that the focal length of your lens is so short. Try shooting some tests in an environment where you can control the lighting. You might really find that f22 on that 7.5mm lens is unacceptably soft. The reason is because the aperture diameter of a 7.5mm lens at f22 is (7.5/22)mm, which is ~0.3mm. This is the size of a pinhole. At such small physical apertures, the wave nature of light starts to behave "sub-optimally". It kind of starts to interfere on itself. The full explanation is beyond me, but its called diffraction, and its a basic feature of all light and optical systems. It affects every aperture, but you really start to notice it the smaller your physical apertures become. If in your tests you find apertures above f8 or so are too soft, your best bet is taping a bit of gel, or perhaps exposed film, behind the rear element.