Originally posted by Lenscap That's all well and good if you're shooting a scene where the horizon is more or less flat. You can apply ND effect to the sky, and not to the ground, or ocean, or whatever.
Yep.
Originally posted by Lenscap But what if the horizon is not...well, horizontal. Mountains, maybe, or trees, or anything poking up into the ND part of the filter. Will they be underexposed?
Yep.
Originally posted by Lenscap Do you fix it in the darkroom or PP?
Yes, with masking but if you are going to have to do that anyway I prefer to just bracket and blend in software.
Originally posted by Lenscap Is there not enough difference to worry about?
Sometimes it just does not make much difference, it depends on the difference in brightness between the sky and the not sky, as well as the strength of the ND grad and whether it is hard or soft.
I use ND grads if there is a reasonably horizontal horizon. If there are rough mountains or other non-horizontal parts I just shoot it bracketed and combine in software.