One big difficulty is that whilst you draw, with the image super-imposed by the camera lucida, your drawing seems true and firm. With the lens out of the way, the inkpen linework seems shaky, amateurish. Setting aside the lens to eyeball it (as Hockney would say) directly, the sketch has been finished off with the Pentel brush pen, a tool that Jack wields so much better. More experiments required!
The camera lucida was invented by William Wollaston in 1807. In Secret Knowledge, David Hockney controversially suggests that the camera lucida, or similar advanced optical techniques, could have been used by artists such as Ingres, Holbein, Da Vinci and so on. Others are skeptical.