Thanks Gunny! Now look what you've done ... he's a scope info monster!
Parallax: The apparent change in the position of an object resulting from the change in the direction or position from which it is viewed. (from Websters)
In nature it is caused by the bending of light by either the atmosphere or water. If you shoot at a fish under water and aim directly at what you see, you will probably miss the fish because it's not really where it appears to be. Just like the sun, which does not really squish into an oval shape before it sets, but it appears that way.
In scope optics, light is bent as it comes through the lenses to cause magnification (which is really just a change in appearance of the object since it really doesn't get closer) and that bending comes to a focal point at some distance from the scope. That distance is the true focus distance for that scope, at that power setting. Objects nearer or farther than that distance can appear to be in focus but really aren't and if you move your point of observation (your eye) behind the scope, the object you are viewing appears to move in relationship to your center of field of view (defined by the crosshairs). The movement distance appears greater as the object is viewed at greater distances nearer or farther than the true focal point. If you could always place your eye in the exact center of the field of view then parallax would be unimportant, but you can't do it. So most scopes are made to be parallax free at approximately 100 yards at their average power setting.
With an adjustable objective, however, you can focus your scope to any distance you want and eliminate parallax at that distance, regardless of power setting. This is very important for precise shot placement and obtaining small groups at any appreciable distance (like benchrest matches).
By the way, DO NOT trust the range markings on the objective bell of most AO scopes. Dial in the distance to your target and, with the scope set to the power level you want to use, sandbag and aim at your target. Move your eye left/right/up/down and look for apparent movement of the target in relation to the crosshairs. Make fine adjustment to the objective until no movement is detected. Look at the setting and record it as being the proper one for that distance. Repeat for each distance you shoot and you'll soon memorize the proper settings. If you forget, look back at your notes.
Target Knobs: Most scopes come with small turrets with screw-on covers. Beneath each cover is a flush adjustment "knob" with a screwdriver type slot for adjusting windage or elevation using a coin for a screwdriver. Target knobs are larger and, mostly, uncovered. They allow you to make adjustments for windage and elevation by hand (just turn the knob).
Side focus: Replaces the adjustable objective lens bell with a large knob on the side of the scope for parallax and focus adjustment. Allows you to make those adjustments easier while looking through the scope.
Heavy duplex: Refers to a type of crosshair reticle. Appears to be a very heavy line at the edges of view but becomes finer at the crossing area. You will also see fine crosshair, duplex (regular), crosshair and dot (dots come in different sizes usually listed in minutes), post reticle and on and on.
Minute: Since I brought it up...means minute of angle (60 minutes to one degree). It is approximately 1 inch at 100 yards. A 1 minute dot would appear to be 1 inch in diameter at 100 yards.
Cams: Devices used in the elevation adjustment knob to compensate for bullet drop at various distances for a specific cartridge. When installed properly, you sight in at a distance corresponding to the knob setting and just turn the knob to any other setting and the adjustment is made for that new distance.
I'm getting hungry now and I think I'll stop here. You have entered a whole new hobby area with scopes and long range precision shooting. It will be a long, methodical process to get where you want to be. Go for it!
Mikey
It was all assault rifles for me, until I got tired of blasting away dirt hills, and now I want more of a challenge.