This is probably the best single resource on the matter:
http://pistol-training.com/articles/vision
It is a detailed article, but here's the gist:
The eyes should focus on the front sight, but converge on the target.
This will result in the shooter seeing "two sets of sights", and may be difficult to focus on the front sight with the vision converging elsewhere.
This is solved by (shock) training and practice.
Being able to do this with your vision is the single largest leap forward in handgun proficiency other than eliminating the pre-ignition push.
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