I'd like to show an example of the difference 2 stops make to an image. by that I mean I took images, one at f5.6 and one at f11. That is 2 stops in total. That means that only a quarter of the light gets through at f11 as got through at f5.6. Stopping the lens down one stop will halve the light getting through, causing the shutter to stay open twice as long. That is not the only result of stopping down, the depth of field will also increase resulting in a totally different image. Why photographers love a fast lens (i.e. A lens with an aperture of f2.8 or faster) is that they can control the depth of field.

This first image was shot wide open on the mZD 14-150, f5.6. Look at the mosque in the background. It is not sharp but also it isn't perfectly out of focus either. A lens with an aperture of f2.8 would render that building as just a blur in the background. 

The image tomorrow will show the mosque taken only seconds after the above image taken at f11. Amazing what 2 stops can achieve.