<p>I may be mistaken, but I recall the built-in hood on the Mk I being considerably shorter than the detachable hood of the Mk II which leads me to wonder if it is less helpful and/or if the larger one contributes to more vignetting.</p>
<p>Not a significant sample size, but FWIW the only Mk I I've used was noticeably less sharp than the single Mk II I previously owned, and had considerably more dust inside from its longer service life.</p>