Identical Lens: However, although a long focal length lens is mandatory, it need not be expensive. The utmost of critical sharpness in a portrait lens is not necessary, or even desired, since considerable diffusion can be tolerated in portrait negatives. Your lens needn't be in a shutter for strictly studio portraits, either. A lens in barrel is perfectly satisfactory, since you can provide yourself with a simple Packard shutter to use behind the lens. Many portrait men actually prefer the Packard to the more costly between-the-lens shutters.
In 1866 two opticians, Hugo Adolph Steinheil of Munich and John Henry Dallmeyer of London, independently and simultaneously designed almost identical lenses composed of two symmetrical cemented elements mounted facing one another with a central stop: spherical aberration was corrected to a marked degree and astigmatism somewhat. Both had a field of view of about 25°, and a working aperture of f/6 to //8. Steinheil named his lens the Aplanat, and Dallmeyer chose Rapid Rectilinear, a name that became generic when the design was almost at once universally adopted and became the most widely used photographic lens until it was replaced by the anastigmat in 1893.
The first lens designed specifically for photographic purposes was Petzval's 1840 portrait lens. The images formed by this lens showed great loss of definition at the corners of the plate-a fault more theoretical than practical in portraiture, where edges mattered little. For outside work, however, particularly in photographing architecture, a lens with a flat field was desirable; and one free of spherical aberration, which caused straight lines to be imaged as slightly curved, was essential.
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Photography
