a hard edge gnd, has the horizon line "hard", and the soft edge gnd has the horizon line "soft"
this is so helpful i realise, so i koped some stuff from wiki
the top one's the soft edge, the bottom's one the hard edge
what is different is the scene you will use it for. for example, if you have nothing which cuts above the horizon that you want details for, e.g. something with a treeline, like upper peirce, then a hard edge would work best. for anything else, soft edge will be good.
i myself use hard edge, which gives rise to very funny halos somethines because i have to place the horizon line higher than it really is for the details i want. hopefully i'll be able to get more variety soon..
by the way it doesn't stop there. singh ray even has reverse gnd. this puts the "hardest" point at the horizon line.. good for shots where the sun is just about to disappear below the horizon.
there is also "gnd full" which means the gradient is from top to bottom. useful for certain types of scenes also , where the brightness is really gradual from top to bottom. as to what, i'm not exactly sure myself.
The soft GND filters give a more gradual transition as compared to the hard at the edge.
A quote from cambridgeincolor : " GND filters come in many varieties. The first important setting is how quickly the filter blends from light to dark, which is usually termed "soft edge" or "hard edge" for gradual and more abrupt blends, respectively. These are chosen based on how quickly the light changes across the scene, where a sharp division between dark land and bright sky would necessitate a harder edge GND filter, for example. "
I currently work with a couple of soft edge GND, but will get a hard edge before my nxt holiday trip