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Error-free depiction of the field of view

Coma and astigmatism

Both image defects always occur on the regularly curved surfaces of spherical lenses and cannot be avoided in the first instance.

Coma occurs when parallel rays of light strike a spherical lens at an angle, astigmatism when divergent rays of light strike the spherical surface at an angle.

Both of these errors can be corrected mainly by the complex combination of several lenses and the use of aspherical lenses.

The technical effort required to correct these errors is reflected in the price of the lenses.

Coma (asymmetry error)

If a collimated ray ("parallel" light) incides on a lens not in parallel but at an angle to the optical axis, the ray will pass through the optic system in an unsymmetrical way due to different surface curvatures.

In case of this imaging error, the rays are not bundled again in one image point. The focal points are therefore also not on the optical axis, but shifted towards the margin.

In the image this error is visible as a drop-shaped, tail-like unilateral distortion of an image spot. The tail is always outwards in radial direction. Coma is caused by spherical aberration.

The origin of the asymmetry error (coma)

Astigmatism (lack of focus)

If a diverging ray of light incides vertically on the lens surface and thus passes unsymmetrically to the optical axis, astigmatism occurs on spherical lens surfaces.

For easier examination, the inherently cone-shaped beam can be divided into two planes perpendicular to each other. (Both are approximately oriented towards the main beam which passes through the centre of the aperture, the meridional beam extends towards the optical axis, the sagittal beam is at right angles to it.)

The cause of astigmatism with an oblique beam is the different local radii of curvature of the circles of latitude compared to the larger constant radius of curvature of the meridional plane on the lens surface.

This results in two different focal points and focal lengths for the different (meridional and sagittal) partial ray paths.

The image point is no longer represented as a single point, but as two image lines. It no longer appears sharp, but pointless. The camera image no longer appears sharp to the viewer. This error can be suppressed by using special types of lenses.

Meridional and sagittal ray

Creation of astigmatism

Important for Machine Vision

  • Coma can be compensated slightly by stopping down the lens, astigmatism unfortunately not. Please stop down slightly!
  • Buy proper optics the lens design of which corrects as many errors as possible.

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