Early Petzval stop location

[erian]

Where is approximately the stop of a Petzval type magic lantern/early type Petzval lens?

I ask this to be able to measure the focal length more accurately.

[Two23]

The first ones had no stops. In the 1850s it was a big washer placed roughly halfway between the lens groups. After ~1860 waterhouse stops placed in roughly same spot. Not sure why you need such precision since wet plate is more than a bit imprecise when it comes to exposure.


Kent in SD

[erian]

The first ones had no stops. In the 1850s it was a big washer placed roughly halfway between the lens groups. After ~1860 waterhouse stops placed in roughly same spot. Not sure why you need such precision since wet plate is more than a bit imprecise when it comes to exposure.


Kent in SD

Thanks. I was not sure that it is more or less in the middle.

[Steven Tribe]

The central stop - or the position of the Waterhouse slot - was never quite in the middle. The “correct” position is dependent on the comparative focal lengths of the front and rear optical groups.

Central mounting was not so common as it required accurate machining (screw thread or bayonet) and was most often used when supplementary central lenses were available or when the front achromat of a Petzval could used separately.

The usual F control was a pill-box system om top of the Lens hood with various sized washers. One French speciality was the “cup” insert stop system. This is where washers are placed quite close to the top surface of the front achromat, deep inside the lens hood. Quite a neat system, but few of these Hermagis, Lerebours etc. lenses from the 1840’s 1850’s still have this attachment as Waterhouse conversions made them surplus to requirements.