So I was comparing my 17-50mm Tamron with the Nikon 50mm F/1.8 to see the differences in bokeh and colour rendition with my not-so-proper test scene below and I stumbled onto the fact that at 50mm, the Tamron produces an image that is shorter than the 50mm Nikon. I got similar results with the 18-70mm Nikon as well. Check out the images below to see what I mean.
So I wondered, how does one measure a lens' focal length? Below are two links that are useful, but technical. Take a read if you're so inclined. Me? I will probably get round to measuring my lenses when I really have nothing to do. In the mean time, I'm going to set up the test scene a little better for the bokeh / colour comparison. Any suggestions?