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Pretty dullard stuff so far,right?Now let's get into what those weird numbers on the binocs all mean.But,first we have to get into a brief biology lesson covering the human eye and how it focuses and adjusts to varying light conditions.As most of you already know the human eye's pupils expand and contract in response to how much light is present and how much light is needed to see properly.In the daylight,or other strong light,the pupils contract to a smaller diameter due to the eye not needing as much light to see.Of course the opposite is true at night or in low light conditions.In those cases the pupils expand or diolate.As a general rule of thumb the average human's smallest bright light vision pupil contraction diameter is about 3 millimeters.In turn the average maximum dark vision expansion diameter is about 7 millimeters.This dark adjusted diameter is important to remember when picking binocs for astronomy just as the daylight adjusted number is important when choosing all purpose/daytime use binocs.Everyone's different and other factors like age can have effects on those numbers.As a general rule of thumb(very general)people under 30 years of age can usually get the whole 7 mm while people over 30 will generally lose about 1 mm per 10 years of age over 30.For our purposes though we'll just go with 3 mm daytime and 7 mm night-time pupil diameters as ideals.
After searching around a bit on the net I did come up with a nifty trick for measuring your pupil diameter yourself.I had even started to type out a description of the proceedure.After a moment of thought I decided to simply post a link to where I saw it instead.You see it involved using metric allen wrenches close to the eyes.It doesn't take eyes in my arse to conjour up the kind of hindsight it takes to see where that's going.Sorry but I'm not gonna be sitting in some courtroom listening to some dumbazz half-wit cry about how it's my fault he's now called "winky".Anyhow,look for the link in the bibliography and for our purposes we'll just stick to guessing.
Now let's get into those numbers.Just for simplicities sake we'll use the Leupold 10X50's I mentioned above as our example.The first number,the number 10,referes to the amount of magnification that will be achived,or how many times larger,or how many times closer,an object will appear.Magnification actually occurs in the eyepiece.
The second number,the number 50 in this case,referes to the maximum diameter,in millimeters,of the main objective lenses,which are the front lenses on a pair of binocs.This measurement is also called the aperature.As another general rule the larger the main objective lens aperature is the more light gathering ability an optical device has thus the brighter and sharper an image you'll be able to see,that is with all other things being equal of course.
Now it just so happens that a "magical" thing happens when you take the main objective lens diameter,or aperature,and divide by the magnification.It gives you the diameter in millimeters of the exit beam,or in other words,the beam of light which enters your eye.In the case of our Leupolds that would be 50 divided by 10 which gives us 5 millimeters.This is a good median average of the typical persons eye pupil diameter at any given time,especially in medium light conditions.
What all of this adds up to is that,if possible,you should try to match up exit beam diameters with what your average pupil diameter might be under any given viewing conditions.Of course if "ifs and buts were candy and nuts it would be Christmas every day",right?Well,anyhow if the light beam is larger than the dark expanded,or diolated,pupil then anything extra is just wasted light which adds up to wasted lens diameter,wasted weight,wasted money,etc,
while too small of a beam may lead to chromatic abberation which is just a fancy term for a fuzzy picture and funny colors.In a perfect world the exit beam from a set of binocs will equal or be just a tad smaller than the diameter of the maximum expanded pupil.At night this is around 7mm(we're assuming it is anyways) as we said before so a combo of main objective diameter divided by magnification which equals the 7 mm night adjusted pupil is our goal.You can lean on this rule quite a bit when you're using binocs with excellent optics as you'll see in my review of my new 100 mm glasses.
Well when we're looking through the catalogs we find such combos as 7X50,8X56,9X60,11X70,etc.,all of which are great for astronomy purposes.All of these combinations also give exit pupil diameters of around 6.5 to 7 mm which puts them in our desirable range.Of course the same criteria can be followed when choosing binocs for other purposes such as hunting,hiking,etc... For instance some compacts are designed for easy of carry and to be lightweight.These types have a relatively high magnification with very small objective lenses.This makes them great in high lighting conditions but poor performers in low light like deep woods or astronomy.There are exceptions to that last rule,to a certain degree,but there's a reason for it and it costs money and a bit of it too.See all of the stuff above again about prisms and coatings and such for the reasons why.
Binocs like this also have a very small FOV or Field Of View.When it comes to astronomy you do want a fairly wide FOV or at least I do.Not that there isn't something to be said for close in,high magnification scenarios but a wide angle view full of stars can be very enjoyable and even a bit messmerizing.This is especially true when you pick out a spot that seems rather void of any features and then train your binocs on it to see it light up with way more stars and such than you ever thought.
