Exhibit 16.1


September 10, 2007

Securities and Exchange Commission
100 F Street, N.E
Washington, DC 20549

Gentlemen:

We have read Item 4.01 of Form 8-K dated September 10, 2007, of Cognex
Corporation and are in agreement with the statements contained in paragraphs 1,
2 and 3 on page 2 therein, except that we have no knowledge of circumstances
regarding the appointment of Grant Thornton as the Company's Independent
Auditors. We have no basis to agree or disagree with other statements of the
registrant contained therein.


                                                       /s/ Ernst & Young LLP
                                                       ---------------------
                                                       Ernst & Young LLP