Some web cameras have a special button to capture frame than button pressed. This function called still frame capture.
In Video Capture SDK .Net you can use the following code to catch still frames.
Set Video_Still_Frames_Grabber_Enabled to true before you call Start in your code.
Use OnStillVideoFrameBitmap / OnStillVideoFrameBuffer events to get all frames.
Use Frame_Save method to save frame to file.
Use Frame_GetCurrent method to get frame as Bitmap.