I posted this on another forum a while back, in response to pretty much the same question.
Hope you find it helpful.
OPTICS FAQ #1
Doing the math. Question: What do all the numbers on the box, or in an advert tell me about the optic I’m buying?
The main numbers will be something like 8X40, or 10X50. This tells you 2 things immediately, once you understand the numbering conventions used.
The first number (8 or 10) is the magnification provided by the scope. 8 means that an object will appear 8X bigger, or 8X closer as you prefer.
The second number is the diameter of the front (or objective) lens. This will control, among other things, the brightness of the lenses, and the field of view.
You can also calculate things from these two numbers using simple math, mostly add, divide & multiply.
If you divide the second number by the first (8/40 in our example) it will give you a number called the “Relative Light”. This will give you a number that represents how bright the scope should be, all other things being equal.
Lets do the math, based on our numbers above.
40/8 = 5
Or:
50/10 = 5
This says that the two different scopes will have the same brightness, but the bigger one will have more magnification. The cost here being it will be bigger, more expensive & heavier.
Lets try a different one. This time we’ll use a compact binocular as an example, a 10X20.
20/10 = 2
This tells us that the compact will give us the same magnification (10X), but the small front lens, which allows it to be small & light lets far less light thru, so it will look much dimmer. The difference can be marked down as 2/5, which equals the difference between the 2 systems. The compact is 0.4 times as bright as the big one.
If we do the opposite and multiply the numbers together, then we can calculate even more.
40X8 =320
10X50 = 500
8X20 = 160
These numbers will give what is called an “Optical Index” by comparing the answers we can get an idea of the size & weight of an optic without ever seeing it! The higher the number is, the bigger the optic will be. Again by dividing out the numbers & comparing the answers we have a reasonable idea of how much bigger one will be than the other.
500/320 = 1.5625
This means the 10X50 will be roughly 1-½ times bigger & heavier than the 8X40.
Or 320/160 = 0.5
In this case the compact 8X20 will be 0.5 or ½ the bulk & weight of the 8X40.
There are also specifications that have 3 numbers such as 3~9X40. These refer to zoom scopes where you can vary the magnification by turning a ring. The first number here is the Lowest power, the second, separated from the first by the “~” symbol is the highest and the third is again the diameter of the objective lens. With zoom optics you need to have a power setting of your choice before doing the math, but again it can be useful. For example: how much brighter is the zoom when set at 3 powers than when it is set at 9 powers?
Like this:
40/3 = 13.333
40/9 = 4.44
13.333/4.44 = 3.009.
The scope is 3 times brighter at 3X than at 9X!
Easy do the math twice, once for each power setting & then treat the results like two different scopes!
Bare in mind these have no relationship to the quality of the optic, they just allow us to play “what if” games without having to go & get a bunch of optics to do the comparisons.
Cool, No?
OPTICS FAQ #2
Understanding the technical terms. Question: what the heck do things like “Aspherical Objective Lens” mean in plain English?
Like any technical field there are terms used that are unique to that area, kind of shorthand for the ideas involved. Here are some of the more common ones you’ll come across when choosing a scope.
American System
The reticule is placed in such a way that the apparent size does not change when zooming. Advantage: Consistent view. Disadvantage: any range measuring must be done at a specified power.
Angle of view.
The same as field of view, but expressed in degrees. You have to do the math to convert it into a feet at a distance number. Usually shown as 1.3 degrees, or something similar.
Aperture / F number
The diameter of the lenses free light-transmitting path. Sometimes referred to ads "D" in telescopes. Usually fixed in telescopes & binoculars, determined by the design. Usually adjustable in camera lenses by opening & closing a mechanical version of the iris in your eye.
Aspheric
Most lenses are part of a sphere. Imagine slicing a piece off of the side of an apple. The curved part with the skin is "spherical" & the flat part where the knife went is "Plane" Aspherical lenses change this curve as you get closer to the edge of the lens. It allows some fairly extreme optical engineering to happen without loss of quality.
Bore sighting
Preliminary alignment of the scope & Barrel. Done by looking down the Barrel while adjusting the cross hairs to meet at the same point that can be seen thru the barrel. Now sometimes done by shining a laser down the barrel & making the cross hairs meet where the dot shows up.
Clicks
Mechanical ratchet that makes a clicking noise when being adjusted. It allows precise adjustments for zeroing by allowing the clicks to be counted. Frequently marked as 1/4 MOA or 1/8 MOA. In this case there is a known amount of movement in the point of impact per click, which allows for rapid, precise changes in zero by going "up 8 clicks". This being a 2" movement at 100 yds with 1/4 MOA clicks.
