Abstract
The intensity distributions of light in the images of a luminous line and a surface boundary formed by lenses with spherical aberration were measured by means of photoelectric photometry, and the following quantities were introduced in order to represent the characteristics of the image quantitatively. They are, for a luminous surface boundary, the distances between the geometrical image of the surface boundary and the points where the light intensity falls to 0.1, 0.02, and 0.01 of that at the bright part; for the luminous line of finite width, the amount of light concentrated in the center of the image core. The quality of an image depends upon the diffraction of light as well as the amount of residual aberration, but these four quantities seem to be sufficient in expressing the performance of a lens in a quantitative way. It was also found that the best focus of a lens is neither the image plane which gives the minimum halo light nor that which has highest resolving power, but the image plane which gives the maximum light concentration in the center of the line image.

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