He is associated with the invention of the first optical telescope and/or the first truly compound microscope, but these claims (made 20 years after his death) may be fabrications put forward by his son.

In 1615 Zacharias was appointed guardian of two children of Lowys Lowyssen "geseyt Henricxen brilmakers" (called Henry the spectacle maker).

Janssen has been given a death date as late as 1638[3] although his sister said he was dead in 1632 testimony[7] and his son Johannes declared his parents had died by the time of his marriage in April 1632.

Janssen's attribution to these discoveries is debatable since there is no concrete evidence as to the actual inventor, and there are a whole series of confusing and conflicting claims from the testimony of his son and fellow countrymen.

[9][8] He had a local magistrate in Middelburg follow up on a 45 year old recollection of a spectacle maker named "Hans" who told a young Boreel in 1610 about inventing the telescope.

[10] Boreel's conclusion that Zacharias Janssen invented the telescope a little ahead of spectacle maker Hans Lippershey was adopted by Pierre Borel in his 1656 book on the subject.

For this to be true (Zacharias most likely dates of birth would have made him 2–5 years old at the time) some historians concluded grandfather Hans Martens must have invented it.

[16] The German astronomer Simon Marius's account to his patron Johan Philip Fuchs von Bimbach about meeting an unnamed Dutchman at the 1608 Autumn Frankfurt Fair who tried to sell him a device that sounded like a broken telescope has led to later speculation this unnamed Dutchman could have been Zacharias Janssen.

[17] The confusion surrounding the claim to invention of the telescope and the microscope arises in part from the (sometimes conflicting) testimony of Zacharias Janssen's son, Johannes Zachariassen.

The people he had the local magistrate interview were trying to recount details 50 or 60 years after the fact and Boreel may have confused the names of spectacle makers from his childhood.

[20] Albert Van Helden, Sven Dupré, Rob Van Gent, and Huib Zuidervaart in their book "Origins of the Telescope" came to the conclusion that Janssen may not have become an optician until 1616 and that the claims surrounding him as the inventor of the telescope and the microscope were the fabrications of his own son, Johannes Zachariassen, who claimed it as a matter of fame and for possible financial gain.