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Message: <blockquote data-quote="typo" data-source="post: 3322389" data-attributes="member: 83808">Sorry guys, the science observations behind the twilight factor is most certainly real. What got lost fron the original German puplications I understand is it's conditional on the pupil/exit pupil.It's probably also true as well that most birders have more complex priorities and go home way before true twilight when the more dramatic differences are evident. I spent a weekend in the company of one of the UK's top birders a few weeks back. He has zero interest in optics and had been using the same10x42 (TF 20.5) for the last 15 years. It was in for a service and he'd been given a 8x32  (TF 16) as a loaner and he was finding he rather liked the lower power and wide view but was concerned about low light use and was considering getting a 8x42 as a replacement. We turned round and headed for base when I reckoned pupil diameters were 3.2mm. He stopped looking for small birds at 3.5mm and went indoors at I'd guess 4mm, but not before he'd compared it to the 10x42 I had with me. He conceded he was still seeing more detail with the 10x but not enough to encourage him to stay out longer. Twilight factor worked as it should, but he didn't bird in twilight. As a comparative novice I was quite content to spend a bit more time watching the flocks fly in to roost and look out for the barn owl I'd seen earlier. For a while the 10x42 was a better choice but as it got darker still the 8x42 had an advantage as the EP was limiting. Unfortunately I hadn't brought my 10x56 so I followed about 5 minutes later. David</blockquote>

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