Originally Posted by
karl french
In 1866 Dallmeyer introduced the Patent Portrait Petzval. Flipping the rear air spaced pair and slightly changing the curves of the elements. Also allowing the two rear elements to be separated to introduce diffusion. Many people copied Dallmeyer's changes. Those copies are rightly called "Dallmeyer copies" as an easy way to identify them in comparison to the "standard" or original Petzval design.
In fact this lens is a Dallmeyer copy. Not by the mere fact that it's an f4 lens, but because it has the flipped rear elements and diffusion control at the rear.
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