I'd like to see a source to the claim that at 40, fully fifty percent of the light that reaches your eye didn't get to the retina.
The abstract of this paper offers a data point that is consistent with the claim. For the purposes of this discussion, it is safe to say that the claim is not too far off.
It might sound too much of a difference, but it's not that apparent (you can try changing an f-stop on a camera for comparison).
(Probably there's some logarithmic sensing involved).
E.g. it's not like having a dimmer for your room lights at 100% and 50%.
I guess it's more like a 1-stop difference in photography (where again, half as much light hits the film/sensor from one stop to the next).
Here's an example with half the 50% less light hitting the sensor between the two pictures. It's not as dramatic as you expect (except in the "depth of field", e.g. how focused stuff is):
http://www.adammoroz.com/wp-content/uploads/2014/10/Untitled...