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Why Do Scopes Have Adjustable Objectives?

The same principles apply to all telescopes, whether for hunters, astronomers or sightseers. Although telescopes were invented more than 400 years ago, their design is still being improved. Some of these improvements come in the form of specialization. The basic principles of the telescope are unchanged, but modifications are introduced to make the telescope function better for a specific application. One of the modifications for rifle scopes is an adjustable objective lens.

Telescope Design
- A distant star acts as a point source for an astronomical telescope. That is, every ray of light that comes from a star is parallel to every other ray. All light that comes into a telescope objective from the same angle is focused to the same point. For distant stars, all those points are at the focal plane of the objective lens. The eyepiece of an astronomical telescope is like a magnifying lens aimed at the focal plane of the objective lens. The eyepiece takes the tiny points of light at the focal plane and makes them large enough to see.
Adjustments
- When a scope is used to look at closer objects, it works differently. The place where light from an extended object is focused depends on its incoming angle and its position. This is not a problem, because the eyepiece can be adjusted to focus on the image created by the objective and magnify it so it can be seen. But this creates an issue for hunters, because one of the modifications that has been introduced for rifle scopes is a reticle --- a crosshair or other targeting aid placed between the objective and the eyepiece. So if the eyepiece is adjusted to focus on a target, the reticle won't be in focus. This introduces a problem called parallax.
Targeting
- Target reticles come in various shapes, but the scope must be adjusted properly to use them to advantage.
  
  The purpose of a reticle is to make for accurate targeting. So in a rifle scope, the objective has two jobs: magnify the image and magnify the reticle. When hunters look through a scope, they expect that when the reticle appears to be on target the rifle will be accurately aimed. But if the objective lens puts the image at a different plane from that of the reticle, this won't necessarily be true. The position of the target relative to the reticle will be different depending upon where the hunter's eye is. If the eye is on the centerline of the scope, there's no problem, but being off-axis even slightly will make the target appear to shift.
Adjusting
- You could ask, why don't manufacturers just set the objective lens correctly at the factory so it makes its image at the reticle? The problem is that the plane of the image depends on the distance to the target. So the need to make the adjustment falls on the hunter. If the eyepiece is adjustable, set it to focus on the reticle clearly. Then pick an object close to where you expect the target to be and adjust the objective lens so the reticle stays fixed on one point in the scene as you move your eye slightly left and right. Now you've put the target image and the reticle at the same plane within the telescope, and parallax won't be a problem.

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