Single lenses give a lower resolution because of lens aberrations: they were reintroduced in the late 19th century for ‘impressionistic’ photography (Dehors Spectacle Lens Sets, 1895), as were simple uncorrected portrait lenses (Wollensak, Pinkham, and Smith) in the early 1900s. Combination lenses designed to produce diffusion included convertible triplets such as John Dallmeyer's ‘Patent Portrait Lens’ (1866; modified 1886, 1909), variable-focus lenses (Thomas Dallmeyer, Adolph Miethe, both 1891), and the uncorrected telephoto Dallmeyer-Bergheim portrait lens (1896-1937). All altered the focus by changing the distance between lens elements, and were popular with pictorial and portrait photographers. Adjustable diaphragms could also be used to control the effect of ‘off-axis’ image rays through the lens, responsible for soft focus.
— Hope Kingsley
