[Sylvain]

Welcome back Sylvain

I have used the ZM 28 in the past, but not for very long, since 28mm is simply not a focal length I feel very comfortable with. However, I know the close siblings (from 25 to 50 mm), some of which I still use today.

I have not noticed the behavior you describe with any ZM lens so far. However, something I have occasionally experienced with ZM lenses is mechanical wear, usually resulting in some play on the focus ring or the lens tube. Is that something you notice on your sample? If so, the softness you see might be a result of partial decentering due to mechanical play.

Hi Markus ! thanks 

Good point ! Actually, I acquired the lens second hand with indeed a little play in the focus ring. I completely overlooked to mention that as I somehow discarded this issue as both close & far focus seem to produce sharp results. I would have thought that mechanical play wasn't coupled to the actual element but you seem to indicate it is a cause.  I will add too that the softness seems even across the field.
 
Markus, I have a lens spanner wrench and I see the rear element has the notches. From your experience, is that "play" DIY fixable?

Toni-a, thanks but I wouldn't venture too far doing this myself ...