This is the location of the Exposure Compensation on the Nikon D3S. You push this and spin the dial on the back of the D3S to under or over expose the photo.

When you make a picture you need to be in the habit of checking to be sure that first photo is exposed the way you want it to look.

In manual mode you can adjust the aperture or the shutter speed to make the photo darker or lighter.  However, when you are in Aperture Mode and you change the aperture or when you are in Shutter Mode and you change the speed the exposure stays the same.

Exposure Compensation Dial on Nikon P7000

The only way to make the photo darker or lighter in Aperture Mode, Shutter Mode and Program Mode is to adjust the exposure compensation.  You can also use exposure compensation in the manual mode and it will adjust the exposure using ISO.

To understand how this works I recommend an exercise.

Take photo with exposure compensation set at 0. Take a few more photos -2, -1, +1 and +2.  Bring them into your editing software to see the different exposures.

If you shot these in RAW and then you made adjustments in your software like Lightroom or PhotoShop you will notice that the dynamic range in one of the exposure you did will give you more desirable results for your personal preference.

[youtube https://www.youtube.com/watch?v=HbV1fckAi5E]