Question about Objectives (B&L)

[watchmaker]

I use several stereo zoom scopes in my work and I understand how those optics work. I recently purchased a B&L centering scope (for lining up work in machines) and the objective is marked 32mm .01. This gives me a very short working distance (maybe 5mm) which is good for some setups, but I would like the option of a 25 mm working distance. What am I looking for?

Thanks. The answers I got here helped me in selecting my microscopes (Reichert 570, B&L SZ7, Nikon SMZ1b and an Olympus fixed power used over my large lathe). I wish I knew enough to pay back the help I got, but if you have questions about precision watches that are not newer than 2000 (Swiss parts restrictions) or machining I would be happy to help.

Regards,

Dewey

[MichaelG.]

Dewey,

'Long Working Distance' objectives are of necessarily complex design and, I regret, generally priced accordingly. ... To complicate matters further: Most of them are now 'infinity corrected' designs.

Here are a some useful links to read whilst waiting for a "windfall"
https://www.newport.com/f/long-working- ... objectives
https://www.thorlabs.com/newgrouppage9. ... up_id=9922
https://www.edmundoptics.com/microscopy ... bjectives/

Some photos of your B&L 'scope would be very interesting
... am I correct in assuming it uses a traditional RMS-threaded objective ?

MichaelG.

[apatientspider]

I use several stereo zoom scopes in my work and I understand how those optics work. I recently purchased a B&L centering scope (for lining up work in machines) and the objective is marked 32mm .01. This gives me a very short working distance (maybe 5mm) which is good for some setups, but I would like the option of a 25 mm working distance. What am I looking for?........

Regards,

Dewey

I'm not all that familiar with Bausch and Lomb's offerings, and I wasn't able to find an image of one of their centering microscopes, but I presume it isn't too different from other makes. Assuming it takes ordinary RMS thread objectives corrected for a 160mm tube length (might also be 215mm tube length) -

You might try an ordinary low power objective. Aside from special long working distance objectives - general, biological, compound microscope objectives, corrected for a 160mm tube length, have a longer working distance the lower their magnification. An objective with an equivalent focus of 32mm is about a 4x lens. Depending on who made it, it might have a working distance of as much as 21mm or about 3/4 inch. American Optical/Spencer made such a lens.

Lower power lenses will have an even longer working distance; for example: American Optical/Spencer - whose line I'm much more familiar with - made a 48mm or 2.2x objective back in the 1940s that had a working distance of 52.5 mm or 2 inches, as well as a 40mm equivalent focus or 2.8x lens with a working distance of 35.2mm. They also made a 30.2mm equivalent focus or 3.5x objective with a working distance of 25.1mm - about 1 inch. On the other hand they made a 24mm equivalent focus or 3.5x lens with a working distance of only 4.2mm. Additionally they offered a 25mm equivalent focus or 5.1x objective with a working distance of 21mm.

Both B and L and AO made a special divisible 10x objective, which was quite common on their lab scopes until the 1950's, that would give you the option of a 4x objective just by unscrewing and removing the front element. This gave considerable working distance for the lower power. The low power element gave a somewhat inferior image compared to a comparable 4x lens, but good enough for much visual work. These are common as dirt on Ebay - both marked and unmarked.

Here are links to some low power American Optical/Spencer objectives presently on Ebay:

https://www.ebay.com/itm/Objective-Spen ... 1438.l2649 and

https://www.ebay.com/itm/MICROSCOPE-PAR ... 1438.l2649 and

https://www.ebay.com/itm/MICROSCOPE-PAR ... 1438.l2649 .

Not exactly inexpensive, but you might submit a best offer and get one for a lower price.

Also, here's a B and L 10x divisible for a low price: https://www.ebay.com/itm/Bausch-Lomb-10 ... 1438.l2649 .

Hope that helps. I'd have included more examples of Bausch and Lomb's lenses, but I don't have a catalog or chart of their stuff with associated working distances. Probably one of the other members here does.

Jim

[watchmaker]

Thank you for the quick responses. Here are two pics. The tube length itself is 90 mm with an added 40mm to the prism. I like this unit because it is so compact ( I use a lathe centering scope mainly on the lathe to line up pinion cutters). I do have a 3 inch working distance Moore(Titan) but it takes up a lot of room. This would allow me to line up the cutter and then back up the slide rest and get the scope out and insert the work. With the longer distance I have to trust the bed and unclamp the slide and move it into position. Works so far, but not satisfying to my OCD.

