This pic is a great illustration of what DOF really means and what it doesn't mean: the plane of focus is surrounded by a zone (here about 3 feet deep) where everything is "acceptably" sharp. The use of calculated circles of confusion are used to define what is standard and "acceptable". What DOF doesn't mean is that everything within the zone is sharp to the same degree.

Focus is maximally sharp in a plane, and everything nearer and farther is less so.

Here, the wing feathers are sharp, but the catchlight in the dove's eye is less so. This indicates that the camera focus was dead on the money, or rather the feathers.

The original pic was shot at f5.6 and 1/500th, ISO 100.

Want a deeper zone of sharpness? Crank up the ISO to 400 and shoot at f11, or f8 at 1/1000th. Whole new ballgame, folks. If there's noise in the dark spots, deal with it in PP.

I'd say your present lens is performing admirably.

gee, a whole lot of stuff to simply say the small the aperture the greater the bending of light and the greater the depth of field. One has greater depth of field at f22 than at f3.5. Real simple.