Everybody knows what a telescopic sight is. Everybody has seen or used one. But not everybody knows how they work or which features are important.
Telescopic sights combine a telescope with an aiming reticle. Telescopes have a system of lenses to magnify the target.
For this system to be useful there are several design objectives which must be met.
Optics For The Hunter
This is a good book on scopes and binoculars.
Eye relief is the distance from the back of the scope to your eye (with the whole field of view visible.) The more recoil your rifle generates, the more eye relief you need. For a center fire rifle eye relief should be in the 3" to 4.5" range.
Handguns require specialty scopes with enough eye relief to hold them at arm's length.
A rifle scope contains a handful of lenses. At the front is the big objective lens. At the back is the ocular lens assembly, also called the eyepiece. In the middle are the erector lenses. The erector lens assembly (called picture reversal assembly in the diagram above) turns the image right side up. In a variable power scope it also moves back and forth to change the magnification.
Your high school physics book probably shows a diagram of a scope where the light comes in one end, bends around through the lenses and comes out the other end all magnified and perfect. The birds sing and life is beautiful.
Nothing is ever that easy. In real life light reflects off of glass and different colors behave in different ways as they go through lenses. Also lenses attenuate light, so the more of them there are, the darker the view.
The first two problems can be reduced with coatings, the third requires well made lenses and careful design.
Top of the line optics will have three or more coatings on every lens surface. As quality goes down the number of coatings per lens and the percentage of coated lenses goes down.
Beware of advertising phrased to sound like all the lens surfaces are coated or multi-coated when only some are.
Exit pupil is the column of light coming out of the eyepiece. The bigger it is, the brighter the scope seems, up to about 7mm. Most humans can't take advantage of an exit pupil bigger than 7mm. Old humans might be limited to 4mm. This is why you have trouble driving at night if you are old enough to have watched John Glenn orbit the earth.
The size of the exit pupil is calculated by dividing the size of the objective lens by the power of the scope. The size of the objective lens is the second number in the designation of the scope.
For example a 4 x 32 scope magnifies the image 4 times its actual size and has an objective lens 32mm in diameter. It will have an exit pupil size of 8mm (32/4=8.)
A variable power scope will have an exit pupil size that changes with the magnification.
For example a 6-20 x 40 will have an exit pupil of 6.7mm at the 6 power setting and 2mm at the 20 power setting.
You will be able to see through almost any scope on a bright sunny day, but that 2mm exit pupil is going to look pretty dark under low light conditions.
Poorly designed scopes may have an exit pupil smaller than the math indicates. Or the exit pupil might not be round. Hold it up to the light at arm's length and measure the diameter of the bright spot.
Even good scopes might have this issue if they have big objective lenses and low magnification. The resulting large exit pupil might get "clipped" by internal parts. If the math says you should have a 10mm exit pupil but it measures 9mm it won't matter. Remember a 7mm exit pupil is about all you can use. But if a theoretical 5mm exit pupil is really 4mm, you are losing light you probably can use.
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A scope must be able to withstand the pounding it will take from recoil, and everyday handling.
There is evidence to suggest that heavy scopes don't stand up to recoil as well as lighter ones. We can thank Isaac Newton for that. He's the one who came up with Force = Mass x Acceleration. I will paraphrase that as Pounding = Weight x Recoil. The heavier the scope and the more recoil generated by the rifle, the more abuse the scope has to take.
Muzzle brakes often stress scopes in ways they are not designed for.
If you only shoot a few rounds a year from a mild rifle, most scopes will hold up. But if you spend hours every weekend plinking with your .458 Win Mag you will need a good scope (and shoulder surgery.)
The mode of failure can be internal parts shifting, lenses coming loose, or springs or reticles breaking. Even the best scopes don't last forever.
The good news is that the major brands (not to be confused with the best brands, although there is a lot of overlap) will cheerfully replace your broken scope. The bad news is they probably won't air-drop it to you in the wilds of Manitoba. Buy a better scope and test it more thoroughly if its failure will be catastrophic.
Even if a scope doesn't break under recoil and handling, a zero shift is nearly as bad. Newly mounted scopes often take a few shots to "settle in". After that if the zero keeps moving something is wrong. It could be the way the scope is mounted or it could be the scope itself.
Good scopes retain their zeros from shot to shot and after transport and handling.
