Paul is right, for hunting, there isn't a need for it. Plus it would add a whole host of its own potential problems in the field. Dirty lens, smeared lens, scratches, fogging, you name it.

If you are talking about target scopes for 3D and spots, there are a ton of options out there.But, like Paul pointed out, its an expensive endeavor. I'm finding this out first hand right now. I am putting together a spot/3D and am looking at buying second hand equipment because its so expensive.