Darlot - Petzval type - 1/2 plate

Daguerre invented photography in France in 1839. Jean Theodor Jamin was a producer of optical instruments since 1822, and like many others, quickly started producing photographic lenses as, all of a sudden, the demand for those was huge. In 1860 Jamin retired and the business was taken over by a former employee Alphonse Darlot. From 1861 onwards the lenses beared only Darlot's name.

This one is a special Petzval lens. Like other Petzvals it is a cemented achromatic front element with another air spaced achromatic doublet at the rear. This illustration I took from a Book published in 1875 and shows the basic scheme. The arrow points the subject not the light direction.

The difference introduced by Jamin in 1855 and continued for many years by Darlot was the "Cone Centralizateur". That is the black cone that holds the rear component. Jamin argued that it helped in preventing internal reflections that otherwise would reach out the sensitized material.

Here is the actual diagram for this specific lens.It covers 13x18 cm (half plate) and the front element diameter is 6 cm. This illustration I copied from Kingslake book (ref. below). The brass barrel is different from mine and seems that it allow varying the distance between the two lens groups. Jamin, and later Darlot, had this kind of flexibility as a point of difference from other makers.

Another feature introduced was a smart mounting that permitted the use of the front achromat alone in a reversed position. That is the meaning of "vis portrait" and "vis paysage". If you take the cone out and screw the lens using the thread marked as "vis paysage" you have a traditional landscape lens with the concave surface facing the scene. With the cone you have a portrait lens.

Aperture control was provided by a set of disks at the narrower part of the cone. This is not very practical compared to Waterhouse stops or iris but sensitive material was so insensitive at that time, that probably in just a few occasions stopping down the lens was needed. This is corroborated by the fact that it is so hard to find a Dalot Cone Centralizateur wih the disks set. Anyway, the charm of using a Petzval lens construction today comes exactly from the beautiful out of focus area, or Bokeh, it provides. That is accentuated when fully open. Modern lenses for amateur digital photography are bound to produce the formerly sought after sharpness. But to many, sharpness looks so boring nowadays.

I bought this lens with a beautifull collodium camera in Bievres in June 2008. The making of the camera and the serial number of the lens put it on the early sixties. As said before, Darlot alone appeared after the retirement of Jamin in 1861, and I have seen some named yet Jamin Darlot with a serial number close to this one. That tells me that it was probably produced not long after the Darlot's take over .

If you like lens history and design you should absolutely download the "A Lens Collector Vade Mecum" it is only 15,99 USD and an invaluable source of information. Go to: Lens Vade Mecum It has more than 700 pages with thousands of lenses description, pictures and diagrams.

Another excelent source is Rodolf Kingslake, A history of the Photographic Lens (Academic Press, INC. ISBN 0-12-408640-3)

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