Coating
A process that applies a very thin layer to the surface of glass. This improves transmission & adds a physically tougher layer to the surface of the soft optical glass. There are dozens of proprietary coatings in existence, all of which have a unique name. Some kind of "Multi Coating' is the current standard.
Contrast
The amount of separation between black & white objects. If they appear light grey & dark grey they are said to be "Flat" or "low Contrast". If they are bright white & dark black they are said to be "Hard" or "high contrast". Frequently used as a measure of quality, but too much is not a good thing in this case, as for example shadows will become black & not hold detail (such as the 12 point buck hiding in them!)
Element
A single piece of glass with one or more curved surfaces. The curve may bulge out "Convex", curve in "Concave" or be flat "plane". Different sides may have different shapes. The basic building block of a lens system. A pair of glasses is a pair of elements.
Erector
The internal lenses that allow the thing being looked at to be right side up & right way round. Usually only found as a separate item on astronomical scopes when using them as a spotting scope.
European System
The reticule is placed so that it changes apparent size as it is zoomed. Advantage: Ranging can be done at any zoom setting. Disadvantage: The change in apparent size is the opposite of what would be idea. (it gets bigger & thicker as the image gets smaller.)
Exit pupil
How wide the actual projected image that you will be viewing is. Very important for low light usage! The average human eye adjusts in diameter as the light decreases. An exit pupil less than the diameter of the iris in your eyes will not get brighter as the light decreases, no matter how bright the system is. This will make the scope poor in low light, or heavy shadow. 9~11 mm is a good low light diameter as this is about as big as the human pupil gets.
External adjustments
The tube has fixed internal components and the adjustments move the whole tube to correct aim by screws outside the optical system. Good for water sealing the tube in extreme environments, but more fragile & likely to get knocked out of alignment than internal ones.
Eye relief
How far away from the eyepiece your eye can be while still seeing everything in the scope.
Eyepiece
The lenses at the back of the scope that you look into.
Field of view. How wide a circle you will be able to see at a given distance. More is better. When comparing 2 different scopes of the same power the one with the wider field of view will allow you to see a wider area. Usually shown as something like 323 feet @ 100 yds.
Flare
Stray light bouncing round inside the lenses, instead of being either trapped, or eliminated. Usually shows up as a loss of sharpness & contrast.
Focal Length The distance behind the front lens groups where a sharp image is formed of a distant subject. Usually longer is more powerful, but has a narrower viewing angle. You have tested a focal length if you ever used a magnifying glass to start a fire by moving it back & forth to concentrate the sun's rays. When the hot spot was smallest & hottest you were at the focal length of that magnifying glass.
Focal point
The plane where an image formed by a lens is tack sharp. There are at least two in all telescopes. Inside the front is the primary where the main lens first focuses the image in front of the scope. To the rear is the secondary, where the eyepiece optics will produce a sharp image of your eyeball.
Ghosting Bright colored blobs of light that wipe out the image as seen without the scope. An extreme form of flare. Usually controlled by good design & a lens shade. The closer you can get to a light before this happens is one measure of the quality of a scope.
Group
A bunch of elements fitted together to do a specific task that one element cannot do alone. The number of groups & elements will vary from lens to lens, but usually more is better, especially in zoom systems. If you put a magnifier in front of normal glasses for close up work this is a group.
Internal adjustment The interior of the scope is a smaller tube, moved within a bigger tube to track across or up & down without any external screws.
Lens shade
A device to keep stray light from degrading the image by shading the front lenses from direct sunlight. Works like holding your hand over your eyes when looking into a bright light.
Light transmission
Not all light passing thru a lens makes it. A small percentage is absorbed or reflected. Not too serious until you add all the glass in a zoom system, then it adds up quite a bit. The higher the number the better 99.5% being about what good modern optics can do.
MOA
One minute of angle. An angle roughly equivalent to 1" @ 100 Yds, or 2" @ 200 Yds and so on.
Mount
Used several different ways. It can mean attaching a glass element to a group, or tube. It can mean the device used to attach the whole optical shebang to another device. Camera mounts allow interchangeable lenses to be swapped on a camera, scope mounts attach the scope to the gun Etc. Basically a generic term. Mounting prisms in binoculars is an important example, clamp mounts are far sturdier than glue mounts Etc.
Objective
The big lens, or group of lenses at the front of a scope that collects the light entering the system & performs the initial manipulation by forming a "real" image. It controls things like field of view & magnification.
Optical axis
An imaginary line drawn thru the exact center of each lens in the system.
Optical Center
The middle of the lenses. This is where the quality is highest. Cross hairs should be optically centered from the factory, but on a used scope you may have to do it yourself. Rolling the scope in “V” blocks while looking at whether the cross hairs "wobble" will detect this, Adjusting the turrets until the "Wobble" stops finds the optical center.
