Confused about scope terminology

[tac-40]

I am in the market for a new scope. My problem is finding one I can see through. No line bifocals are the requirement, otherwise I can't even see to load the rifle.:D

I know from my research that there are several brand named scope out there in my price range. However, I keep running across some terminology that is confusing to me. Like the following terms are used to describe the same thing, but I think they have diiferent meanings:

Side focus
Rear focus
Parallax adjustable

So I am asking all of you knowledgeable ghurus out there, what does each term mean and how will it affect my ability to look through the scope when wearing my prescription glasses. Better yet, what should I look for in a scope.

[brownknees]

The thing to remember with scopes is that they focus at both ends.

The parallax is the (optically) placing the reticule in the same focal point as the target. This is just like focussing a camera.

The second thing is making adjustments for the rear where your eyesight is compensated for.

Now as you're wearing corrective lenses (like me) the combination of your eyesight & the correction from the glasses is used to project an image from the rear of the scope into your eye/glasses.
This image (called a "virtual image" in the trade) needs to SEEM to be a long way away. This is done to prevent eyestrain.

The parallax correction is needed to stop the reticule from seeming to wander round on the target, this will reduce accuracy.

These can be adjusted by a knob on the scope.

Most of the terms being used describe the location of the knob that adjusts for parallax. The side mounted ones are supposed to be easier to adjust in the field as you just reach to the (usually) opposite side from the windage knob to make the change, instead of all the way to the front of the scope.

The parallax should be changed every time the range changes, but the eyesight correction should be set & forget (unless your prescription changes)

Here's a post I did a while back explaining a bunch of these specialist optical terms. It's a bit long, but should be an answer to most questions on which scope to pick.

OPTICS FAQ

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 single 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. Extended eye relief is a good thing if you wear corrective lenses.

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 on 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.

Part two, what the numbers mean.

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?

[Douglas S Graham]

Nice reply, Brown.............!!!!!

[ggiilliiee]

here is the only termnology ya need to know ..."scratch/ dig" rating .......if the lenses arent opitcal grade & ground by gunter in germany .....all the above is meaningless ...grab a ziess ...look into the sunset ...try it with another scope .....tell me what ya see ..he he........ if ya wanna go deeeeper .order a EDMUND SCIENTIFIC CATALOUGE....all the formulas are in the book ...diopters, focal length for various lens forms .. ect ...ect ...good info

[shootist87122]

I'm also in no-line bifocals (nearsighted), but switch to single vision (distance corrected only) shooting glasses at the range - especially for irons, but also for scopes. The reticule focus just works better with the single vision "shooting" glasses. I use Decot Hy-Wyd - the corrective part of the glass is huge and is made to work with your head up or down on a stock.

Talk to Decot about your shooting glasses problem - that's all they do.
http://www.sportglasses.com/

[brownknees]

Single correction lenses work very well, I'm using no-line trifocals, the type I'm using is called the "computer screen" model, this puts the area where the zones switch out of the way for most scope use.

all the above is meaningless ...grab a ziess ...look into the sunset ...try it with another scope
The German lenses & baffling are designed for resolution. Most of the others are designed to give an impression of resolution thru high contrast.

Now all of the info above is assuming an apples to apples comparison, if you check a $45 scope out against a Sowarski then it just ain't a fair fight.

The Japanes optics are getting better, but they're (for the most part) marketed to upper-middle quality & sell on price.
If you doubt this pick a high end Japanese scope & price it against a Zeiss, or Leitz one with the same sprecification.:biggrin:

[MasterGunner]

A couple of other items to file away about scopes.

Occular. The end you look through. Most scopes have a focus ring for your shooting eye located at this end.

Target Knobs. Most sporting scopes have caps that must be removed to do windage and elevation adjustments. Tactical scopes have large target knobs so you can adjust w/e directly (no caps to fumble with and lose). The same thing applies to the side parallax (focus) scopes like the Leopold Mk 4 and Tasco 10x42M Super Sniper.

Optical Quality. The purity of the optical glass used in the scope directly affects how well it transmits light and how bright and clear the image appears under various light conditions. Coatings applied to the glass help with light transmission and some manufacturers use coatings to compensate for poorer quality glass used in cheaper models of scopes. Likewise, a larger objective lens can be used to compensate for lesser quality optical glass. The bottom line is you usually get what you pay for.

Here's one way to check a scope for optical glass quality: look through the objective end and out the occular (reverse of normal). What you'll see when viewed against a light background is the kinds of imperfections that are present in the glass. The fewer you can see, the better the glass. Also, a larger diameter scope tube transmits light better than a smaller one; that is, a 30mm diameter is better at light transmission than a 25mm diameter.

Scope Descriptions: Here are two examples: (1) Leopold 3.5-10x40 w/illuminated reticle and (2) Tasco 10x42M w/Mil-Dot reticle. In the case of the first, this is a variable power scope that goes from 3.5 power to 10 power and has an objective lens of 40mm. The scope has a battery and on/off switch that powers an LED to light the reticle of the scope in dim light conditions (dawn or dusk). For the latter, this is a fixed 10 power scope with a 42mm objective lens, and military or side parallax (focus) adjustment. The Mil-Dot reticle is a crosshair with round or oval dots that are a "mil" in diameter. They are used for ranging the target. The Army and Marines use two different Mil-Dot systems. If you've got a scope with this reticle, you need to know which kind it is to use the ability designed into the scope.

