Diopter and field of coverage (SORRY!)

SORRY, to bring up an over-discussed topic, but I just cant' find my answer by searching.

I know that I diopter lessens the distance a lens can focus, but what effect does it have on the angle of coverage the lens has?

Say the Nikon 18-70 DX covers about 70ish degrees on land with no diopter. So when you put on a +2 diopter, what is the angle of coverage? Does it change, or just the focusing distance? How much of the "wide" is lost?

Thanks a ton.....!!! ;-)

Happy diving,

Scott

Adding a close up filter decreases the effective focal length. So it should makes your wides wider.

How much the focal length changes?

EFL = 1000/((1000/FL) + DP)

1000/((1000/18)+2) = 17.37 | 18mm lens and +2 diopter

at 18mm it becomes 17.37mm