Why doesn't TTG spell out “light”-related limits?

Because for almost any common kind of “light”-related change, it is possible to picture a situation in which that change would be necessary for the photograph to meet P7.

For example, there aren’t any “light”-related changes more radical than changing every color in a photograph to its opposite—

—and yet that is what has been needed to produce a trustworthy result from billions of photographs recorded onto film negatives.

(Converting “negative” images to “positive” is of course allowed by TTG.)

•

Summary: When discussing allowable “light”-related changes in TTG photographs, what matters is not the nature or degree of the change, but whether the result meets P7.

See the Background brief on light if it is unclear why TTG severely limits non-“light”-related changes.