Measuring Focal Length - 50mm f1.8D AF Nikkor
Prepared 2005-11-15 (146/42925) by Bill Claff (Revised 2006-01-18)

Chart

Data was collected using a set of Kenko extension tubes. A chart of the data follows:

Results

The focal length is 52.8mm. Above the 50mm published value but in agreement with anecdotal evidence regarding Nikon lens design.
The lens focus extension is 8.3mm. This checks by inspection.
The lens focuses entirely by extension. This is why the blue and cyan data points are nearly identical and is obvious by inspection of the lens.

Conclusion

Since the lens focuses by extension there are no real surprises.