I had a pair of Leupolds, absolutely loved them (stolen). I currently have a pair of Steiners. Bought them cheap when they discontinued the line & wanted to use the name on a different design. I won't by standard binoculars anymore, the fixed focus is such a pleasure. Pick an intermediate distance & focus there, everything from 50% of that distance to 200% should be in focus. Instructions from both manufacturers recomended focus @ 2 or 300 yards but if you're going to sit on the back porch & watch a few feeders, several hundred yds would be unworkable.
So it's a function of depth of field (the distance in front of, and behind, the focal point that remains in focust).
Deeper focal fields require a smaller aperture, meaning less overall light. It also allows the mfgs to be a little more sloppy in their construction and in the precision of the focal mechanism as a smaller aperture has the inherent property of making things appear more in focus (like squinting your eyes to sharpen up the focus on something). In daylight situations that's usually not a problem given the relatively large objective lens sizes for most binoculars.
In low light, though, you need a wider aperture to maximize light transmission. That shortens the focal field and requires more precise focusing mechanisms. Given a large objective, large aperture, good glass, and high end anti-reflective coatings, you can get units that gather and transmit available light so well they will give cheaper NVG's a run for their money. Problem is the depth of field can be so short as to be measure in mere inches, requiring constant focus corrections.
Trades offs. Always trade offs.
Brad