Optical Glass
Special kinds of glass used to control the way light is bent by the individual lenses in a scope. Frequently several different types may be used in a single optic. Types such as "Flint Glass" are common. Nowadays there are even more specialized types in use such s "Fluorite Glass" frequently found in camera lenses. The list goes on and on. Basically its an indication of a higher quality lens.
Optical plane
Not a cheap flight for photographers! To make the math easier the front to back of everything from the subject to the eye is divided into slices, just like bread, but much thinner. Each of these slices is a plane. Some of them have vital things happening in them (such as the reticule being positioned there.)
Parallax The differing position between the image of the reticule & the image of the target. When correctly eliminated the two appear as one. When incorrect the center of the reticule can appear to move across the face of the target, causing dispersion of the shots. It can be checked for & adjusted if the scope has an adjustable objective (AO) lens.
Power
Usually the magnification of the optic. 10X means it magnifies 10 times, or makes things seem 10 times closer. Frequently over stated by optics manufacturers. A rule of thumb is you can have only 20X per inch of objective diameter. So that 40mm X 200 power scope will not be real sharp.
Prismatic Using a prism shaped glass block to bend light round corners. Usually refers to binoculars, but spotting scopes are frequently prismatic to reduce bulk. Very few sights are prismatic, one of the exceptions being the "Trilux SUIT"
Range finding reticule
Divisions, or marks on the reticule, known as “Stadia” that can be compared to an object of known size & used to estimate the distance to that object. There are many systems all of which rely on the same principal: If the size of an object is known, and the power setting of the scope is known then 2 sides of a triangle are known. The scope is “programmed” to do the trig. Needed to calculate & display the 3 rd number (the distance to the target.) by doing the Sine, Cosine, Tangent thing we all hated in high school! The accuracy of all of them depends on a couple of things, (the correct estimation of the size of the target, the ability to very accurately place the object exactly within the stadia.) The stadia can be circles, lines diamonds, dots, lozenges or indexed curved lines, but the technique used ids the same A unique one is the "Sheppard" where 2 reticules are used. They are made to appear to be together by clever optical design.
Rangefinder
Device to measure distance. Some use moving mirrors, but the newer ones bounce a laser off the target & calculate the delay to be very accurate.
Relative light
The amount of light being sent out the back to your eye. The higher the better for low light conditions. Works with the "Exit Pupil"
Resolution
One measurement of how sharp an optic is. It is the number of black/white lines the lens can detect at a given distance. More is better. Especially if you are trying to see a .223" hole at 500 yds.
Reticule
The framework that you sight with inside a scope. It can have dozens of types, including, but not limited to: Post, Post & horizontal wire, cross hair, 30/30 (cross hairs that thicken towards the edge of the scope & thin towards the center) Range finding (many types) & Mil Dot (another range estimating system) The functions of the reticule are mainly A: put an aiming point on the target. B: indicate whether or not the gun is level. C: Have some system to pull the shooters eye to the center.
Sighting in
Same as zeroing.
Telephoto
REAL MEANING: A lens built in such a way that its optical length is greater than its physical length. Example a 1000mm lens can be as little as 450 mm long. Done by using clever internal lens design, prisms, or mirrors. COMMON MEANING: a lens that makes things look closer than they are.
Turret
The mechanical screws, dials, springs & indicators that tell you where you are setting the cross hairs. Usually one for elevation (up & down) & another for azumith (left right.)
Twilight factor
How bright the image will appear to be under low light, such as dense shade. Does not really apply under bright lighting, you will not be able to detect a difference of twilight factors under normal lighting.
Wide Angle
REAL MEANING: a lens that compresses more than we can see with the unaided eye into the same space we see with that eye. COMMON MEANING: having a wider than normal field of view for that type of optic.
Wide Field
Same as the common usage of wide angle.
Zeroing
Getting the path of the bullet & the optical view to meet at a given point. The sight is mounted away from the barrel. The bullet’s path is curved, & the optical path is straight. Correcting for all these things is the “Zero” at a given distance. The appearance is that the bullet will strike the target exactly where the cross hairs were pointing when the cartridge fired. There are actually 2 zeros at any setting, one as the bullet climbs up after leaving the barrel, and another as it drops back under gravity much further away. You can use this to sight in for a distant range when there is only a closer distance available. If you “Zero” at the first point, you can calculate the second out if you know the trajectory of the bullet. It is approximate, but better than guessing. Making adjustments to the scope’s mounting & adjustment turrets does it.
Zoom
Changing the magnification by manipulating the internal optics. Common ones are 3~9 or 4~12 meaning you can change the "power" between either 3X to 9X, or 4X to 12X. Popular because magnification & area viewed are exact opposites & you can trade off between the two quickly & easily.
