Chocolate Cows and Lens Aberrations

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This mnemonic was how I was taught to remember the seven basic aberrations in lenses. Is this information a must know to be successful as a photographer? Of course not. But when researching the purchase of a lens or wondering why that unexpected effect is in your image, this info might come in handy. The extent to which lenses are corrected for these faults is the prime determining factor for the cost and quality of lenses. Some lenses purposely leave some aberrations in the lens or even exaggerate them such as in some of the aftermarket soft focus lenses.

Chromatic Aberration is the inability of the colors to focus at the same point on the image plane. An Apochromatic lens is one which is corrected for three colors. An Achromatic lens is one that is corrected for two colors. This lens fault must be corrected by lens design.

Chromatic Difference of Magnification is the fault caused when one color is magnified more than another color even though they both may focus properly. This fault is usually corrected in most lenses for a specific distance only (about 25’). Apochromatic lenses have the maximum correction for this fault. This lens fault must be corrected by lens design.

Coma is a fault due to the fact that a lens will produce slightly different images from it’s various areas, and the image will emerge not as a point, but rather a tear- drop-shape or elongated, or some other shape. This fault can be corrected by proper placement of the diaphragm and also by stopping the lens down.

Spherical Aberration is caused by the light rays passing through the center of a lens focusing further back than those rays entering through the edge of the lens. A lens that has spherical aberration present will produce an image of a point source of light which appears to have a halo around it. This fault can be corrected using aspherical surface lenses and also by the proper design of additional elements.

Distortion is a difference of size in various parts of the image. Distortion is usually due to the fact that the lens is not balanced, that is, symmetrically designed on either side of the diaphragm. Barrel or pincushion are the most common distortions. Distortion can be corrected by constructing the lens of two identical cells with the shutter and diaphragm between.

Curvature of Field is when the focused image of a lens does not lie in a perfectly flat plane. This is not enough of a problem with most lenses to really bother. This fault is the reason the film plane on some of the older, cheap box cameras is curved.

Astigmatism cause an effect of both horizontal and vertical lines not being in focus. These are usually called radial and tangental lines. This fault must be corrected through the lens design.

 Lens Aberrations

Lens Aberrations