Also not mentioned but reasons to upgrade in it's own right. Are features & controls! Geeze people pixel peep to the micron level. And then don't even consider the real world benefits of a more capable camera.
As mentioned a built in motor for AF,AF-D and G lenses is a big plus. A Bigger Brighter viewfinder. Dedicated controls to make adjustments to capture without the need of your eye even leaving the viewfinder. No More take camera down from eye,Menu dig for changing setting and hoping your not in too bright of light where you can't even see the damn screen.
Throw in superior bracketing,faster AF and FPS. And you got a serious tool D90 to grow into. The only thing you can do with a D40,D60,D3000,D5000 is grow out of. My D40 became too frustrating to use after about 9months. And for me part of the joy of this hobby is to eliminate frustration and add ease and joy on getting the pic!
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