The D300s is another model in Nikon’s lineup to include a “Quiet” mode (I’ve seen this feature in the lower D5000 as well). Selectable on the camera’s drive mode dial as “Q,” the mode opens the mirror and lets the shutter do its thing as normal when the shutter button is pressed, only it slows the mirror’s return to a lowered position.
The sound that the shutter itself makes is actually pretty quiet. It’s the mirror quickly slapping down that creates the D300s’ very loud and authoritative “shutter” sound during normal operation. In practice, I’ve found that slowing the mirror’s return in “Quiet” mode significantly reduces the noise that the D300s makes. On top of this, the mirror’s return is tied to the shutter release button in “Q” mode, meaning that the mirror will only return to its lowered position when the shutter button is released. So if, for instance, a shooting situation calls for as little disturbance as possible, you could conceivably press and hold the shutter release, taking the shot, and keep the button pressed down until you remove yourself from the situation, release the button, and let the mirror back down at a slower rate.
How well does it work? Well, Suki is a pretty light sleeper, and those big triangles she has for ears are pretty sensitive. I’m very close to her in this shot with my 35mm:
We’ve been using the heater a lot this winter, and Suki’s favorite place to be is right over the vent when it’s on. I figured I’d use her heater nap-time to see how close I could get to her with the camera without her noticing. I woke Suki up with my first shot, so I went into “Q” and waited for her to drift back to sleep. I then took at least 15 images without her noticing!
Staying in “Q” also hushes the mirror when switching live view on and off. Very cool.
Jonathan Fleming's Blog 
Wow, recent digital cameras have such cool mode! It helps a lot to take sleeping cats & dogs:)) And when not sleeping, some of them are sensitive and upset by the sound cameras make, so the function is really nice I guess:))
Suki-chan ホンマにかわいい〜〜〜！
That is what I desire most. A quiet mode! Especially for my type of shooting being street a good bit of the time. Nice going and nice shots here. To take images of a sleeping dog says alot about how quiet it is.