Depth of Field Control
Why do some lenses offer better control of depth of field than others? For instance, my Nikkor AF-S 18-70 f/3.5-4.5 performs very poorly in the this regard, while my humble little 50/1.8 is excellent. About the only way I can blur the background with the zoom is to shoot wide-open from the absolute minimum focus-distance from the subject, and even then the effect is mild. Shooting portraits, even wide-open at 70mm, is hopeless. Yet the fixed AF 50mm offers plenty of depth-of-field control. Educate me, please.