Welcome to Nikonians! Congratulations on your new D5100.
>Experimented in green Auto mode, but camera >insists on popping flash on some images even in full sun... >why does my unit not understand it has a 1.8 f lens to work >with?
In Auto mode, the camera maintains complete control of exposure and when the flash pops up, for some reason the camera thinks fill flash is required. To prevent the built in Speedlight from poping up, set the mode selector to flash off mode. You can also try using Program (P) mode where the built-in Speedlight will only pop up when you turn it on by pressing the flash button on the front side of the camera.
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