Most newcomers to photography are confused by the F number setting. To put it quite simply follow this decision tree :
1. Decide what parts of the image you want to have sharply focused. Here is a picture of a boy at nightime :
Since I wanted the model to be in sharp focus with the background nicely blurred I chose F1.8 as my aperture. Larger the aperture gives a smaller depth of field (DOF).
2. Smaller the F number bigger is the aperture. This is because the aperture size is measured as a fraction with the denominator as the F number. Hence aperture of F/4 is smaller than F/2.8.
3. Checkout the picture below :
I wanted all the pillars to be in sharp focus so I increased the F number to F/6.3. Both foreground and background is sharp.
4. Larger the F number gives a bigger depth of field.
PHOTOMANTRA : Small F numbers for portraits.