To clearify further the issue, a 75/1.8 and a 50/1.8 has very different depth of view. Both blurblok and ST1100 gave a good explanation of the 1.8 issue.
Assuming you have a subject 2m away from the focal point in the lens, using a fix aperture of 1.8, the results with 50 and 78mm lens are as follows:
focal length,depth of view
As you can see, if the lens does become a 78/1.8, the subject will have sharp nose and super blur ears at f1.8. That is not the case when you shoot such an object. So you will only see 78mm equivalent of image and lens performance is still a 50/1.8. It maybe a bit confusing to state that a 50/1.8 lens become a 78/1.8 lens when put on a DSLR.