I bought a Vivitar 70-210 Macro Focusing Zoom lens for my Nikon D3000. After using it for a while, I've noticed that a lot of my shots are slightly out of focus - a side effect of my inexperience with manual focus lenses, as well as the small viewfinder in the D3000.
The d3000 has an "Electronic Rangefinder" feature that uses the autofocus sensors and the exposure meter display to tell you whether to focus closer or further. Unfortunately, this feature doesn't work in Manual (M) mode, presumably because it wants to display the metering info in manual mode. I do get the rangefinder when I go to P,S, or A. Since I don't get any metering info anyway (this is a non-CPU chip, so the camera has no idea what my aperture size is), I don't get much help at all from the camera in focusing or exposure. (It does, however, light up a dot in the lower-lefthand side of the viewfinder when I'm in focus.)
So, is there a way to force the camera to display the rangefinder information in Manual mode?