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Though it tends to oversimplify things I still prefer to refer to exit pupil in such a comparison. Exit pupil can be determined by dividing your magnification into your objective diameter. The larger the number, all else being equal, the greater amount of light that is available for your eye to utilize. In low light conditions, dawn, dusk, heavy tree canopy a larger exit pupil can be very beneficial in allowing darker areas to be viewed more easily.

Also, a larger exit pupil also tends to allow your eyes more "room for error" when it comes to having them lineup with your own pupils.

With both those thoughts in mind I would opt for the 10x42.

Just my opinion.

DeerHunter2000 To spend the money you will for quality optics I would recommend a power that can be used after the hunt. The 10x and 12x are a good choice for open terrain like Wyoming and New Mexico, but if you live and hunt in thick cover you are overpowered. Quick requisition of a target in dense cover is next to impossible with the high power optics. I would suggest 8x for all round hunting optics and a variable power spotting scope for open terrain.

my neighbor has the nikon monarch 10x42 and the several times i have glassed through them they where extremely clear i think that nikon makes some exceptional optics

Of the two mentioned I would recommend the 10s for all around use. The 12s will excell when longrange viewing is needed. But in the woods they will be a lil tough to find stuff quickly with. If you aren't the steady type you will likewise find yourself giving your eyes a fit with shakiness. I personally prefer and use two types, a 15x for field/longrange use and a 9x for woods and closer range use.

Like I always say, the Monarch series binoc is THE best optic on the market for the money. QUALITY stuff!
RA

I wouldn't go with the 12X42 unless you really, really need the extra magnification. A 10X42 will be better in 98% of the hunting situations out there today. Just my opinion

10X42 Advantages:
1. Brighter
2. Steadier
3.Better Eye-Relief
4.Lighter
5.Larger Feild of View

12X42 Advantages.
1.More Magnification

i live out west, northeast wyoming to be exact. i hunt a lot out in the open prairie, where you can see forever. i myself use 8 power binos and that is plenty. some of my buddies use 10 power, and that isnt overkill at all, but i dont really need it. where you are in PA, i would go with the 8's, they will gather light better, and wont be too much magification.

bonus, 8 power might just be a bit more affordable too!

ACtually most experts recomend the 8X as an all around power range. Unless you are using a stand or a tripod, the 12X binocs get a little shakey. I even find the 10X to be pretty shakey. remember you will be looking through them for long stretches. Ever try to hold your rifle steady with the scope cranked all the way up to 14X?
With optics, it is more a question of image quality than image size. A great pair of binocs at 8 power will show detail that a cheap pair of 12X's won't show. Not that the Monarchs are cheap!
I use a pair of German Optolyth 10x50's and although they are very nice, I am stepping down to an 8x40 pair as soon as possible because they are too shakey being held offhand.
okcmco

I tend to agree with the others regarding 8 power. It is the best all around compromise when it comes to optics for hunting or other outdoor activities though you didn't seem to keen on hearing it from your original post.