I am a young man who still has a lot to learn about firearms and you may have noticed that I ask a lot of questions periodically.  I think that WTA is a great educational resource when it comes to the shooting sports and firearms technology.

Here is a new set of questions I have about scopes and how they work:

I know that a lot of the big scope brands today include Swarovski, Zeiss, Leupold, Trijicon, Bushnell, etc. but I want to understand how exactly they work.

What is parallax? And what does the parallax knob on the scope do?

What is windage? And what does its knob do?

What does magnification mean (as in a 4x for example)  Would a scope with a larger number have more or less magnification?

Is it true that modern scopes are filled with special gasses to prevent fogging?

How are scope bases tapped and drilled to be mounted on receivers?

Are there scopes designed to operate specifically in low-light areas or vice-versa?

And finally, where can I learn more about MOA and milradians?

Thank you for your time.

MOA is "Minute of Angle," or 1/60 of a degree. 60 minutes equal 1 degree.
http://en.wikipedia.org/wiki/1_in_60_rule

We use a similar, if less precise, formula in navigation. We call it the "60 to 1 rule" For every 60 units of distance (range), one degree difference in direction will move the end point one unit. So as shooting goes, if you fired a bullet from 60ft, and your scope was off by one degree, your bullet would hit one foot from the bullseye. Since that's quite inaccurate when it comes to shooting, they measure it in Minutes of Angle, which is 60 times more precise. Now instead of being off by 1 foot at a range of 60 feet, you're only off by 0.2in.

The wiki article above says its only for air navigation, but its applicable to so many other things. At 60mph, you travel one mile in one minute. And it's completely scalable. 120mph is 2 miles per minute, and so on. One degree off for 120 feet will put you 2 feet off. 2 degrees over 60 feet also put you 2 feet off. etc. I'll stop. I could seriously talk about the uses of the 60 to 1 rule all day.

Let me clarify Paralax in rifle scopes.
When shooters are talking about Paralax they are talking about the scope's reticle being on a different focal plane than the target.  So like T-man said, you get that movement.  You could put your rifle in a vice, move the position of your eye, and the reticle would look like it's moving too.  Shooting like this, you just can't get the best accuracy out of your gun.  When you focus the paralax, either with the newer style Side Focus knobs or the traditional Adjustable Objective, you bring the reticle and the target together and you can greatly reduce or eliminate that movement.  This allows the shooter to be much more precise...
All scopes have a Paralax focus.  Most are focused internally at the factory, typically on your 3-9 scopes that focal range is at about 150 yards.  Some, like say the Leupold 4.5-14 VX-III are set further, about 200 yards.  In most scopes Paralax isn't an issue until you increase the power of magnification above 9x power.  Some better glass, like Swarovski for example, it wont come into play until you get above 12 power.
Is Paralax important?  To most shooters - no.  They'd never know the difference because they act like Duffers and just crank it to the INFINITY setting an leave it there - in effect putting the scope as far out of whack as possible.  But for shooters that are going for precision - Paralax becomes important because you can't shoot with precision unless it's focused.  You can be good, you can be consistent, and do a decent job... but without a focused paralax you wont have that precision for long range shooting.

I know a bit about it.
And I'm not the only LRI Graduate here.

If you want to learn it - Go to LRI.  Let's just say that I thought I knew how to shoot before I went.

I know a bit about it.
And I'm not the only LRI Graduate here.

If you want to learn it - Go to LRI.  Let's just say that I thought I knew how to shoot before I went.

Didn't we all. 

The goal of the LRI's instructors is to take you and your rifle as far as you can go.  The Long-Range Hunting I class will take you to 1,000 yard shots on steel, but that is based on a broad base of skill application developed as you work up to it in 100 yard known distance increments starting with a 100 yard zero.  They will teach ranging and firing on a course of unknown distance targets (which sucks in the pouring rain with a 102 fever).  They will cover many other skills, both shooting and observing, over the 4 days.  it is well worth the trip.

Interestingly, parallax becomes an issue at close range too.  Sticking a scope with a 150 yard parallax on a .22LR will mean that the reticle will appear to move differently in your scope than targets at 50 yards appear to move.  Its even worse if you stick a "normal" scope on an air rifle.

In answer to another of your questions, the light transmission ability of a scope is dependent on a few things (the quality of the glass, the coatings on the lenses, the quality of the lens grinding, the number of lenses, etc) but the largest effect on light gathering is the size of the objective lens (the front lens).  Light enters the scope through the objective lens (the front of the scope) and passes through severl lenses on its way through the tube to your eye.  The light exiting the scope is called the exit pupil and you can see this "exit pupil" by pulling your head back from the scope and moving around to see a dot of light.  When your eye is within the correct eye relief or distance from the rear lens of the scope the exit pupil appears to fill the entire scope.  If you're too close or too far away the view won't appear to fill the whole scope ... it'll be a dot that moves with movement of your eye relative to the scope.

This exit pupil changes with magnification ... at lower magnification you have a wider field of view and a larger exit pupil, while at higher magnification you have a narrower field of view and a smaller exit pupil.  The size of the exit pupil is critical in determining how bright a scope is .... if the exit pupil is smaller than the pupil in your eye, the view through the scope will be dimmer than viewing with the naked eye.  The balance between objective size, magnification and exit pupil is all part of the black magic of scope design.  The quality of the lenses and coatings and the manufacture process is all part of why high end scopes cost more than cheapie "made in China" units.

While you lose precision, an extended eye relief scope mounted in that "Scout" configuration allows for faster target aquisition and engagement.  They are generally low power.  And the problem I have with most of them is that they all have too small of a field of view... which makes them not as fast as they should be.

That happens a lot with cheap scopes.  Good scopes, the point of zero should not change.  Now, of course you will always have changes - drastic ones - when you use a BDC type reticle, and zoom out or off of where the scope maker says you should use it... Generally at max magnification or at a set point, usually at 10 power.   This is what makes PFI scopes so bloody good - they have a first focal plane reticle, so you can zoom in and out all you like and the reticle is still smack on every time, all the time.  You also get that in a 30mm tube.  And you get those features for 1/2 of what the other guys would sell the same scope for... if they made anything like it.
Ahem... I happen to be a PFI dealer and if you want one:
https://wethearmed.com/index.php/topic,118.0.html
 

RIFLE:  Kimber 8400 Classic, .300 WSM.  A thousand bucks.
SCOPE:  BSA Cat's Eye.   Fifty bucks.

Showing everyone in the Gun Store how much of a moron you are - Priceless.

I told him not to put that NcStar on the AR, at least not in public.

Now I'm the a__h___ for some reason. 


-T.

No, the real a__h___s are the designers at NcStar. Not only are those scopes cheap, they're uglier than sin!

What to look for then?

 Only time I ever looked through glass was once when I used a SuperRedHawk in .44 Mag.  It had a scope mounted directly over the barrel.  Since we were shooting many guns that day I never really payed attention to the brand of scope nor what magnification it was.  I was so excited shooting a .44 Mag for the first time I forgot to ask.

 

Looks like I need to get myself a good rifle then.

I consider myself lucky that I don't have a good rifle to mount glass on.

Actually, I'm taking George's advice on red-dots for my AR. I'll get something with my bonus check (read "desert money") when I get back from the deployment.