Sorry, but that's just not true. The STF is not a soft focus lens, it does not have deliberate spherical aberration, in fact it has remarkably low SA and is one of the very sharpest 135mm lenses you will find. It introduces no softness (reduction in resolution) to any part of the image, either in-focus or out-. It was originally a Minolta AF lens and Sony continues to make it, so it (including the current Sony version) will work with any Alpha/Dynax/Maxxum 35mm cameras people might have.
What it does is have an apodisation filter (dark at the edges, light in the middle) very close to the aperture. This means that when shot wide open, the bokeh has a gaussian rather than circular form, which means that you no longer have blur circles with sharp edges but big soft blobs with no visible edge whatsoever, which means that backgrounds become beautifully smooth.
With any lens (ignoring aberrations), a point source appears on the film as an image of the lens' aperture, scaled by how far out of focus it is. So a focused point is a point, a defocused point will appear as a circle/septagon/whatever with sharp edges. If you then put the apodisation filter next to the aperture, that circle is dimmed at the edges and you lose the sharp edges on your bokeh.
As to the original poster - your vision gets sharper because when you squint, you're manually stopping down your aperture. The eye is a very simple lens that works poorly wide-open. Our vision is sharp in bright sunlight (small pupils, high f/number) but in dim light, the pupils open up, expose more of the poorer-performing part of the lens and your vision gets softer.