John Smith succeeded to the optical instrument business of his better known father-in-law, John Marshall, at his premises in Ludgate Street, in 1723. Marshall had upheld a long-running feud with his neighbour in Ludgate Hill, John Yarwell, as to who invented a new method of grinding lenses in batches. Smith retained his father-in-law's use of the Royal Coat-of-Arms, although not the warrant, and his shop sign. However, he seems to have expanded the business to include a number of navigational and mapping instruments.