Purge Valve. On high end scopes and tactical scopes, there is a small "knob" built into the body of the scope (usually on the bottom left side) where the w/e knobs are located. This is a cover for the purge valve used to fill the scope with an inert gas (dry nitrogen) that prevents internal fogging of the scope from temperature changes. Charging the scope with nitrogen is done at the factory and should not be a factor unless the scope starts to leak nitrogen and starts fogging internally. Then it has to be returned to the manufacturer for repairs.

Kill Flash. A honey comb-like insert that screws into the objective end of the scope. The honey comb (Kill Flash -- a brand name) prevents sunlight reflections off the objective end glass but does not interfere with the view of the target through the scope.

Sun Shade. A tubular extension that usually screws on the end of the scope's objective end. (On some scopes, the tube is a stamping that slides forward.) The tube also prevents sunlight reflections off the objective lens of the scope.

Lens Caps. Rubber or plastic caps that fit over the objective and occular ends of the scope to protect the lenses when the scope is not in use. Butler Creek produces some of the most popular lens caps that open with the flick of a finger to use and snap closed to protect the lenses.

Other ways to check scope quality. Use a cardboard box that can support the scope tube between the occular and objective ends. Cut a 90 degree V notch in both sides so that the scope is supported in the V notches and the scope can rotate 360 degrees while sitting in the box. Center the crosshairs. Now put the box with the scope on a level surface so you can look through it against a bright background. Watch the reticle. The center of the reticle should remain centered as the scope is rotated through 360 degrees. If the reticle crosshairs do not stay centered -- that is, they move in a circular motion -- pass on the scope.

Shadow: To shoot a scope accurately, the shooter's eye must be positioned directly behind the scope in the same place every time. When you look throught the scope you should see a clear picture of the target with a sharp circle. If the side of the circle appears shadowy or fuzzy, this means your eye si not centered in the scope. A shadowy or fuzzy edge at the top means the bullet will strike low; if at the bottom, the bullet will strike high; if on the sides, the bullet strikes opposite of the shadow or fuzz.

Scope Adjustments: Scope adjustments should be positive. In an ideal situation you will hear and feel a click as the knobs are moved to adjust windage or elevation. Cheaper scopes do not provide this feedback to you. Worse still, you cannot be sure that the crosshairs actually moved where you meant them to go. Some cheaper scope crosshairs will lock-up for several adjustments and then move to where they're supposed to be set. In a high quality scope you can actually watch the crosshairs move around a zeroing target as you adjust the scope. That's repeatability. Also, you should know what kind of movement you can expect for each click. Some scopes have 1/8 minute clicks and some have 1/4 minute clicks. A minute is an angular measurement. One minute of angle equals 1 inch at 100 yards. Therefore, a 1/4 MOA is 1/4 inch at 100 yards.

Bore Sighting. Aligninging the axis of the bore (line-of-bore) with the axis of the scope (line-of-sight). The vertical plane of the axes (line-of-bore and line-of-sight) are identical. The horizontal plane of the axes (LOB and LOS) are parallel. Here's a simple example using a rifle, scope, rifle vise, level, and zeroing target.

1. Put the rifle in the rifle vise and make sure the top of the scope blocks are plumb (horizontal level) and adjust as required. Install or check the scope on the rifle for plumb (horizontal level) and adjust as required.

2. Install and turn on the laser boresight. Put the target at 25 yards. Position the target so the red dot is at the center of the zeroing target crosshair and use the level to make sure the target crosshair is plumb (horizontal level). Adjust as required.

3. Look through the scope and compare the scope vertical crosshair with the vertical crosshair of the zeroing target. The Vertical crosshairs should match. Now compare the position of the laser dot with the location of the horizontal crosshairs. Compare the horizontal crosshair of the scope with the horizontal crosshair of the zeroing target. They should be parallel. Now adjust the horizontal crosshair so it is about 2-1/2 inches above the laser dot. This is a rough zero. Take the rifle to 100 yards and check your sight setting with live ammunition. You should be on target within a few shot groups of 3 rounds.

:fal:

[tac-40]

Thanks guys for the info. Some of it knew, but most of it was new, I know what scope I want to get, so now I know what the spec sheet is saying.

ggiilliiee, I sure would like to get a Ziess, or Schmidt&Bender, Zawarski, Night Force or any of the other high end scopes. But my scope is going on a rifle that will only see 1-2 MOA at best. Not the rifle's fault, but that's the best I can shoot unless I get lucky. So no need to spend a couple of K for for overkill.

[brownknees]

MasterGunner.
Nice info.

The one thing I'll differ on is the ARD (or killflash) units.
I'll NEVER use or buy one of these again.
I was kind of skeptical once I discovered how they were made. This was because this is the same hardware as the things used in studios to "diffuse" pictures (damage sharpness intentionally)
To see a good example of diffusion look at the Select Comfort bed advertisments, the images of Lindsay Wagner are so diffused they are fuzzy!

The Killflash ARD DOES kill reflections from the fromt optic, but at a heck of a price in lost image sharpness, especially at higher magnifications.

[anymanusa]

good information