Petzval lens
Encyclopedia
The Petzval lens was developed in 1840 by Joseph Petzval. It consisted of two doublet lenses with an aperture stop in between. The front lens is well corrected for spherical aberrations but introduces coma
. The second doublet corrects for this and the position of the stop corrects most of the astigmatism
. However, this results in additional field curvature and vignetting
. The total field of view
is therefore restricted to about 30 degrees. An f-number
of 3.7 was achievable, which was considerably faster than other lenses of the time.
The speed of the lens is ideal for portrait photography
since the exposure time could be reduced.
Coma (optics)
In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...
. The second doublet corrects for this and the position of the stop corrects most of the astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
. However, this results in additional field curvature and vignetting
Vignetting
In photography and optics, vignetting  is a reduction of an image's brightness or saturation at the periphery compared to the image center. The word vignette, from the same root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a photographic...
. The total field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
is therefore restricted to about 30 degrees. An f-number
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
of 3.7 was achievable, which was considerably faster than other lenses of the time.
The speed of the lens is ideal for portrait photography
Portrait photography
Portrait photography or portraiture is the capture by means of photography of the likeness of a person or a small group of people , in which the face and expression is predominant. The objective is to display the likeness, personality, and even the mood of the subject...
since the exposure time could be reduced.