How you focus your eyes can make a huge difference to your mental focus and productivity.
When you focus your vision on a narrow field, directly in front of your face and bring your eyes to a single point of focus, it creates a heightened state of alertness, focus, and cognition.
How do you achieve this focus? You want to keep your work or whatever you are focusing on directly in front of your field of vision. A book, a standard piece of paper, a smartphone, a tablet, or even a small laptop could fall into this category. If you are using a large monitor, you can still optimize this visual focus trick by keeping the document or spreadsheet you are working on in the center of the screen, not to extend past the point of your ears (roughly).
This type of focus does require a lot of energy, however, and this is why your eyes can get pretty tired after focusing on a narrow field for too long.
For every 45 minutes of this intense and narrow focus, it is helpful to spend at least 5 minutes allowing your visual field to relax into a broader gaze. Going for a walk and looking out into the horizon will achieve this kind of relaxation.
Allowing your gaze to shift from pointed to relaxed, can help to optimize the time spent in that focused and productive state without straining or burning yourself out.