If you could buy only one portable binocular for terrestrial viewing would it be the Sport ED 10×42 or Ultra 10×50?
For myself, it would be the 10×42 Sport ED. Has center-focus, all magnesium/aluminum construction so is MUCH lighter and smaller, and chromatic aberration is almost non-existent due to the ED objectives. This is the only binocular I take when traveling light, because it can do just about everything, including astronomy, very well.
On the plus side for the 10×50 Ultra, while it is individual-focus (somewhat awkward when hand-held), the depth of field is quite good, so for objects beyond 50 yards, everything is in crisp focus- therefore not much focusing is required. The view will be a tad brighter due to the extra light-gathering of the 50mm objectives. The optics are superb (albeit with some chromatic aberration). It’s built like a tank, and should last forever. If you’re thinking about the Ultra, another to consider is our new 10×50 Deluxe. Has center-focus, is lighter than the Ultra, and optically is surprisingly-close to the Ultra considering the relatively low cost ($169). But for portability, my preference would still be for the 10×42 Sport ED.