Jamin and Darlot produced for a few years around 1860, a couple of versions of Petzval Portrait Lenses with adjustable spacing between the front and rear group. (Not to be confused with Dallmeyer's 1867 introduction of variable spacing in the rear group for increasing depth of field via spherical aberration.) The adjustable spacing was on at least some models of the Cone Centralize, but also on a more conventional model of their Petzval. An Eastman House publication on the Cone Centralisateur states:
"Although basically nothing but a Petzval objective, this lens embodied several interesting features, the most distinguishing characteristic being the conical shape of the rear half of the lens barrel (cone centralisateur) which was intended to prevent internally reflected light from reaching the photographic plate. Also the separation between the front and rear components could be varied, either by simply drawing out the front portion of the barrel or by use of a rack and pinion mechanism, the purpose of this being to enable the user to obtain the best possible aberration correction for any particular object distance."
Does anyone know any more detail about which aberration corrections were possible by varying the spacing? Or know of any information on using the feature?
BTW, here's a photo of the more conventional Jamin Darlot Petzval, showing how the feature can vary the length by about two inches. The smaller knob is a friction lock allowing the front section to slide in and out independently from moving the rear section:
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