Hi there, I'm Andrew, and I wrote the SLRgear review that no one cares about.
I stumbled upon this thread looking for something else, and thought I'd chime in on a few points.
People have been asking about Blur Units. We use a software program called DxO Analyzer to produce our results, made by the same folks who created DxO Optics. They came up with the Blur Unit (or more properly, "BxU", or Blur Experience Unit). Here's an excerpt from an
article describing our testing process:
"BxUs are a mathematical measurement that correlates very well with human perceptions of image sharpness. One BxU roughly corresponds to a "blur more" operation in Photoshop."
With that out of the way, I should underline that it isn't a great unit of measurement, and it's widely misconstrued, but it's the only one we have. We go through a
great deal of effort to get what we think are the most accurate results for a given sample of a lens, but even then, it's important to note that you're not likely to get the same accuracy we are. Focusing systems are surprisingly inaccurate, you're likely not using a rock-solid tripod, and your subject may be moving.
To the original poster, I would actually suggest the 35mm f/1.8 DX, which provides an equivalent field of view of around 52mm on the D300's sensor, which I'm assuming is a factor in the equation (the 50mm would offer a 75mm equivalent field of view). If sharpness is what you're after, the 35mm is actually sharper than the 50mm at f/2 (assuming you put any stock in our testing), but as has been stated, as you stop any of the three lenses down, they reach a point where they're all very sharp. Re the 35mm f/1.8, if you're ever interested in moving to a full-frame camera, it actually works surprisingly well on an FX body (there's just a bit of vignetting when the lens is focused at infinity).
I appreciate the point that has been raised concerning shooting in the real world, compared to shooting test charts, which is why we have a standard test scene that we have shot to accompany all of our reviews. This gives you additional information concerning how a lens renders out-of-focus elements, colour rendition, and what those blur units actually mean. The fact is that 3-4 blur units isn't unacceptable, in fact, it's fairly sharp.
If anyone has any questions concerning our tests, I'm happy to answer them, and you can email me through this forum.
Best regards,
Andrew Alexander