wel, that didn't take more htan one google serach - my old (and dirt cheap) Nikon D40 has this setup option:
Quote:
USB Menu
This selects how the camera behaves when plugged into a computer via USB.
I leave it at mass storage, which means my D40 appears as an external hard drive between which you can drag and drop images and folders in Mac OSX Finder or Windows Explorer.
PTP is used if you want to control the D40 as an external device, for instance, via Nikon Camera Control Pro for remote camera control. PTP makes the D40 look like a device instead of like a drive.
Use whichever works best with your computer and workflow.
Nikon and many others have created software for it. Have not really looked for the right solution but here's a youtube clip showing the basics:
'
From what I can gather, the software used in the video is free
I'm sure if I had a real goal, I could probably build my own webcam app using these tools. In fact, I bet there's a lot of people out there reselling stuff based on these libraries.