Sight alignment and sight picture are two different things. This holds true whether you are talking about marksmanship, high-level precision shooting, competition shooting, or defensive shooting, and whether you are shooting a rifle or pistol.
This is the relationship between the front and rear sights on your rifle or pistol. There should be equal amounts of light on either side of the front sight post inside the notch of the rear sight, and the sights should be even across the top. That gives us an alignment of the gun along what we’re looking at. And what are we looking at? The threat … and this is where sight picture comes in.
Now we take the front sight and superimpose it over what we want to hit (the threat). We still have the alignment in the background but now our focus is very specifically on the front sight. So we have a hard focus on the front sight but we still want equal amounts of light on either side of the front sight post and the sights even across the top, all superimposed on what we want to hit.
When doing sighted fire, we want to maintain our sight picture throughout a smooth trigger press, whether during firearms training or practice, or actual tactical, competitive or defensive use of the gun.
IN THREE DIMENSIONS
Looking at it in 3D, we have the two rear-sight posts, the front sight, and the bad guy. Again, hard focus is on the front sight. The rear-sight posts will be somewhat blurry, and the target/threat will be out of focus because we can only focus on one distance at a time.
The other important concept to understand is that if we are going to try to align these three objects in space in front of our eye, we must close the support-side eye (the eye that is not behind the front sight). Otherwise the input from two eyes will give us a double or ghost image of the threat, and we may have a difficult time obtaining even sight alignment.
Practice this during your regular shooting drills so it will be natural if you ever need to shoot in defense of your life.
In conclusion, to have both sight alignment and sight picture working for you in a defensive situation, you’ll want to close one eye when aiming and firing.