It depends on what you want to photograph, assuming you need
this and not just "view" (that's what binoculars are for). I
suggest a 200mm telephoto as a minimum, ideally 400mm. As price
increases considerably, you can use an adaptor to increase focal
length. A 2x adaptor will provide 400mm instead of 200mm, but you
will lose on quality. Also, considering the distance, you will need
a tripod to shoot steadily at such a distance. Some telephotos will
have stabilization (VR, IS etc depending on manufacturer) but best
results will be provided using a tripod.