With apologies for subverting the Cooke thread, describing soft lenses and how they work kinda gets into semantics. All pathways through the lens are producing sharp images, but they're on different focal planes. The outside areas of the lens are on a focal plane closer to the lens, getting progressively farther away as you approach the center.
On a side note, soft lenses were first produced not to get a pictorial effect, but to get a deeper depth of field, done by spreading the focal plane. It was a bit later that photographers began to use the softness that produced as an effect.
Yes, one must always refocus, but especially so with dial-in-the-softness lenses. As you change the spacing between the elements to adjust the softness, you also change the focal length slightly, but enough to throw the whole image out-of-focus, in spite of the spread depth of field. I suspect this, coupled with faulty instructions to "focus, then dial in the softness" has led to a lot of confusion about the difference between soft focus and out-of-focus as photographers using that method got simply out-of-focus images.
I'll shut up now...
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