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View Full Version : What type of lens did I just buy?



williaty
18-Jan-2019, 10:06
I took a risk and bought a bigger lens on eBay. I'm hoping you guys can help me figure out what optical formula of lens it is. I have no experience trying to decipher this from looking at a lens so I snapped some bad cell phone pics as the mail lady dropped it off. The lens is marked "18in E.F." on the barrel. The holder for the lens is marked "Bausch & Lomb Optical Co. Rochester, N.Y. U.S.A. 1327". I think at some point in the past there was a rack and pinion setup to allow the lens barrel to be focused relative to the holder. I assume this implies it was once a projection lens.
https://i.imgur.com/81umwnj.jpg

The optical barrel is black along the entire length. The front group separates on threads from the main barrel and the rear group. NOTE: for the whole thing, I'm making the assumption that the "front" group is the one on the left in this picture since it has more glass in it.
https://i.imgur.com/eBiA8ij.jpg


What I'm assuming is the front group appears to have either 2 or 3 elements in it. Here's the front group shot from the front/outside:
https://i.imgur.com/Pue2NSD.jpg

Here's the front group shot from the rear/inside:
https://i.imgur.com/JkLdzhm.jpg

Split because of image rules...

williaty
18-Jan-2019, 10:08
I *think* the rear group only has one element in it but it's hard to be sure. Here's the rear group shot from the front/inside:
https://i.imgur.com/xkPVYx5.jpg

And finally the rear group shot from the rear/outside:
https://i.imgur.com/EaueXFE.jpg


Any idea what type of lens this is?

Steven Tribe
18-Jan-2019, 14:26
A triplet (the Cooke design). These eventually replaced projection Petzvals. Cheaper as there as just three pieces of glass and just 6 surfaces to prepare. - rather than four lenses and eight surfaces for a Petzval.

williaty
18-Jan-2019, 14:28
A triplet (the Cooke design). These eventually replaced projection Petzvals. Cheaper as there as just three pieces of glass and just 6 surfaces to prepare. - rather than four lenses and eight surfaces for a Petzval.

If it turns out to be a decently well figured example of the formula, what should I expect from it? Coverage? And weirdness like a petzval? Some of this I can test out myself once I get another person to help. Right now, it's too long of a focal length to hold it in front of a ground glass and look at the same time.

williaty
18-Jan-2019, 15:05
A triplet (the Cooke design). These eventually replaced projection Petzvals. Cheaper as there as just three pieces of glass and just 6 surfaces to prepare. - rather than four lenses and eight surfaces for a Petzval.

I have an additional question.

I looked up the Cooke Triplet since I'd heard of it but I'm not familiar with it. The reference materials all show the Cooke design having a front element, a large air gap, a middle element just forward of center, a larger air gap, and then a rear element.

I managed to get some of this lens further apart. The front cell is 2 elements with a small airgap (half inch) between them. The frontmost element is plano-convex and the middle element is biconcave. Then there's a long airgap (about 4") before the rear element. The rear element is definitely convex on the rear face but the front face is so far down in the tube that I can't tell if it's plano or just very weakly curved. So the shape of the elements matches the Cooke diagrams but the middle lens is way, way towards the front in mine. The Cooke diagrams show the middle element just barely forwards of center. Given the location of the middle element, does it still qualify as a Cooke Triplet?

goamules
18-Jan-2019, 15:13
The Triplet term is fairly vague, it's a lens with three elements, spaced somehow. A particular seller on ebay a few years ago, and I'm sure today still, liked to call these black B&L projector lenses "Cooke Triplets." They have very little to do with Cooke, except that Cooke also made Triplets. As did Wollensak, Leitz probably 10 others. I'm not sure how Cooke could have patented a triplet. But if they did, B&L sure couldn't have made it the exact same way. But this seller cashed in on people hoping it was like a Cooke. By just sticking their name on his ads.

Yep, they're still doing it: https://www.ebay.com/itm/6-inch-152mm-f-4-B-L-Bausch-Lomb-Cooke-Triplet-Lens-6x6-6x9-2x3-3x4/153341204554

It would be like calling any other company's V8 engine a "Ford V8", even if made by Chevy.

Steven Tribe
19-Jan-2019, 05:22
The original triplets had the middle lens mounted in the barrel. The position of the lens is quite critical for achieving sharpness. The later triplet systems moved the middle lens forward so as to become part of the deep front cell and modified the optical design. Patent protection was for only a short time in the 19th century.

John Kasaian
19-Jan-2019, 06:46
Coverage? In a darkened room hold your lens about 18 inches in front of a light colored wall pointing it at window on the opposite wall or a subject illuminated by a bright light. The projected image ought to give you an idea

Amedeus
21-Jan-2019, 13:15
This is a traditional B&L projection lens that can also be used for image taking of course, shoot wide open and enjoy a wonderful bokey.

Have a few of these and used one on 8x10.

They've come in different configurations. One of my physical larger ones with rack and pinion is a Petzval and I have a triplet also, not sure if the front has a doublet in it or not, this post opened up an interesting question.

Cheers,