Every lens, like a golf ball, has a sweet spot. I've found maximum sharpness on a given lens is actually between f8.0-f11.0. While manufacturers make all sorts of claims regarding sharpnes, factory techs would be inclined to say differently. I suggest you run some tests at maximum aperture, then in the mid-range and at minimum like f16 and f22.

Try this with different lenses with appropriate shading and without changing the camera position at all. Use a tripod, a cable release make; sure your camera is plumb and level to the newspaper; use a medium shutter speed like 1/125 or 250 and if you can, pre-release the shutter on each exposure; make sure the newspaper is flat against the wall and evenly illuminated. RECORD your exposure info for each frame. Use slower film, iso 100 or less processed normally.

Then take a high magnification loupe and a light box and look at the negs. Not prints. See what you've got then.

I'd lay odds that a really close analysis will reveal the sharpest f-stop won't be at the bottom or top end of the aperture ring but more in the center. There are so many variables to lens sharpness. Your new found 1.4 could have a miniscule separation of lens element(s) or coating issue, camera shake (even at 1/500th from mirror slap). I think the true key to this kind of analysis is knowing where that sweet spot lies in each of your lenses and work within the limitations of every piece of glass you shoot with.
Take it light ;>)
Mark