Just need to make sure we don't confuse lens sharpness with depth of field. A lens can be pin sharp at f1.4 as @ashenfie has pointed out. It's amount of what is in focus in the picture that will be pin sharp that you need to think about. If you photograph a portait of someones face with an 85mm at f1.4 you can get clean focus on the eye but the eyelashes may be out of focus because the depth of field is so shallow. Shooting wide open isn't always the solution.While very expensive lens will be able at fully open it will always perform better by simply providing more light. A common FF portrait lens in the 85mm F1.4, now if you at 1.8 then depth of field is only 13cm. Now at f4 you would have 29cm, further the extra stops means the lens is sharper.
The solution is make sure you have enough light or the equipment to add light as necessary (fill light).