While on the subject this would be a good time to take a second to explain FOV.Despite there being two completely different ways of expressing FOV it's still simple really.The first method is a linear FOV.First thing to remember is that the industry standard for measuring linear FOV is at a distance of 1,000 yards.FOV basically indicates how wide of an object,or a section of turf,will fit in your picture when viewed at that distance.Let's take an average pair of 10X50 binocs for example.Most binocs in this size have a FOV of around 340 feet at 1000 yards.In other words if you are looking at a building from 1,000 yards away that is 340 feet long then it should just barely fit in the "sight picture",or the picture you're seeing.Generally the more main objective diameter you have and the less magnification you have the wider a FOV you have.In fact many compact binocs have such a small main objective lens(usually 25 mm)that they don't gather enough light to even see much at 1,000 yards so they don't even get rated on FOV.
The next method of measuring Field Of View is an angular method often refered to as "degrees of field" and it too is measured at 1,000 yards.I'm gonna try to explain this in my own way so please try to bear with me.First lets use our 340 foot long building and our 10X50's again.Let's also position ourselves in an imaginary spot where we can look down upon ourselves looking through our binocs at our 340 feet long building that is 1,000 feet away.Now strike an imaginary line from the outer edge of the left lens to the far left side of the building.Now do the same on the right side.The angle or degrees of span between the two lines is the Field Of View in degrees.If we know the FOV in feet(340 in this case)we can easily find the FOV in degrees by simply dividing that number(called the linear,remember?)by 52.35.So if our FOV is 340 feet at 1,000 yards then we just divide 340 by 52.35 and we get our FOV angle in degrees,which is 6.5.Just for reference a view of five degrees is about the width of ten full moons,a very wide field indeed.Got all of that?I know this is one of those cases where a picture is worth a million words so here's a good one to check out showing what angular FOV is: http://www.nightskyinfo.com/binoculars-terms/field_o
f_view.gif
O.K. y'all we're approaching the end of the techno stuff so just bear with me while I cover the last few remaining points that you'll find important to know when shopping for a quality pair of binocs.I reckon the next thing I'll cover is waterproofing.Many binoc manufacturers offer some of their models in non-waterproof form.The only reason whatsoever that I can see for this is price as non-waterproof models are a bit cheaper....initially anyhow.In my humble opinion if you are absolutely 100% sure that your binos will always be used in a steady,non-fluctuating temperature indoors with low humidity then by all means they may be a viable choice.However,if you are ever gonna step outside with your binocs in any kind of humidity or if your ever gonna move your binocs from a cold A/C chilled house to a warm and muggy outdoors then you will indeed want waterproof and maybe even fog proof units.I suppose it kinda goes without saying that if you ever anticipate any marine or nasty weather use then they'd be the choice too but,as you probably already gather,falling rain and splashing waves aren't the only enemy of water sensative binocs.The very best waterproof binos are not only well sealed but have been nitrogen filled.This process used gaseous nitrogen to force out all air and,along with it,any moisture.This ensures against fogging up when moving from one temperature extreme to another.According to one article I read lately which focused on binocs for rainforest use exclusively(see Skyguy I told you I'd read EVERYTHING on binos)nitrogen filling also wards off the growth of humidity spawned fungus which actually feeds on binoc coatings.Nitrogen filled binos are also refered to as fogproof.If you buy a pair of these and they still fog up don't panic because the nitro filled units will clear themselves very quickly.
Have you noticed how I haven't said anything about non-nitrogen filled waterproof binocs?I'm not going to either because IMO they're simply not worth it.Do yourself a favor and go the extra buck and splurge for the good ones.It's really a matter of pay me now or pay me later.While the non-waterproof and the non-nitrogen filled binocs may seem cheaper now but in the long run you'll pay more for repairs and/or new binocs.If you absolutely must skimp then at least get the basic waterproof.
One bit of terminology used in bino speak is one which has never really impressed me although it's suppoded to do just that.I'm talking about the words "rubber armored".Whenever I see this in a bino's description the first thing that enters my mind is "oh cool,they have a nice rubber coating to make them easier to grip" and that's it.I always envision myself holding a pair of binocs out at arm's length and dropping them onto concrete.Somehow in that scenario the rubber coating just doesn't add a lot to my faith in their ability to survive.I'm willing to venture a guess and say that most bino drops are from about chest to waist hieght then I'm sorry but the fact that some binos have a thin coat of rubber just doesn't do anything for me.Maybe,however,the rubber will help to keep you from dropping them in the first place.If you are worried about dropping them then well built binos from a quality manufacturer is a good place to start.
BTW,just because I feel this way about rubber armor doesn't necessarily make it true.Truth is you just see a lot of binos labeled this way yet I've never seen anything about rubber armor anywhere including forums,manufaturer websites,etc. so the consumer is pretty much left to speculate about it's effectiveness.In it's defense it does help cut down on unsightly scratches and makes them quieter in the woods.
Finally we come to the last bit of terminology and it is the word "baffling".All good quality binocs should have effective baffling to cut down on glare and unwanted reflections.Try looking through a set of binocs towards a brightly lit area or surface.The image should be surrounded on all sides by total darkness with no unwanted reflections or bright spots.Normally this isn't a problem with binocs made by a reputable manufacturer.

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Binoculars 101:A Primer for Those Who Like to Keep Both Eyes Open-Pt.2

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