To elaborate a bit on what others have said, the lens to which you linked is substantially overpriced. Similar lenses have sold on eBay recently for $10-$28 (mostly $10-$13) plus shipping. The Nikon f/4 50mm lens is a 4-element design, which is inferior to the 6-element design of the Nikon f/2.8 50mm lens. I happen to own samples of both, along with two other 4-element 50mm lenses (a Russian Industar-96U and a Durst Neotaron) and a 5-element lens (a Russian Vega-11U). Of these lenses, the Nikon f/4 is dead last in quality, whereas the Nikon f/2.8 is tops. This is based on side-by-side tests I've done myself. That said, the differences are small enough that they aren't really objectionable in modest (say, 8x10-inch) enlargements done at f/5.6 or f/8. The differences become more noticeable at wider apertures and/or larger print sizes (or greater enlargement, to be more precise). Differences show up most in edge and corner sharpness. Also, sample-to-sample differences in enlarger lenses can be significant, and used lenses (as most of mine are) may have been abused. Thus, it's entirely possible that my Nikon f/4 is a dud by Nikon f/4 standards.
Nikon's 6-element designs are generally considered excellent, as are 6-element designs from other major manufacturers. (Rodenstock, Schneider, and Nikon are the most popular enlarger lens manufacturers, but Fuji, Minolta, and various others make well-respected lenses, too.) Some, but not all, manufacturers use -ar suffixes for their 4-element designs and -on suffixes for their 6-element designs. Rodenstock and Schneider both do this, but Fuji and Nikon don't.
