I'm not sure exactly what you are asking.  but indeed light and dark
adaptation is exactly the reason that the human eye is able to function
over a range of approximately 5 log units of light intensity.

anyway a little web searching will give you the answers that you seek.
here is a site you should look at first as it is easy to understand and
quite succinctly accurate.

http://webvision.med.utah.edu/light_dark.html

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While the iris is tied to some measure of overall brightness, the
"adaptation" of aach photocell is nothing more than the amount of
pigment (rhodpsin for rods, one of three photopsins for cones) that
the cell can keep "in stock".  

The pigments are "destroyed" by the process of catching a photon, and
must be regenerated.  Add the regeneration rate to the destruction
rate (i.e. the number of photons hitting per unit time), and you get
some specific level of pigment.

So is your suggestion valid?  To some degree, almost certainly.  But
pigment regeneration does take time.  A full 30 minutes for the rods
to be fully stocked with rhodopsin after leaving a bright room (once
full, the rod will fire from a single photon hit).

For any given ambient lighting level, normal visual scanning will tend
to average out the pigment levels of your photocells on the whole. Any
increased dynamic range you get from individually adapted photocells
will have to be due to however much pigment is created in the few
moments you spend looking at one particular spot.  

The effect can probably be noticed if you find a particularly detailed
view where one half is in light, and one in shadow.  After fixing your
gaze on the border, details in the shadow half will become clearer as
time goes by.