A glass adapter, which means that it has a 1.4x optic glass pieces inside it to make up the focal flange distance difference between the 2 camera systems (distance between the lens flange and film/sensor plane), allows an older lens to focus to infinity properly on the newer camera body.

     Glass adapters can sometimes degrade the image quality of the lens; because you are adding a piece of optic glass that is not specifically designed for that particular lens, sometimes there can be an adverse reaction in the optics. This can cause soft focus, chromatic aberration, image artifacts, etc.

     Unfortunately, it cannot be narrowed down to certain lines of lenses; any reaction is going to be specific to the particular lens. Some lenses may not have an even interior coating or they have a flaw in the optic glass that under normal use you would never notice, but when combined with the glass adapter the optics bounce the light differently and it can bring out these flaws. The only way around this is to have an optics master design the glass specifically for that individual lens, which can be rather cost prohibitive and not possible for a off the shelf adapter.

     We don't want to scare you off from trying a glass adapter out, on about 80% of the lenses there is no visible effect and no problem with the adapter; but every now and then we do come across a lens that will react badly. The only way to know for sure is to try it out on your lens and see if the results will work for you or not.