If by knowing the FOV of my glasses and the average wingspan of the bird in question, would I be able to get a reasonable estimate of distance away from me by using the following calculations ?
Let's use a hypothetical FOV of 300 ft/1000 yds and a bird with a wingspan of 3 feet. If the bird fills my field of view, then I would estimate that he would be 1/100 (3 ft / 300 ft) of the 1000-yd distance, or 10 yds, away from me.
Also, let's say the bird is far enough away that I guess-timate that 20 of them would fill my field of view. At 3 feet each, that would be 60 feet (3 ft x 20 images) of bird-span. That works out to 1/5 (60/300) of the 1000-yd distance or 200 yds away.
Does this sound right ?
Bob & Jane - birding since April 2009
Lifer count : 110
Newest : female Belted Kingfisher
