Correct, the MTF of a lens is meaningless unless the lens and sensor are aligned (aka focused) correctly. Contrast in the image is directly related to the square-wave geometrical MTF. Once you factor in focus/alignment error, it can be difficult to tell the difference between a "3MP" and "12MP" rated lens. We're talking about <10 micron accuracy.
This is why companies like Apple use 6-axis active alignment machines to manufacture every camera module.
The "Megapixel" rating of a lens is based on the nominal as-designed MTF and image sensor Format. There's a lot of manufacturer fudging that goes on with these ratings. And some in China literally print whatever they want on the side of a lens. I've seen the same lens with both "3MP" and "5MP" printed on it.
Depth of field isn't really an issue with wide angle lenses for smaller format sensors. The EFL is shorter which boosts the depth of field. The bigger problem is field curvature if the page is closer than ~6" - most of these lenses weren't designed for short object distances.
This is why companies like Apple use 6-axis active alignment machines to manufacture every camera module.
The "Megapixel" rating of a lens is based on the nominal as-designed MTF and image sensor Format. There's a lot of manufacturer fudging that goes on with these ratings. And some in China literally print whatever they want on the side of a lens. I've seen the same lens with both "3MP" and "5MP" printed on it.
Here are a few tips on how to focus a camera. Regardless, it'll be hard to get within 2-5 microns with a CS Mount 1"-32TPI thread. https://commonlands.com/blogs/technical/how-to-focus-a-camer...