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 Optics 101 What do the numbers, etc. mean?
27Beck  [Member]
9/10/2009 9:05:29 AM
Complete noob to optics. Have searched, but did not hit the answers I was hoping for.

Need help with optics terms.

What does say 3-9x40 mean?

Is a 5-20x50 better than a 3x9-40?

What is parallax?

I'm sure there are also other term that go with optics, but answers to the above will be a good start.
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KavonTN  [Team Member]
9/10/2009 9:25:24 AM
Originally Posted By 27Beck:
Complete noob to optics. Have searched, but did not hit the answers I was hoping for.

Need help with optics terms.

What does say 3-9x40 mean?

Is a 5-20x50 better than a 3x9-40?

What is parallax?

I'm sure there are also other term that go with optics, but answers to the above will be a good start.


3-9x40 is just how the scope is. in that example, the magnification is variable from 3x to 9x and the objective is 40mm, good for gathering light on a dark day
5-20x50 is not necessarily better, just more magnification, different tool for a different job i guess
Market_Garden  [Member]
9/10/2009 9:52:02 AM
The larger objective lens isn't going to do anything for you in daylight. Where the larger objectives make a difference is when the light starts getting dim.

Here's a good writeup on parallax... http://en.wikipedia.org/wiki/Parallax
DevL  [Member]
9/10/2009 9:58:14 AM
As stated 3-9x40 is 3-9X zoom 40mm objective.

Exit pupil, in the simplest terms is the size of the beam of light coming out the back of the scope. If the exit pupil is large you have to be further off center before seeing a black cresent come into your field of view. Also anytime its dark enough that the exit pupil is smaller than your eye's pupil your image will be dark. For this reasn high magnification and a small objective lens is a bad combo for low light. A Nightforce 2.5-10x24 will have a 2.4mm exit pupl at 10X becasue exit pupil is objective size in mm divided by the magnification. Generally your pupil will get into the 5-7mm range in low light. This is why you get a brighter picture just oafter sun down by lowering magnification. In bright light your pupil is very small so you dont notice any loss of brightness in day light becasue your pupil is smaller than the exit pupil.

Parallax is the distance the reticle is focused at. Scopes with a fixed parallax will produce a reticle that is focused at one distance... say 200 yards. If you get very much closer or furhter the reticle OR the target will be in focus but not both. Also, when the reticle and target are in different focal planes you get more reticle drift for a given amount of your head being away from the centerline of the scope. If you take a scope with adjustbale parallax and set it very close and sight in on a target very far as yo umove your head in a small circle yo uwill see the reticle drift in a small circle on target. THis reticle drift causes a loss of accuracy and does not occur if the prallax and target are focused at precisely the same range. There are two types of adjustable parallax... an adjustbale objective will work by focusing at the objective the side focus parallax adds a lens so you can adjust from the left side of the scope but it much easier to see and reach from the shooting position and is what is most commonly used... the light lost by adding the lens is insiginifcant vs the improved ergonmics.

There is also a focus ring at the back of most scopes. This is a diopter adjustment and allows a user to ficus a scope to thier own eye so you dont need to use glasses. Yo uwill also note it can increase or decrease magnification slightly as it is adjusted. It chages the focus of the optic... it does not change the distance the reticle is focused at as that is the purpose of the adjustable parallax knob.

Also... as magnification increases yo uget a decrease in the depth of field or the range in front and behind your target that is in focus. This makes focusing the scope and reticle more precisely very important as you dont want to focus on the mirage itself but on the target. If you will shoot very far (most scopes have fixed parallax in the 100-200m range) with very high magnification you want adjustbale parallax. If you dont use very high magnification and dont shoot very far you dont need adjustbale parallax and can eliminate it by just staying well centered on the optic nor will you need a large objective lens.

Finally the eixt pupil that is larger than 8mm has NO bearing on low light performance as your eye cannot dilate more than this. However is does give more leeway for head position before you see black come into your FOV.

Eye relief is the optimal distance for viewing a clear sight picture without being too close or too far. As you get too close or far you will get black encroaching on your FOV when looking through the scope.

Hope this helps. Forgive the typos.