Sounds interesting but other factors do effect the readings - time of day for instance.

For the cost of a roll of film and processing I took a day and from my upstairs bedroom window took photos every hour from sunup to sunset, in early June and again in August as the arc of the sun does change. I then made a sketch of my yard and location of planting beds and was able to calculate hours of sunlight on each section. Some areas got only midday light, others only early am or pm and actual amount of light was considerably less than I thought.

But anything that helps pick plants appropriately is very appreciated!

I'd say it's pretty useless. During the course of the summer the position of the sun swings so widely that a bed that now gets sun almost from dawn to sunset will get almost no sun but around noon because as the season progresses the bed is protected by a bog tree on the east and another on the west.
It's best to know the arc of the sun and stop to notice what will shade what bed and when.
Of course I feel much the same way about those moisture meter things some people use in their pots.
Linda C