posted on May, 6 2006 @ 10:37 PM
CX, you will find that as the power goes up the field of view goes down and the harder it becomes to point accurately at a specific target. Further,
if you take any pair of binocs and divide the magnification into the objective lens diameter you will get a number that tells you about the diameter
of the exit pupil and if you square that number you get a figure for relative brightness of the binocs. For example, 20X60 binocs have an exit pupil
3mm in diameter and a relative brightness of 9. Whereas 7X50's have an exit pupil just over 7mm and a relative brightness of 50. So what do those
numbers mean? Well, the pupil of your eye has a diameter of around 7mm and if the exit pupil is only 3mm then only a small portion of your eye
actually gets to see the image coming through the binocs. Further, your eye will perceive that image as relatively dim simply because your entire eye
doesn't get to be used to look at it and because the image is dim (remember it only had a relative brightness of 9 to begin with).
7X50 binocs on the other hand are often called night viewing binocs because they have a high relative brightness (50) that just so happens to match
the pupil diameter of most peoples eyes and thus allows them to make full use of the entire eye. You will find the larger exit pupil diameter much
more comfortable to look at as well. What you want is an exit pupil diameter that matches you pupil size and has the largest objective lens that you
can comfortably hold & carry around (glass lenses are heavy).
Stated another way, if you take the square root of the objective lens diameter, that number should normally be the highest magnification you would
want for that particular pair of binocs (for hand held viewing). With a steady mount of some sort you can increase the magnification somewhat, but
remember, the brightness of the image will decrease as you do so and so will the field of view.
Looking at the night sky is fun and rewarding, but not if you have a hard time holding the binocs steady and not if they are heavy. I will also say
that you will actually see less stars with 7X50 binocs than with 20X60's, but they will be much easier to look at. Stars are very dim objects to
begin with and your naked eye can see stars to an approximate magnitude of 6 to 6.2 (if you are out in the country away from city lights). With a
pair of good 7X50 binocs you can see much fainter stars (to around magnitude 10.5 or so). As the objective lens diameter goes up the more light they
can capture and thus the dimmer the objects you can see; however, as you begin to magnify the image remember that you are also spreading what light is
available over a larger & larger surface area and as you do that the dimmer any particular portion of it will become.
60mm objective lenses will capture 44% more light than 50mm objective lenses and you can thus see objects that are dimmer. 80mm objective lenses can
capture 256% more light than 50mm objective lenses and thus you can see even fainter objects. A good telescope--say with an objective mirror 8 inches
in diameter--can capture still fainter objects yet, since an objective that size can capture around 900 times more light than your naked eye and about
1600% more light than a 50mm objective lens. BTW that would only let you see objects with a relative magnitude of around 13.5 to 14.0. However, the
number of stars you can see with your naked eyes is about 4000, but the number you could see at the above magnitude is around 2,000,000.
I hope that has helped answer some of your questions. There are a lot of books available on the subject of optics and astronomy and if you are really
serious about looking at the night sky then I suggest you get a couple.
[edit on 6-5-2006 by Astronomer70]