What is the cheapest yet still good zoom lens?
You are in a range where cheap means poorly made optics and there are NO inexpensive, high quality lenses. Nikon makes an excellent, relatively inexpensive 200-400 mm f/4 ($5,000). With the new low noise at high ISO’s cameras like the D3 and D700, means this lens has taken on a new life. 400 mm f/2.8 lenses ($7,600), while still an excellent lens, is no longer necessary under low light conditions.
