I don't know if its because i am new to photography or there's something wrong with my camera but my question is,is there anyway of testing the camera to see if its working right? the problem i am having is on some days when i use the camera my photos are bright,clear with vibrant colours and most days they seem dull and flat I am using manual mode and metering which i think i am using right but the pictures seem dull and flat most of the time. i am using the 18-105 kit lens but seem to get diserpointed with the results.Any help or suggestions will be appreciated.Thank you.
Most likely your pictures are over or under exposed. If you are getting quality images some days but not others I would assume that your camera is functioning fine. I would suggest shooting in aperture priority or shutter priority for a while and pay attention to what your camera is doing with the non priority setting. Shoot in aperture priority if you want to control DoF and shoot shutter priority if you want to stop motion or blur motion. The D90 is a great camera and should treat you well.