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Today I tried out my 50mm f1.8D in PC fair. I found out that when I move my focus point to the red circled points (outermost), the image area that i focusing on tends to out of focus. (very obvious when using "Single Point" in AF-area mode)
This problems seems unsolved and not very sharp when I used 3D-tracking (11 points) also. Orange circled area also blur a bit?
When I use the middle focusing point only can get very sharp image.
My friend told me that that's the problem of 50mm f1.8D. Only the middle focus point is the sharpest. Is that true? If that so then i cant take full body portrait or half body already(focusing on face which using the red circle)
I was using Nikon D90,SB-900, 50mm at F2.2 or 2.5
Is it the weakness of 50mm f1.8D? or something wrong with my lens? or the camera operating mode?
Sifu-sifu please enlighten me on this please....

I notice this problem again when doing Christmas shooting on yesterday.....all focus points produced blur photo except the middle focus point
I want to do some test to see whether is my lens problem, or is my D90 body problem, or is my hand problem?