Why can't I take a photo??

Question

I'm dorking around with a Nikon D80 and I am a total beginner. I am trying to shoot some light trails but the darn thing doesn't even take a picture. I am in manual mode with the bulb setting. For some reason, when I hold down the button, it does not take a picture. I can hold the button half way for the lens to auto focus (is that what it is doing when it is moving?), but when I press down all the way, the camera refuses to take a picture. When I go back to automatic mode, it happily takes a photo. What am I doing wrong? Thanks!!

Edited post: I am thinking it has something to do with how far/close the subject is. If there is something close, the camera doesn't want to take a picture, but if there is nothing right in front of it, it will take a pic in manual mode... (Generally hypothesizing right now.)

Answer: I think I found out what is wrong. The lens was on auto focus. I guess I had to change it to manual to use manual mode..... It works now (taking a pic of something close up that is..) Could anyone give a more detailed explanation as to what happened here? :)

Answer

Re. your answer - you don't have to have the focus set to Manual just because you're in Manual mode, but autofocus systems generally don't work in the dark. Therefore the camera will fail to focus and refuse to take a picture. By switching to manual you remove that problem.

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