Why does the vision change when Ant-Man shrinks?

The visual change when Scott shrinks for the first time reflects multiple things.

1. Scott is immediately disoriented by a (relatively) large drop, warping into Ant size, and the new perspective he would have based on size. Shock would daze him. Think of how people get blurry vision while drunk or after a hit to the head.

2. Scott and the Audience are given a new perspective, a point of view that is not normal. Visual tricks like Foreshortening will have a greater effect. Look at how people or cars look when you see them from a great distance/height. Small sure, but they no longer seem life-like. Basically Macro photography, in reverse. Tilt-Shift photography also provides a similar look. From Wiki:

The filmmakers made extensive use of macro photography. Production designer Shepherd Frankel said, "It's more visually interesting to depict things from Ant-Man's point of view instead of seeing him from a normal perspective. But we wanted a realistic realization, not Honey, I Shrunk the Kids with its oversized set pieces." Rebecca Baehler served as the director of macro photography, taking cues from Carpenter. Carpenter said vibration became "a tremendous problem" when moving the camera during the macro photography because "one inch off the ground is like fifteen feet in the air. From an ant's perspective, you move four inches, to a human perspective, that's a football field!" The solution required the filmmakers to think outside of the box so they turned to Baehler, who had a background in "commercial 'tabletop' photography." In order to add Rudd's performance as Ant-Man when in the macro world a Centroid facial capture set up was used, with a 5-camera array of Alexas surrounding Rudd.

3. Scott's perspective is very Foreshortened, because of the angle of view, the physics behind smaller lenses, and possibly the Ant-Man helmet.

Foreshortening is the visual effect or optical illusion that causes an object or distance to appear shorter than it actually is because it is angled toward the viewer. Additionally, an object is often not scaled evenly: a circle often appears as an ellipse and a square can appear as a trapezoid.

A more acute viewing angle introduces much more Perspective Distortion. Looking at the Empire State Building from it's base is way different then from a mile away.

Even just using a different lens (In this case, A normal eyeball vs an eyeball that's fraction of an inch wide) causes distortions as well:

But most of all, that's basically how the view will look for an ant. As Pym says in that scene: The world sure seems different from down here, doesn't it, Scott?