This looks like veiling flare, a symptom of the Aspheric lens element in the lens separating.It certainly is not blooming :-( Sorry Patrice...
Thank you all for your opinion...I'll probably consider a digital-optimized 18-80 or similar (don't know yet which brand and model), not necessarily compatible with my MZ5n (as I have the Tokina 19-35 and a PTX 80-320), and a used M-50 1.4 that will fill the gap between 35 and 80mm and allow macro with extension tubes.
Thank you again. Patrice
