How to Check for Chromatic Aberration in Scopes

Light rays entering a scope are broken into three constituent colors of red, blue and green by the objective lens. The separate colors intersect at a convergence point called the focal plane. Images formed at the focal plane are then brought into focus by adjusting the ocular lens. Due to differing wavelengths of the colors and the quality of the optical glass of the lens, the convergence point of each color may vary minutely. One color is in perfectly true focus at the focal plane while a slight discrepancy exists in the focus of the other colors. This aberration, also known as "color fringing," usually manifests as a fringe of one color along the edge of a light image where it contrasts against a much darker image. Because chromatic aberration is most pronounced at higher magnifications, a scope should be tested at a location where it can be adjusted to maximum magnification.

Things You'll Need

  • Paper target with black bullseye
  • Stick-on white spotting disk
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Instructions

    • 1

      Take the scope to a shooting range or other location with sufficient open viewing distance to use the scope at maximum magnification.

    • 2

      Hang up a standard paper bullseye target with a white spotting disk stuck over a portion of the black bullseye.

    • 3

      Adjust the scope to the highest magnification.

    • 4

      Focus the scope on the bullseye.

    • 5

      Look for evidence of color fringing around the edge of the white spotting disk where it borders the black bullseye.