Egrets, I echo Chuck's words. Good on you for looking after your Dad. I am sure best wishes go to you and your Dad from all on BF.
You are so right about light (and so is Chuck) and it can be changing by the minute and can make comparing binos really tricky. But another way of looking at it is if you need to search and scrutinise and agonise over whether a difference exists then it probably isn't important when using bins for nature observation.
If you can afford bird food then this can massively increase the variety of birds in your garden and if you can't (it can be expensive) then at least dig over a small patch so it is disturbed earth. This will at least bring Blackbirds, Robins, Dunnocks and quite possibly Song Thrush.