How many pixels does it take to see the fine print in the book within 3-5 meters?

This is not only related to the pixel, but also to the quality of the sensor, the resolution (that is, definition) of the lens and the focal length of the lens (involving optical zoom or telephoto lens). If the sensor is good enough, the focal length is long enough, and the lens is clear enough, it only needs 2 million pixels.

But in general, it is unrealistic to achieve this effect through mobile phone shooting, at least a mobile phone without optical zoom can't do it, because first of all, the optical sensor of the mobile phone is small, and according to the current technology, no zoom can only be noise (equivalent to 26-28mm focal length).

However, even with optical zoom, you can enlarge the contents of books 3-5 meters to full screen, and you will also face a problem, that is, the resolution of the lens. Once the resolution is not enough, even if the zoom distance reaches the limit, the photographed text is still unclear.

A telephoto camera (zoom factor is about 30x) or a single-lens camera (single shot, micro shot) with a high-quality telephoto lens of more than 300mm may achieve ideal results. If you do this, the photographic equipment will be very large, then it will be very expensive, and then there will be problems derived from the high anti-shake requirements.

So I wonder if you have any more specific environmental factors or requirements to consider besides this requirement? If not, I suggest taking two more steps and taking pictures in books. Ordinary mobile phones are not bad at present.