Spherical aberration

By comparing the wavefront of the electromagnetic radiation with the reference sphere, it is possible to determine what aberrations are present in an image and how severe they are.

Aberrations Physics

Aberrations are errors in an image that occur because of imperfections in the optical system. Another way of saying this is that aberrations result when the optical system misdirects some of the object’s rays. Optical components can create errors in an image even if they are made of the best materials and have no defects. Some types of aberrations can occur when electromagnetic radiation of one wavelength is being imaged (monochromatic aberrations), and other types occur when electromagnetic radiation of two or more wavelengths is imaged (chromatic aberrations). The origins and consequences of chromatic radiation were discussed in the previous section.

*The reason why focus should be adjusted at the highest power : The same as a camera, at lower power focal depth becomes deep and it comes into focus at a wide range from the front of the target to beyond. Therefore, when you zoom in at higher power the focal point may shift and the image may blur. On the other hand, at the highest power focal depth is shallow and it comes into focus at a narrow range. When you decrease the magnification, the focal point will not shift and the image will not blur.

Spherical aberrations occur for lenses that have spherical surfaces. Rays passing through points on a lens farther away from an axis are refracted more than those closer to the axis. This results in a distribution of foci along the optical axis.

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Aberrations 5e

Before adjusting the focus, you would need to adjust the diopter of the eyepiece. Diopter should be set at the lowest magnification. Please see HERE for detailed instructions.

A reference sphere isn’t a physical structure; it’s just a mathematical construct that the wavefront of the electromagnetic radiation is compared to. If the electromagnetic wavefront has the shape of the reference sphere, then the wavefront will come to a perfect focus at the center of the sphere. Remember that the definition of a ray specifies that rays are drawn perpendicular to the wavefront. All of the rays associated with a spherical wavefront will intersect at the center of the sphere. If the wavefront is not spherical, some of the rays will pass through the center of the sphere.

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Aberrations in optics

When you focus on the target by adjusting the side focus turret, you are actually moving the focus lens back and forth. By moving the focus lens back and forth, the target image formed on the first or second focal plane will move back and forth. Reticle image is at a fixed position regardless of the focus adjustment. (For FFP (first focal plane) scopes, even though there is no reticle on the second focal plane, reticle image will be formed on the second focal plane as well.) When the reticle image and the target image match, you have focused perfectly. When adjusting the side focus turret, please make sure to do it at maximum magnification. If you adjust the focus at low power, when you zoom in at higher power the focal point may shift and thus the image may blur.

Monochromatic aberrations can be grouped into several different categories: spherical, coma, astigmatism, field curvature, and distortion.The idea of reference sphere is often used in discussions of aberrations. For all spheres, a ray drawn perpendicular to the sphere’s surface will intersect the center of the sphere, no matter what spot on the surface is picked.

Some rays on an aberrated wavefront focus to a different point, W, than do rays that are perpendicular to the reference sphere.