I have a 1600mm lens for a nikon camera?

How can i tell the distance? Its a 420 to 800mm, till i put the 2x converter on, then it becomes 840 to 1600mm. So how would i be able to know how far out i'm looking? I tried converting length but its not giving me the right one. Lol its saying 5 feet.lol. So please help me tell the distance. Thank you

Added (1). I mean i wanna know how many feet or miles i'm looking out.

Depending on how you bought the lens, new or used, there should be a dial on the side of it. If you are using a manual zoom, or automatic, that dial will still be there. If it has worn away, you may be lucky enough to have one of the lenses with the little tic marks between each major number.
The reason I say if you bought it used the dial may be worn away to where you can't see any numbers or tics.

"how far out i'm looking?"

Actually, the same as any other lens: infinity. But you're getting closer to it than with a shorter lens.

Distance will depend on the size of object you are looking at - you can see a jumbo jet aircraft a lot further away than a motorbike. Since you already have the lens you can tell simply by fitting it to your camera - that's the beauty of SLR cameras. That lens is quite powerful, so think of hundreds of yards, even miles. It is big and heavy, so should be on a tripod for steady pictures.

The optics built into that converter should maintain correct focus with the distance scale on the lens. Try to focus without the converter and compare with the converter. I just checked mine. It didn't change.

There's nothing in the camera that can tell you what you want to know, once you hit infinity focus. That is probably not more than 30 feet. After that, it depends on what you are shooting.

IF you are shooting a fixed object that you can find on a map, it's simple enough to locate your position and the other objects position.

If it is something not on a map, then you get to do the math… You have to know your focal length and angle of view at that focal length. You also have to know the size of the object you are looking at. With that information, you can correlate the size to the angle, giving you the distance. Perhaps someone here has the formula to make that work…