Tt&h cooke convertible lens questions

[Craig Tuffin]

Thank you too Jim. I think I knew it was too good to be true

[Roger Hesketh]

Mark to be honest I am not sure how the extension lens was used whether it replaced the rear lens in which case with it being a triplet lens I would expect it would throw everything out of kilter. Or as a supplementary lens which would I suppose would have meant that it would have had to have been negative. The Verito is the only other lens I know of that you can swap single lens elements in that case the front element which is only their to reduce the power of the rear group anyway so changing it or removing it does not affect the optical performance greatly.

I agree about what you say about convertibles in general which is why I pointed out to Craig that his lens is not one that we would generally think of as a convertible lens even though I strongly suspect it has two aperture scales. Whereas many of the lenses we have been discussing do not as they were never sold as being convertible or rarely they were once sold as convertibles but were later on in the production run not sold as such.

One such lens being the Wollensak Series IIIa Wide Angle which has a very similar optical formula to the Lee designed Cooke Series XV Convertible lens That of two air spaced doublets either side of a central stop.

See this link fellow forum member Desertrat explains much more eloquently than ever I possibly could.

http://www.largeformatphotography.in...at-By-Another-
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I don't believe Ernst Gundlach is necessarily afforded the respect he deserves as a lens designer. People speak of his Turner Reich as just having an extra element to get round Zeiss's Protar patent. I don't believe so. I think he put it in their for a reason. Especially considering as we can see here he was perfectly capable of designing and building his own unque Anastigmat and one which was much easier to build. In many ways a very similar design the air space acting as an element. Quite an acheivement to design two unique anastigmat lenses which were still in production 60 years later.


Thanks for the info on RR and Petzvals Mark I do believe you have inspired me to have a play to see what sort of images can be made from bits of them Thanks

[Jim Galli]

Mark to be honest I am not sure how the extension lens was used whether it replaced the rear lens in which case with it being a triplet lens I would expect it would throw everything out of kilter. Or as a supplementary lens which would I suppose would have meant that it would have had to have been negative. The Verito is the only other lens I know of that you can swap single lens elements in that case the front element which is only their to reduce the power of the rear group anyway so changing it or removing it does not affect the optical performance greatly.

I agree about what you say about convertibles in general which is why I pointed out to Craig that his lens is not one that we would generally think of as a convertible lens even though I strongly suspect it has two aperture scales. Whereas many of the lenses we have been discussing do not as they were never sold as being convertible or rarely they were once sold as convertibles but were later on in the production run not sold as such.

One such lens being the Wollensak Series IIIa Wide Angle which has a very similar optical formula to the Lee designed Cooke Series XV Convertible lens That of two air spaced doublets either side of a central stop.

See this link fellow forum member Desertrat explains much more eloquently than ever I possibly could.

http://www.largeformatphotography.in...at-By-Another-
Name











I don't believe Ernst Gundlach is necessarily afforded the respect he deserves as a lens designer. People speak of his Turner Reich as just having an extra element to get round Zeiss's Protar patent. I don't believe so. I think he put it in their for a reason. Especially considering as we can see here he was perfectly capable of designing and building his own unque Anastigmat and one which was much easier to build. In many ways a very similar design the air space acting as an element. Quite an acheivement to design two unique anastigmat lenses which were still in production 60 years later.


Thanks for the info on RR and Petzvals Mark I do believe you have inspired me to have a play to see what sort of images can be made from bits of them Thanks

Was Gundlach the designer at Wollensak in that time frame? Never heard his name mixed up with Wollensak lenses before.

[Roger Hesketh]

Wollensak were principally shutter makers. They bought the Rochester Optical Company in 1905 for it's line up of Royal Anastigmats.

The Rochester Optical Company was founded by Ernst Gundlach and was renamed when he left the coumpany in his day It was named something along the lines of the Ernst Gundlach Son and Co.

Gundlach designed the Royal Anastigmat which is what early Wollensak series 1 Convertibles are. The later ones when the Zeiss patent expired are Protar VIIa 'copys' .

Gundlach is also credited as patent holder of the Wollensak wide angle which is a stretched varient of the Royal Anastigmat. Stretched with wider glasses similar to the way the Watson Wde Angle Holostigmat is a stetched version of Watson's Series 1 Holostigmat.

Jim, Gundlach never worked for Wollensaks they bought his old company He worked pretty much everywhere else though. When he first came to the US he worked for B&L founding their microscope department.

Other lenses in the Wollensak line up were Rochester Optical products for instance The Vitax was previously available as the the Royal Portrait lens

[Mark Sawyer]

Mark to be honest I am not sure how the extension lens was used whether it replaced the rear lens in which case with it being a triplet lens I would expect it would throw everything out of kilter. Or as a supplementary lens which would I suppose would have meant that it would have had to have been negative. The Verito is the only other lens I know of that you can swap single lens elements in that case the front element which is only their to reduce the power of the rear group anyway so changing it or removing it does not affect the optical performance greatly.

I'm not too sure about it either, Roger. My suspicion is that it's a positive single achromati that shortens the focal length when screwed on the front, but it could also be a negative that lengthens it, and that would make more sense with the word "extension". I don't think it would replace an element, though I could be wrong...

There were a few other lenses that had removeable or interchangeable elements. The Portrait Plastigmat had a front element and could be used with or without, similar to the Verito. The 18" Verito's were available with a second front single-meniscus element that could be swapped out to make it a 22", and the 16" Vitax was available with a second front achromatic doublet cell that could be switched with its regular front cell to make it a 20" f/5 Petzval. And of course there were all sorts of casket sets around, so there was a fair amount of lens-element-swapping going on...

[Hermes07]

From the 1912 Cooke Brochure

"The Series III., IV. and V. Extension lenses are designed to
replace the back glass of the normal Cooke lenses, and the Series II.
and 11 A. are designed to replace the front glass. The exchange can be
effected quite easily, being simply a matter of unscrewing the normal
lens and screwing in the Extension."

Happy to email or upload it for anyone interested.

[Mark Sawyer]

Oh, well if I'd known we were actually going to look things up so we knew what we were talking about...

[Amedeus]

I can confirm this for the Series II. I have a 8.2" Cooke Series II and the 12.8" extension lens for Series II. Just unscrew the front and screw in the (positive) extension lens for the new combination.

From the 1912 Cooke Brochure

"The Series III., IV. and V. Extension lenses are designed to
replace the back glass of the normal Cooke lenses, and the Series II.
and 11 A. are designed to replace the front glass. The exchange can be
effected quite easily, being simply a matter of unscrewing the normal
lens and screwing in the Extension."

Happy to email or upload it for anyone interested.

[Roger Hesketh]

Oh, well if I'd known we were actually going to look things up so we knew what we were talking about...

+1

A two stop decrease in effective aperture to gain a 50% increase in focal length seems rather a lot.