Thursday, 20 March 2014

Lens Aberrations

What is Lens Aberration?

In an ideal optical system, all rays of light from a point in the object plane would converge to the same point in the image plane, forming a clear image. The influences which cause different rays to converge to different points are called aberrations.

What Causes Chromatic Aberrations?

Chromatic aberration is caused by lens dispersion; it occurs when light of different wave length travel through a lens at different speeds. This causes the image to look blurred and also causes noticeable color cast (red, green, blue, yellow, purple or magenta) along the edges, especially of bright objects or where there is high contrast in an image. There are different types of aberrations:

1. Axial Chromatic Aberration

Axial chromatic aberration is the failure of a lens to focus at least two out of the three primary colors to the same point on the film plane causing color fringing.

axial and lateral chromatic aberration of camera lens
axial and lateral chromatic aberration of camera lens

2. Transverse Chromatic Aberration

Transverse Chromatic Aberration is most likely to occur with telephoto lenses. It is noticed as color fringing and occurs when the lens magnifies different wavelengths differently.

3. Spherical Aberration

shperical aberration
shperical aberration

Spherical Aberration is the failure of a lens to focus the rays of light passing through the centre and the edge of that lens to the same point. This causes a blurring of detail across the whole image.

4. Coma

coma illustration courtesy wikipedia

Here point sources of light to appear as smeared comma shapes on film. It affects the image more towards the edges and progressively less towards the centre, so when using lenses which have such a problem try to keep bright light sources away from the corners of the frame.

5. Curvature of Field

Field curvature
Field curvature illustration courtesy wikipedia

Curvature of Field is the failure of a lens to focus its image onto a flat plane, as a result when focus is on the center of the image, the corners will not be sharp, and vice versa.

6. Astigmatism

Astigmatism: Illustration by Sebastian Kroch

Astigmatism causes a lens to form a linear image from a point source of light, with the line changing from vertical to horizontal as the lens is focused. The effect of residual astigmatism in a lens is its inability to focus equally horizontal and vertical lines, as a result vertical lines in the frame may appear sharper than the horizontal ones.