Help identifying Dallmeyer No. 3 Compound Tele-Photo Attachment

[Fragomeni]

Looking for a little help clarifying what exactly I have here.

This is marked JH Dallmeyer No. 3 Compound Tele-Photo Attachment Patent Dec 1891.

It came attached via a period correct custom flange on a Dallmeyer 2B lens. What’s tripping me up is that I cannot get it to work with the lens at all so I’m wondering if this was an attachment for a different Dallmeyer lens that somehow founds its way to being attached to the 2B that I have. It’s all quite confusing though because the mounting flange looks to all be original Dallmeyer work and is made to fit to the 2B.

Referring to an 1896 Dallmeyer catalog, a few versions of the tele-photo attachments are mentioned with illustrations but none quite match this one. Here is the catalog. The tele-photo attachments are toward the end:

http://www.thedallmeyerarchive.com/Records/catalogues/1896Dallmeyer%20Catalogue.pdf

The tele-photo attachments shown there all have larger negative elements much closer in size to the front and rear elements of the 2B and other portrait lenses that they were made for. The one I have has a much smaller negative element (see image below).

When attached to the lens in all configurations I can think of i.e. with and without the rear elements attached (I’ve even tried without the front element as well as reverses the negative element and mounted it inside the cone rather than the outside as pictured) to the portrait lens, it all fits together nicely but does not focus and image on the ground glass at any bellows extension. The further out the bellows is extended, the more out of focus things seem to get. The more I shorten the bellows, it seems to start getting a little more in focus but never anywhere near making an image. There is a limit to how collapses I can make the bellows because at a certain point the rear element of the attachment touches the ground glass.

So I really have no idea what’s going on with this thing. I’ve also been unable to find any photos or documentation matching the design of this particular tele-photo attachment. Any insight would be appreciated.

 

[Fragomeni]

Update.

I’m a little dumbfounded by this thing and have no idea what it would have been used for but I’ve made a bit of progress on figuring out how the attachment works. I was looking at it this morning and it struck me that the design of the housing around the lens element looks a lot like a loupe. So, I attached it to my 2B and put the thing up to my eye and low and behold, it turns the lens into a low power telescope. The drive shaft knob on the lens body changed the focal length and works as a zoom (it would do the same with the normal tele attachment as well). This explains why it won’t throw an image on a camera— it’s an eyecup and is meant to be held to the eye and viewed through.

If anyone has information on Petzvals being used as low powered telescopes please let me know. I’d love to know what this may have been used for.

 

[Bronson Dugnutt]

It's probably some sort specialty afocal teleside converter, akin to those found as accessories for modern cameras with fixed lenses.

There is a reference to Dallmeyer optics on this page, but might only pertain to the well-known models in the catalog you've referenced.

 

[reddesert]

Rear mounted telephoto attachments of this type are a negative lens that goes behind the main positive lens to make the rays converge slower, so that the combination has a longer focal length. That positive-negative arrangement is also how a Galilean telescope works (negative lens as eyepiece), which is presumably why you can use it as a telescope, but I doubt that was its original intention. The telescope eyepiece has a shorter focal length than the objective, while for telephoto lenses the converter should be longer focal length than the objective, so that's a clue.

The telephoto effect of greater focal length decreases as you increase the distance between the lens and negative attachment.

If the attachment has a shorter focal length than the primary lens (for example a 300mm main lens and an attachment with focal length (-)200 mm), then the combination won't work as a positive lens - the attachment is too strongly diverging.

My guess (not really knowing anything about Dallmeyers in particular) is that the attachment is intended to go with some other lens. You could probably figure out what's going on by measuring the focal length of the attachment, but measuring the focal length of a negative lens is a bit of a pain. One way to do it is to measure the magnification of the Galilean-telescope configuration, which is the ratio of the focal lengths of the objective and eyepiece.

 

[Fragomeni]

Rear mounted telephoto attachments of this type are a negative lens that goes behind the main positive lens to make the rays converge slower, so that the combination has a longer focal length. That positive-negative arrangement is also how a Galilean telescope works (negative lens as eyepiece), which is presumably why you can use it as a telescope, but I doubt that was its original intention. The telescope eyepiece has a shorter focal length than the objective, while for telephoto lenses the converter should be longer focal length than the objective, so that's a clue.

The telephoto effect of greater focal length decreases as you increase the distance between the lens and negative attachment.

If the attachment has a shorter focal length than the primary lens (for example a 300mm main lens and an attachment with focal length (-)200 mm), then the combination won't work as a positive lens - the attachment is too strongly diverging.

My guess (not really knowing anything about Dallmeyers in particular) is that the attachment is intended to go with some other lens. You could probably figure out what's going on by measuring the focal length of the attachment, but measuring the focal length of a negative lens is a bit of a pain. One way to do it is to measure the magnification of the Galilean-telescope configuration, which is the ratio of the focal lengths of the objective and eyepiece.

Thanks for this info. The adapter cone is fitted to the lens via what appear to be all original Dallmeyer hardware and machining. Metal, machining, color, wear, and even details down to the ornamental knurled edges all match the lens exactly which is why I'm thinking this was all done by Dallmeyer for the lens (as opposed to something from another lens that found it's way on this one. There are usually signs of a mismatch in cases like that but does not seem to be the case here. I've done a bit more digging but have not come up with an understanding of what this might be. What continues to strike me is the much smaller size of the lens in the adapter relative to the size of the cone. The cone, as I mentioned is perfectly fitted to go over the much larger rear objective of the lens and screw into place (the same as all of the Dallmeyer tele-photo attachments I've seen documented. Typically though, the objective in the adapters is about the same size as the rear lens objective. This one is much smaller. Also the size of the cone that it is built into is much larger than could fit any smaller Dallmeyer where the objective sizes might be more proportional. The cone definitely seemed fitted to the lens and the small magnifying objective is perfectly fitted into that cone. It's just hard to see how it's not intentional.

I have also removed the magnifying objective and attempted to measure a focal length for it but it alone does not throw and image as would be expected (perhaps this is what you mean by the difficultly of measuring the focal length of a negative lens). I've compared it to the eyepiece objective in a telescopic scope and it behaves the same way as that. Thats what made me continue to think it was designed as some kind of eyepiece for viewing through the lens.

Anyway, the search continued. Thanks for your insight.