Hmmm, actually that the pupil generally dilates in certain distance range, especially for distant viewing, is totally new to me. I would more likely assume it depends on the "attention" the viewer pays to the object of interest, as also Wikipedia cites it here:
The Task-evoked pupillary response is the tendency of pupils to dilate slightly in response to loads on working memory, increased attention, sensory discrimination, or other cognitive loads[4].
4. ^ Beatty, Jackson; Brennis Lucero-Wagoner (2000). "The Pupillary System". in John T. Cacioppo, Gary Berntson, Louis G. Tassinary (eds.). Handbook of Psychophysiology (2 ed.). Cambridge University Press. pp. 142162. ISBN 052162634X.
which can also occur when viewing near objects!
Lighting is a totally different issue, however also here the retina does the main work, as is usually assumed, as the pupil area in older subject might only change by about 1:8^2 ~ 1:60, while the visual system can adopt to lighting conditions over say 1:100000!
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