There is a fundamental difference in image quality between a fixed focus head and a zoom head.

50 1.8 In shooting close-ups, the aperture 1.8 plays a very important role, the large aperture makes more light intake, soft in-focus, out-of-focus images soft as a creamy feeling. In terms of the effect of the film, it is closer to the visual. This kind of fixed-focus head shooting film is set of zoom head can not be compared, much, much worse.

And 18-200 its aperture and imaging, far less good than 50-1.8, of course, refers to the subject of shooting close-up. But the range of use is wider, and more subjects can be photographed. It's just a high usage rate.

The main difference is the image quality.