Six Element in Two Groups Lens Design

[Hunter_Compton]

I recently acquired a National Graflex Series II camera, it came with the Bausch and Lomb 140mm Telephoto accessory lens.

I don't see this lens talked about much, not sure if they're just scarce. However, I had to take the one I received with my National apart to free the sticky aperture, I was surprised to see it's a six elements in two groups design. Each a cemented triplet. This is especially surprising given the low speed (f/6.3) of the lens.

I looked through some of my lens books, but found nothing referencing a design similar to this. Anyone familiar with this design or where it originates? It seems highly expensive.

 

[Nicholas Lindan]

The 6-element, 2-group lenses include the Goerz Dagor and non-super Schneider Angulon.

 

[abruzzi]

thats my first thought as well, but the Dagors I've seen are closer to symmetrical., and I think the Angulon is as well. There is a pretty significant size difference between the front group and the rear group in the photo.

 

[Hunter_Compton]

thats my first thought as well, but the Dagors I've seen are closer to symmetrical., and I think the Angulon is as well. There is a pretty significant size difference between the front group and the rear group in the photo.

That's what I'm seeing as well. However, looking up Dagors put me on to the Voigtlander Collinear, which looks much closer in terms of cemented element shape.

I assume the difference in size between the front group and rear group is in some part due to the fact that it is a telephoto.

.

 

[Dan Fromm]

thats my first thought as well, but the Dagors I've seen are closer to symmetrical., and I think the Angulon is as well. There is a pretty significant size difference between the front group and the rear group in the photo.

Hmm. Not directly relevant, but some manufacturers, e.g., Berthiot, made casket sets using cemented triplets of a variety of focal lengths. One of the possible combinations was a pair of identical cells. Other possibilities include unequal cells, one larger than the other.

As to whether the lens is a true telephoto or just a plain long focal length lens is an interesting question. Teles' front elements are positive, their rear elements are negative. The OP can answer this question for us.

 

[Hunter_Compton]

As to whether the lens is a true telephoto or just a plain long focal length lens is an interesting question. Teles' front elements are positive, their rear elements are negative. The OP can answer this question for us.

Distance from the front face of the front element to the film plane is approximately 120mm and from the film plane to the rear of the rear element is approximately 82mm at infinity, so it's definitely a true telephoto, if the stated focal length is to be believed.

 

[reddesert]

You can estimate the focal length of each group by holding it away from a surface (like a wall), moving it to get a focused image of a distant source (even a window on the opposite side of the room will do), and measuring the distance of lens group to image. This isn't the exact focal length, but usually close enough. If the lens is a true telephoto design, the rear group will have a negative focal length (diverging) and won't form a focused image when you try this.

It's not uncommon for older telephoto lenses to be designed with something like two cemented doublets, with the front much larger, like the Wollensak Raptar Telephoto f/5.6 whose diagram is shown here: https://alphaxbetax.com/Wollensak Lens and Shutter Compendium/#Raptar-Telephoto-5.6

Somebody at B&L may have decided that they needed triplets in this design for whatever reason. I didn't see anything that looked like it in skimming one of the old B&L catalogs at cameraeccentric.com, but I didn't look thoroughly.