Geena,
This is part of ther problem with digital cameras , you don't know how to relate focal length unless you know the size of the sensor or the actual image size on the sensor .
18mm is fairly wide angle but 55mm is not much over standard lens , possibly a small telephoto . Also the smaller the image area the less pronounced depth of field is , that's why most video cameras have a deep depth of field .
Looking at your pictures again I can see a compression of the image on the 55mm setting , the rails on the balustrade appear closer and thicker .
