probably because a spotting scope has a huge objective hols... measure it, what's yours?... A spotter lets in a lot more light.
A 56mm scope has twice the lens area almost of a 40mm scope to let in more light. An 80 has twice as much as a 56.
It's also a fixed mag, so has around 4-5 less lenses. With each lens losing say even 1% a fixed mag will lose around 10% in transmission
but a zoom has another say 4-5 lenses so loses another 5%
And that's assuming 1% is correct, and that's assuming that the 1% stated is across the whole visible spectrum as well, which is probably not correct.
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