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The Ocular Lens, is the lens or collection of lenses which is closest to your eye. The lens assembly typically consists of a plano-concave lens, and a combination of bi-convex lenses, spacers, and double concave/double convex lenses. The lens is housed in the ocular bell also known as the eyepiece.
It is the Ocular Lens that is typically adjusted to alter the focus of the reticle for the viewers eye.
The journey of the image to your eye
The Objective Lens
As we discussed in our article on the Objective Lens. Light travels from it’s source, reflects off the object we are looking at and is viewed by our eye.
These lights waves are considered in physics to be travelling in straight lines.
The objective lens collects these light waves and bends them (refraction) to one point, this is called the focal plane.
Due to the nature of this process, the image is inverted (upside down). See the following diagram:
The Erector Lenses
The image needs to be inverted once more (rectified) so it appears upright at our eye. This is achieved by the Erector or Reversing Lenses.
In fixed magnification scopes the Erecting lenses are attached and immovable in the tube. On variable magnification scopes the Erecting lenses are typically attached to framework which can move forward and back.
This changes the distance of the focal plane to the Ocular lens, thereby altering the magnification.
More on this in an upcoming article on scope magnification and zoom. We will also cover focus in the same manner.