What type of lines in linear perspective show that objects are increasing or decreasing in size?

increasing: diverging

decreasing: converging

In Photography, the effect is only an illusion and is not totally dependent on this. Relative size (and perhaps overlapping images) can do more to produce the effect. Want to have a bit of fun with this? There is a shot I used to show my students to illustrate the point of linear perspective that you can re-create. It works really well when done with slide film, since the image size can be made much larger.

Imagine 3 different colored pool table balls on the table, arranged so that none of them overlap yet are close together left to right, and placed at different distances from the camera. Let's say a red ball is the furthest away. The maximum separation should not be much more than about 8 inches from front to back, and no identifiable border or markings (such as the table side, markings/spots or nearby wall) can be shown. Take the shot about 45 degrees above and from one end, and all balls must be in focus with no reflections that produce identifiable images. It is a picture of 3 colored spheres on a sea of green (or whatever the surface is). Show the picture to different people, with the red ball at the bottom, left, right and top while asking them to tell you which is right side up (without any cheating such as looking at printing on the back). See how many of them get it right. Let them hold it after you've shown it in all four positions. The minor variation in the size of the objects they intuitively know are the same size will drive some of them nuts when they have no other clues to go by! If their brains don't translate the information (the ball farthest away is slightly smaller, but MUST be closer to the top of the photo if it is farther away) they won't be able to figure it out, or if they do, won't be able to tell you why. Below is an overhead view of the setup; the arrow being the direction the photographer points. Ignore the dots, they are only placeholders here as empty space is stripped out and the alignment gets messed up.

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