When the Exposure Delay Mode is set to ON, D90 introduces a delay of about 2 seconds between the mirror flip and the shutter release. The purpose of this function is to eliminate the vibration caused by the mirror flipping. When I use a tripod to take pictures of ‘stationary’ subjects, I always use this function along with my wireless remote. More than once, I made the mistake of leaving the setting ON, causing delay in normal shooting conditions. If you use this feature, make sure to turn it off, at the end of your shooting. “You learn from my mistakes”.