What happens when you put a cheap lens on a expensive body?

I"m thinking about getting the Nikon D800, though since the body is 3 grand, I won't be able to get nice lenses right away. I heard that lenses are more important then a camera body, though the camera body I have right now is old and is going to crock anyday. Is is it true that any Nikon lens will work with the nikon d800, unlike canons 5d mark iii that will only work with lenses that are EF, not EFS? That is the only thing that put me off from the 5d mark iii. By cheap lens i'm talking about lens that are from 300-600. I probably won't be able to afford one that is anymore expensive then that for some time after the body purchase.

Added (1). By 300-600 I meant dollars, not size.

There are NO inexpensive 300 mm to 600 mm lenses (cheap indicates poor quality).

All but for a few under $400 AF-S lenses will provide you with excellent images when using the D800. Make sure that you buy FX lenses so you can fill the frame of the D800. If you use a DX lens, the cameras will automatically crop the image to prevent vignetting.

I'd assume it's the same with the D800, but not sure. I know the other Nikon fx cameras work with the dx lenses, but not the same as they do with the fx lenses. What the Nikon does is crops the image on the sensor only using part of it. This lowers you resolution and results in the same image and crop factor you would get with a crop sensor camera. Useful if that is the only lens you have, but I don't see any reason to buy it for that reason. The canon 5D does only work with EF lenses, but there are plenty of cheap EF lenses. EF-s lenses are just smaller and have less glass. It's not really that they are cheaper. I believe Nikon fx lenses do cost more, but not sure. I shoot with a canon 5D.

If you buy that camera you are going to want really good lenses. You can start with the 50mm f1.4. If you are going to get into this camera you better be ready to spend more money because if not you will be wasting the money you spent on the body by shooting through bad glass. It's like buying a Ferrari and putting a little 4 cylinder Chevy engine in it. The 70-200 f2.8 is a great lens for that body.

If you are not prepared to spend this kind of money. Buy a cheaper body and better lenses. The lenses are so important.

What happens when you put glasses with cheap lenses on over your eyes?

Same thing.

Nikon calls their EF-S equivelant DX.

You could stick a 50mm f1.4 on it. Or the 24-85 f2.8-4. Those are pretty sharp, full frame and reasonably affordable on the used market.

$300 isn't cheap to me at all when I'm able to buy $10 lenses for my dSLR. It's not the cost that matters. It's the mount. As long as it fits and works then you shouldn't have any problem. Then again, I'm not totally sure about that.