I hope someone can help me. I've bought a Nikon D810 for my wife who's a keen amateur. Can someone help me with choosing a lens? She takes portraits and landscapes primarily, and I have to say I'm confused at the volume not to mention jargon out there. It's her birthday present so I can't ask her and I have a budget of £600 max.
I'm with Matt. A lens is a very personal choice based not only on what one wants to do with it but also how one chooses to balance performance vs. cost vs. convenience.
She'll be much happier if you let her pick the lens. Unless she's already mentioned a particular lens that she's just dying to get neither you nor anyone here will guess correctly to surprise her with that "perfect" lens. This is especially the case since we don't know what Nikon compatible lenses she may already have. And since you're here asking I'm guessing she hasn't discussed her next desired lens.
Those are two very different things for Portraits a 50 or 85 is often considered the best options. For landscapes it varies greatly from wide angles, ultra wide angles, to telephoto depending on what you're trying to achieve.
First step will be determining if you want to get her a single lens that can do both decently, or two lenses. Or you could just get her one for now.
Nikkor 20mm f/1.8
This is right in your budget and would be excellent for landscapes. It can do portraits as well but would distort basically. It would hold up a little better with group portraits but still its a bit wide for that.
Nikkor 24-85mm f/2.8-4
Won't excel at either portraits or landscapes but will do both well enough especially for an amateur. This would probably be the lens I'd go with. Here's a Flickr pool for it.
Another solution is to buy older lenses. For example I've got an old 24mm Nikon lens that won't have some of the features of these but can certainly take great shots. That's the thing about lenses, if you take care of them they last and last. At worst you might need a tripod but then you need a tripod anyways.