But don't forget that other factors can cause your zero to shift, including wet wooden stocks.
Unless you only shoot indoors or on warm sunny days your scope will need to be weather proof. Most are (or at least claim to be), but some only with the turret caps on. This could be a problem if you need to adjust the scope in wet weather.
Again, how important it is to have a weatherproof scope depends on where you will be when it fogs up.
For critical applications submerge it in warm water before you put it on the rifle. If bubbles come out and don't stop after a few seconds it probably leaks.
If it passes that test, dry it off and let it come back to room temperature for several hours. Then put it in the freezer for an hour. It will fog if moisture was sealed in at the factory.
Obviously scopes need to be adjustable. You have to get the cross hairs to match up with the bullet strike.
But not all adjustments are created equal. Some have little caps over the adjustment turrets and little screws to accomplish the adjustments and little lines to help you guess how much to adjust.
If your application is of the "set it and forget it" type, this will work just dandy. But if you routinely shoot at different distances, under varying conditions, with multiple loads, you need some good, calibrated turrets with resettable knobs.
The calibration to beat is MOA (minutes of arc) which translate to about 1 inch at 100 yards. Some scopes are calibrated to actual inches per 100 yards. Both work equally well.
Resettable knobs allow you to set the knobs to zero after sighting in. Then, if you change the zero for some reason it is easier to go back to your original zero.
Most scopes allow you to turn the eye piece to bring the reticle into focus. Some have fine threads and a lock ring. This is the "American" system. Some have a fast focus eye piece. This is the "European" system. American and European manufacturers produce both kinds.
The "American" type doesn't change by accident. The "European" type is faster to adjust for different shooters.
The purpose of this adjustment is to get the reticle into sharp focus. Do this with the scope pointing at a blank light colored surface and while wearing any corrective lenses you will be wearing when shooting with the scope. When you think you have it right, look away for a few seconds and then look through the scope again. The reticle should be in sharp focus. If not, adjust it again, look away again and check it again until you get it right.
Parallax is the apparent misalignment of objects not in the same plane, when viewed from different angles. Say what? Like the needle on your gas gauge. From the driver's seat you see you have an 1/8 of a tank, but from the passenger's seat it looks like you're down to the fumes.
This happens in a telescopic sight when the aiming reticle is not in the focal plane inside the scope. The problem is the focal plane moves as the distance to the target changes.
At low magnification this is not too bad of a problem. Most manufacturers solve it by picking a range and eliminating parallax at that range. There will be some parallax at other ranges.
High power and variable power causes parallax to be more of a problem. Parallax can be eliminated by moving the objective lens which moves the focal plane.
To check for parallax at a specific range:
Reticles come in a heap of different types. Every manufacturer has their own special reticle design. But they all also have the old standbys: cross hairs, duplex, and mil dot.
Reticles can be placed in the first focal plane (FFP) or in the second focal plane (SFP). Each location has advantages and disadvantages.
First focal plane reticles get magnified along with the target. This makes the cross hairs look fat at high magnification and skinny at low magnification. On the other hand the reticle elements always subtend (compare to) the same percentage of the target. In other words an element one mil wide is one mil wide at any magnification. This makes some reticles, like mil dot reticles, useful for estimating range.
Second focal plane reticles always look the same size even as the target looks larger or smaller. This means reticle elements will cover up more of the target at low magnification. They can also be harder to see at high magnification. On a mil dot scope the dots will be one mil apart at only one magnification that the manufacturer chooses. This makes mil dots harder to use.
In the case of telescopic sights mil stands for milliradian. A milliradian is 1/1000 of a radian. A radian is a unit of angle. There are two pi radians in a circle. In other words there are 2 x 3.14159 = 6.2832 radians in a circle.Therefore there are 6283.2 milliradians in a circle.
This means that one mil subtends one yard at a range of a thousand yards. Range can be estimated by comparing the mil dots to an object of known size. Range (in yards) = Size (in yards) divided by mils and then multiplied by 1000. So a 2 yard (six foot) tall man who subtends 5 mil dots is 400 yards away. 2/5 = .4 and .4 x 1000 = 400.
A minute of angle (MOA) is also a unit of angle. There are 360 degrees in a circle and 60 minutes in a degree so there are 21,600 MOA in a circle. Thus there are 3.438 MOA per mil.