unexpected results!!! 50mm 1.8D vs 1.4G - explanation challenge

Started Oct 2, 2011 | Discussions thread
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Alex Notpro
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unexpected results!!! 50mm 1.8D vs 1.4G - explanation challenge
Oct 2, 2011

I'm upgrading my D5100 to a D7000 and I figured I'd partially fund the upgrade by liquidating my 50mm f/1.4G for a 50mm f/1.8D (netting $300)... since I found myself shooting more at f/2 to f/4 anyway to get decent DOF, I thought, "why pay more for the same results".

So I've got both lenses with me right now... I did a little controlled test, and I'm shocked by what I found:

When shooting at f/1.8, the 50mm 1.4G gives me "almost 1-stop" better performance versus the 1.8D: with a fixed shutter speed of 1/50s @ f/1.8, the camera selects an ISO220 on the 1.4G and for the same composition, ISO400 on the 1.8D.

I could not believe the result so I repeated the test in a different composition, but the result was the same.

Is there a logical explanation for this? What specification on these lenses explains the almost full extra stop of light-gathering performance at the same aperture and focal length??

I also got different White Balance behavior between the two lenses, but this was not consistent (1.4G warmer in one case and 1.8D warmer in another).

Please consider my post an "Explanation Challenge" I'm hoping for something more than "duh, it's a G lens, of course it's better than the D lens".

Thanks

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