What is the F number?

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.