Hi welcome to the forum.
From what you said, you seem to select binoculars in a scientific way. Actually, you can get more detailed information in internet like Wikipedia:
http://en.wikipedia.org/wiki/Binoculars
For me, choosing binoculars have the following concerns:
i) Quality. This is easy to know by observing other birders. Swarovski and Leica of course are the best choices but also more expensive. Minox and Olympus are medium brands and good too. The good brand makes better lens with better coating and mechanism, just like Nikon and Canon lens usually provide better image quality.
ii) Weight. Though binoculars are not extremely heavy stuff, prolonged birdwatching may cause tiredness, especially when you just hang your binoculars by your neck.
From my understanding, the expression A x B has the following meaning：
A = Magnification, B = Objective diameter
The above 2 parameters make the third and fourth criteria.
iii) Magnification - the greater the better, as the images will get closer. A larger magnification leads to a smaller field of view.
iv) Objective diameter determines how much light can come into the binoculars to make brighter images. It's very important as it directly affects performance. For other parameters kept constant, your 8 x 25 should have a dimmer and vaguer image than a 8 x 42 binoculars.
For me, FOV is not so important in binoculars. In photography, we may need wide angle to take shots that can give a wider view to exaggerate the object taken like scenery of mountain ranges, grassy terrain or desert. But for birdwatching, we seldom need a wider view to see more birds because they are always so distant and we hope to be as big as possible!
So, to answer your questions:
1) May be a wrong concept, as (iv) said.
2) (iii) explains your observation.
3) Refer to (1).
4) Choose a lower magnification but a bigger objective lens - yes you can. But usually there is a common range for the ratio of B/A, around 3-8.
5) I also don't know what is twilight factor. But if you want to have a better lens that best truly reflect the color of the real objects, good brands usually have to be considered. Like those big brand lens, they have many measures to prevent the optical flaws like diffraction and dispersion, e.g. specific coatings, aspheric lens design, lens made of fluorite, etc. So the color distortion problem is not from A and B but the lens quality. Of course, for same quality of lens, with a greater B, the image quality should be better.
Hope the above helps.
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Last edited by thinfor at 8/02/2013 10:08 ]
