What is focus?
Focus means "to cause to be concentrated, to pay attention to the task at hand and not let your thoughts wander". You can focus on what you see or what you hear, sometimes both. Rather than think of focus as a whole you can split it into two categories.

Automatic Focus
This is fun, focusing on something you enjoy is easy, a game of football or a good movie for example. This kind of focus does not make you tired or bored and you can keep your focus for a relatively long time.

Concentrated Focus
The more difficult of the two as you have to concentrate on what you are doing which is focusing on the task at hand. Whether it is sitting at a seminar and listening to a lot of speakers or sitting at the office trying to finish your paper work.

Why cant i focus?
For how long can you focus on any given subject or object?

Focus period
You focus period can be very short, it varies from person to person and what the subject is. But whatever you are doing your focus period comes and goes, it fades in and out. Imagine you are trying to get a picture on your television and you have a bad signal, the picture comes and goes, just as your focus period.

Noise
Noise will also distract you, not necessary a loud one but a constant noise that is there all the time and eventually it iterates you so much that you start to focus on the noise. How many times has that happened to you?

Day dreaming
You are driving your car; focusing on the road ahead then all of a sudden your car turns suddenly. You have started to mix together other thoughts, fantasy and reality become one while driving. If you look at something long enough it starts to change shape, you are getting tired. Back up