For $20, the 16mm objective is worth the risk. Thanks Need to make a post and a cell for the reticle. The alignment thumb screws are nicely done and they are covered by a rotating shield. It is stamped B&L 17 on the back.

[MicroBob]

the lens formula is 1/f = 1/i + 1/o

where f is the focal lenght, i, the image length (160mm or OO) and o is the distance to the object.
The term 1/i is very small so we can overlook it.

1/f equals roundabout 1/o
So f is roundabout equal to o

BUT 32mm is not roundabout equal to 5mm!

I think that the 32mm is something other than the focal length. Maybe front lens diameter? Or lenght of the objective? Or distance from screw flange to object?

I think an objective with just 5mm working distance is unusual for this job. From a Mitutoyo measuring microscope I know that the objectives (though special long distance versions) are 2x 5x an 15x and the weaker ones have a much bigger working distance than 5mm.

According to Nikon typical working distances are:

Achromat Correction
Magnification Numerical Aperture Working Distance (mm)
4x 0.10 30.00
10x 0.25 6.10
20x 0.40 2.10
40x 0.65 0.65

So your objective will be something around a 10x.

I would look for a 4x or 3,2x . This gives enough magnification for the job and is easily available.
Even weaker objectives are much harder to find.

Bob

[apochronaut]

Your 32mm B & L objective is around 3.5X more or less. I own the same objective, as well as numerous other low mag. B & L objectives and if you like, in the next couple of days I can sit down and determine it's actual exact magnification by comparing it to other B & L objectives, with marked magnifications. I can also check and see if there is a better alternative for your use.

One of the reasons for having as short a working distance as possible, is that the quality of the image, can generally be improved, due a higher N.A. being more easily achievable. Longer working distances cause a potential loss in N.A., although various designs, usually costing more money, can work around that problem.

B & L , similar to JIm's description of Spencer's offerings , produced numerous low magnification objectives, geared towards specific applications. The 32mm objective you have was widely used in shop microscopes, inspection microscopes and some hand held measuring scopes etc., mainly I believe, because it produces a reasonably flat field.

I do have, and if you are interested in it you can p.m. me, a 4 /10x objective for B & L, that rather than being divisible, has a swinging front element, which by rotating a little knob on the front, can be swung away, for 4X and swung back for 10X. It's very convenient . I will check it's w.d.

[apochronaut]

I made a quick inventory of B & L 160mm tube objectives I have on hand; 40mm(about 2X), 32mm/.10( like yours), 30mm/.09( 3.5X), 28mm/.08 (3.2X), 24mm/.20, 22.7mm/.17(6X). The Bifocus objective aforementioned, is marked 16-32mm 0.25-0.10 N.A. 10-4X. It is 26mm from the mounting shoulder to the tip.


I can check out the performance of these, as well as the working distances for them by Monday, and let you know about them. Any you might be interested in are available, if needed.

[watchmaker]

So if I want more room, I should go with the 40MM? Thanks!

[apochronaut]

So if I want more room, I should go with the 40MM? Thanks!

Usually,within the confines of a certain objective design, the lower the magnification( thus a longer focal length) and the lower the N.A.; the longer the w.d. will be but that is a rule that can be deliberately altered for special purposes by the mfg. and always requires some manipulation of the optical design with extra cost involved. Longer or extreme working distance objectives will often have very sophisticated optics inside the barrel and are also often double or more the cost of standard working distance objectives of the same focal length. A conventional 40mm objective can give you 50 mm or so of w.d., normally but my experience with these older ones is that the curvature of field and lack of planarity( field flatness) can be quite severe, so if field flatness is of concern, it may take some experimenting with various objectives, to find the right combination of magnification,field flatness and w.d. I have quite an array of 160mm, older low power objectives : 1.2X up to 6X in various magnifications, N.A.s and working distances. When I check into the B & L ones for you , I can compare a few others. I've done this kind of thing before for other forum members, with success.

What working distance would be ideal for you and how much of the field needs to be flat and in focus? Older objectives that are not marked as plan, vary quite a bit in their planarity, from almost completely plan to not very plan at all. Makers of tool room and shop scopes were aware of the necessities and sought to incorporate the requirements into their optics but many times w.d. wasn't a necessary requirement, just inspection.

Also, what kind of eyepiece is on that scope? I may have one the same and it would assist in assessing the objectives, if I used the same eyepiece as you have.

