Coolpix P6000 and 20-60X eyepiece (at 20X)
The camera has a zoom lens (4X) that ranges between 6mm - 24mm (35mm equivalent is 28mm - 112mm). Vignetting is visible w/ the 20-60X zoom eyepiece, at 20X, until you zoom the camera to about 18mm (or 72mm in terms of 35mm equivalent). Using Mike McDowell's equation the Effective Focal Length of the digiscoping rig is 800 - 3200mm.
Nikon V1 and 20-60X eyepiece (at 20X)
Nikon V1 and 40X W eyepiece
The effective focal length of this digiscoping rig is ~1900 - 5800 mm. Vignetting is eliminated at about 18 mm zoom on the 10-30 mm lens.