Dictionary - Lens Information

Common Optical Terms

Spherical Aberration
	SPHERICAL ABERRATION If a lens was perfect , it should give its highest resolving power at full aperture, but in practice this is not the case. The best definition is normally found at approximately two stops below full aperture. If the lens has to be stopped down much further to achieve good resolution , then spherical aberration is indicated. This aberration is concerned with the inability of the lens to give a point image of a point source accurately over the complete negative area due to the light striking the lens at different angles upon the outer curved surface .
Chromatic Aberration
	CHROMATIC ABERRATION Many lens descriptions refer to the colour correction of lenses . This is done mainly to suggest that the lenses are suitably corrected for colour photography. The advantages of colour correction are not , however, solely limited to colour since this correction is equally important for black and white photography. Unless all colours come to focus at the same point , the definition will suffer on any colour sensitive emulsion .
Astigmatism
	ASTIGMATISM This occurs mainly at the edge of the negative . Lines running in different directions are not defined equally sharp . Astigmatism is revealed when photographing a dart-board pattern , radial lines running in one direction will appear darker than those running at right-angles to them. Lens design may give priority to correcting spherical aberration leaving a slightly higher degree of astigmatism by comparison .
Coma
	COMA In theory each image point should be reproduced as a very tiny circle of light. With a lens set at maximum aperture , at the edges of a negative these tiny circles distort and become elongated. this is known as coma . If it is not well corrected the edges of a negative can be undistinguishable. Stopping the lens down to a smaller aperture will improve the situation . PLEASE GO TO, Dictionary of Optical Terms , BELOW FOR MORE INFORMATION.