[watchmaker]

You would ask me that. Glass??? Seriously no markings, but it is heavier than a normal 10X eyepiece. Almost as heavy as my AO/Reichert 15X utlrawides. So a lot is going on in there.

Yes, field flatness is essential. You are really helping me avoid some costly experiments.

I CAN live with the W.D. given my sliderest. But the more I listen, the more I think this was purpose built. No reticle came with it so I have no idea it's last use.

Thank you again.

And here was me, thinking an objective was an objective. So much to learn and so little time left. At least I won't be bored.

[apochronaut]

The eyepiece possibly has field flattening elements in it. If you could take a few pictures of it ; eyelens end, field lens end and from the side; I might be able to tell something from that. Nevertheless, I have the same 32 mm objective as a reference.

I don't recognize the instrument but all companies made a lot of industrial microscopes, after w.w. II. Is the actual instrument marked B & L, or just the objective?

[MicroBob]

Hello Apo,

I just had a look at my Leitz EF 4 /0,12 objective with 45mm objective design. It is a semiplan achromat of typical small design for this type of objective. The free working distance here is 24mm. It is parcofal with the stronger objectives. So 5mm for a 3.5x objective is a quite small working distance! I think my Lomo ca. 3x objective of pre-WW II Zeiss design is not parfocal with the other 33mm objectives, it had a too long objective design lenght. So it should be possible to find an objective of this magnification with a longer working distance.
For normal application the short w.d. of the BL objective is no disadvantage - but do you have an idea why they made it this way?

Bob

[MichaelG.]

[ ... ] 5mm for a 3.5x objective is a quite small working distance!
[ ... ] but do you have an idea why they made it this way?

I agree with your comment, Bob; and I have a nagging suspicion that the answer to your question might be "they didn't".
Until someone finds another unit with the same configuration and performance; we have to accept that either (a) this one does not have its original optics, or (b) it was intended for a different purpose.

It is not inconceivable that the unit is configured as a telescope, for something like a "Travelling Vernier".

... Checking the optics 'from first principles' would seem to be the best approach.

MichaelG.

[watchmaker]

Goes to show what changes when you have it right. I was hand holding it before. Attached are photos of how it is used and the post mount I machined. It is a 40mm W.D. (Fl?). About 1/2 of my Titan which it far more acceptable for setting for gutting gears and pinions. I have to make the reticle cell and it will be usable.

Apochronaut, I WOULD be interested in cutting this in half or so. As you can see, I have a very dependable slide (bought if from a friend who purchased a tool crib stock of unused Schaublin). Turns at the mass was the thick wall AL housing. Here are a couple pics; field lens looks simple, there are two lens in the eyepiece lens.

I put a short post on it so I need the least amount of space to remove it. With the current eyepiece and the and hopefully a shorter F.L. one, I have enough travel to setup, back out the top slide, remove the scope and insert my work. It is a bonus it is finished in machine gray. I agree it may have been used in some esoteric application.

Along with the photos of the mount, I am included the bracket it came on. It is marked with a drawing no. and date (1967). You can see it was built up and not cast or machined from a block, suggesting it was not a production item. Very expensive way to make something unless it is very low production and not worth tooling up.

[MichaelG.]

Dewey,

l think you deserve a Red Herring award for that one !
5mm working distance certainly had me thinking.

Glad it all turned out well for you though.
... It looks well on the lathe.

MichaelG.

[watchmaker]

Egg on face. &>)

[MichaelG.]

No problem, Dewey

All's well that ends well.

MichaelG.

[watchmaker]

I did figure out how to collimate it. Made a small aperture disc that slid into the bore. Rotated prism housing to bring a precentered (mechanically) target to get the horizontal axis centered. Then saw the screws on the inside of the casting that thread into a plate (one missing). Brought the aperture down but I need to buy a box of 3-56 screws so I can fill the empty hole. Looks like it is minimally off depending on position in collet. Will put witness marks on collet and scope so I can minimize reticle alignment at each use. For stuff that has to be abolutely perfect, I will use the target. But it looks like min repeatability is going to be .01mm or less. Once I install a real reticle I can measure this (will not repeat guesstimates)

After I do that and make the proper size cell I will install the reticle. Tried that first and it was a complete waste of time.

I will probably sell my Moore since I do not do a (w)hole lot on the vertical mill that I cannot easily center by other means.

Thanks for all the help!