The numbers 1.8, 4.5 and 5.6 refer to the aperture of the lenses. The
smaller the number, the bigger it's light gathering power, and the
smaller the minimum depth of field you can achieve with it. The 50mm is then a
lot faster (i.e. gathers more light) than the zoom, by almost an order
The pros of the 50mm are then:
- better ability to take pictures hand-held in low light
- better ability to get a shallow depth of field.
One of the reasons the pictures taken with the 50 look so good is the
fact that with this lens you can throw the background out of focus. The
background-defocussing ability of a lens is proportional to the diameter
of it's entrance pupil, thus:
- for the 50mm/1.8, the entrance pupil is 28mm across
- for the 55-200 @ 55mm, it's 55mm/4.5 = 12 mm
- for the 55-200 @ 200mm, it's 200mm/5.6 = 36mm
This means that the 55-200 will also allow you to defocus the
background. You will, however, need to go to the long end of the lens,
which in turn means a long working distance: you have to stand far from
your model. With the 50mm you would stay closer.
Although I don't know this particular zoom, it's also common for this
kind of zooms not to provide a very good sharpness when used at their
long end and full aperture. In summary I would expect the 50mm to
provide nicer images and the 55-200 to be more comfortable to use.