The Nikon web site has a page of useful information about eyepiece correction lenses here: http://www.nikonmall.com/buynikon_assets/Eyepiece_Compatibility_Chart_ 040917.pdf Some of the more interesting info there is: With the standard, clear glass eyepiece, the viewfinder has a base diopter value of ?1 The marked value of correction lenses is the combined diopter value of the base viewfinder diopter value plus the actual diopter value of the correction lens. (e.g. a +1 correction diopter has an actual diopter value of +2). A neutral correction lens has +1 diopter value and gives the viewfinder a +/- 0.0 diopter value. It is not the same as the standard eyepiece lens. The apparent viewing distance of the viewfinder image is 1 meter.