The human brain is not restricted by a small 2D screen. It senses distance/3D by blending the slightly different images from both eyes in to one. Eyes never sit still, they constantly move. We are standing at the inside of a gigantic sphere. Eyes compare the size of objects, to assess dimensions.
Handing out 3D goggles to your audience is not an option (at least not today), so the presentation designer has to resort to tricks to create 3D effects.
- Pay attention to camera position (earlier post)
- Put a known object in the image so people can relate the size of the whole to the familiar dimensions of the object (earlier post).
- Or use effects like the one used in the image below. Stitching together multiple photographs to create on large, distorted image that gives the illusion of standing inside a sphere. Your eyes are really running up and down the image, just as you would do when you would stand inside the cathedral for yourself. Huge image by balondrotor here. (Earlier post on a similar but less spectacular version taken in the Notre Dame)