Indeed. There is no such thing as a "sphere of vision". Electromagnetic radiation in the visible spectrum which bounces off objects casts energy onto the water molecules in the retina of your eye. It's not like some video game where you have a sight range of a hundred feet. As long as that radiation reaches you, you can see it with proper magnification.
So say, for a moment, that your optical theory is correct. We can see oh, the 4000 miles away to the 35 mile diameter sun, but we can't see the five miles to the end of our 'sphere of vision' (called by us round-earthers the horizon)? If a continent is 20 miles away over the horizon, and we can't see it due to our "sphere of vision", why can we see 4000 miles to the 35 mile sun (35 miles being much less than the size of a continental coastline)? Why doesn't our vision just cut off?
If your sphere of vision is nonexistent, but the earth is flat and the only reason we can't see is due to our poor sight range, shouldn't a telescope be able to see that far? We have some very powerful telescopes these days. Try investing in one, and see if you can spot the Hawaiian islands from the California shoreline. You won't have much luck.