Odd behavior: Bausch & Lomb 0.8x MicroZoom objective

ray_parkhurst · Post by **ray_parkhurst** » Tue Nov 13, 2018 10:15 am

This objective has behavior I don't understand...

First, I am trying to use it as a finite objective. Is the MicroZoom an infinite microscope?

Second, it provides approximately the same magnification, regardless of working distance. This seems to indicate telecentricity, correct? But by keeping the same total extension, I can focus the subject across a wide range of working distances, with little or no mag change. I can go from almost touching the subject (maybe could actually touch it...) out to as high as my setup can go (~12" WD) and I can get the subject in focus with just a small change in the extension. At 12" the magnification is ~0.2; at 4: the mag is ~0.25; and at very short WD the mag is ~0.28. In all cases there is significant vignetting.

Has anyone worked with one of these? It's clearly of a different design than any other objective I've worked with, so I'm not sure what I'm looking at here or how to make it work properly.

rjlittlefield · Post by **rjlittlefield** » Tue Nov 13, 2018 10:50 am

Caveat: I have never touched one of those objectives.

But from your description of how it behaves, it sounds like one of the fixed-magnification lens setups that is telecentric on both sides.

That design is popular for industrial applications because of exactly what you describe: a wide range of focus positions with no change in magnification.

Internally they consist of two positive groups separated by the sum of the focal lengths, with one aperture that is located at a point one focal length away from each group. When used with collimated light, the system acts as a beam expander/compresser, but with finite conjugates it gives fixed magnification real images.

Narrow fields naturally come as part of the package, since the field on each side cannot be wider than roughly the diameter of the lens elements on that side.

The one thing that's odd in your description is that your lens is rated at 0.8x but you're seeing 0.25x. I have no clue why that is.

--Rik

ray_parkhurst · Post by **ray_parkhurst** » Tue Nov 13, 2018 10:51 am

Thanks Rik, I figured it had to be something weird like that. Odd that the other lenses made to work with this microscope don't behave the same.

phil m · Post by **phil m** » Mon Nov 19, 2018 10:20 am

Does the MIcrozoom have a compensating lens just above the nosepiece?

ray_parkhurst · Post by **ray_parkhurst** » Mon Nov 19, 2018 10:26 am

phil m wrote:Does the MIcrozoom have a compensating lens just above the nosepiece?

I have worked with the 0.8x, 2x (or maybe it's 2.25x), 3x, and 8x objectives for the Microzoom. The 3x and 8x work like regular finite objectives, and give rated mags. The 0.8x gives a lower mag, and so far I have not been able to get a usable image from the 2x at all. So it may be that there is some option in the MicroZoom that switches in additional optics required for the 0.8x and 2.25x, or some other switcheroo. Maybe someone has a MicroZoom and can elucidate us.

phil m · Post by **phil m** » Mon Nov 19, 2018 11:03 am

I thought you might have a Microzoom but I guess, just the objectives.
I have never had a MIcrozoom in my hands but have had lots of Balplans.

My understanding is that the Microzoom head and the Balplan head are basically the same, except that the Microzoom has dual diopter adjustments and I think also a slightly larger f.o.v. If it is true that they are interchangeable, they both have a travelling telon lens internally, which converts the infinite image to a convergent image.

In the Balplan, there is a negative telescope lens located just above the nosepiece. It is in fact, the cemented rear lens element common to all of the objectives, so the objectives are in fact partial objectives, completed about 15mm above their apparent rear element. The magnification factor in the telescope lens appears to be 5X, so not too far off what you are experiencing with the .8X microzoom objective. Perhaps the Microzoom was only 4X, for some reason.

In the Balplan, the objectives are therefore about 60mm parfocal( I haven't actually measured them) and the system is infinity from the telescope lens up.

It is possible that the microzoom was designed similarily but that doesn't explain why the higher magnification objectives are behaving like normal finite objectives. It may be some aberration phenomena that is decidedly more pronounced with small magnification/long focal length lenses?

I did discover something similar with the Balplan. Bausch & Lomb also made a series of no cover objectives for MET use, that had large 25mm threads. I don't know what microscope they were used in but the design and size of the objectives would lead one to believe that they are from the Balplan/Microzoom era. There is no tube length marked on them. I bought an adapter to convert the 25mm to R.M.S. and trialled the two I have in a Balplan. The 80X .90 planfluorite works perfectly in a Balplan but the 5X planachro produces a very distorted image, with tons of lateral ca and flare. It focuses at a weird distance, when compared to the standard Balplan 4X planachromat, whereas the 80x .90 planfluorite has a w.d. within expectations. Both of those objectives came from the same lab and were being used in metallurgical research. I have no reason to believe that they were not being used in the same stand.

The telescope lens in the Balplan is threaded into place and is used for all objectives.

Scarodactyl · Post by **Scarodactyl** » Mon Nov 19, 2018 11:24 am

I have a Microzoom II (not sure what the difference is, if any). The head is in two parts (or three if you have the optional extra mag attachment). The top with the eyetubes and trinocular port looks much like a Balplan head and has no lens in it I can see, aside from a flat piece of glass (which slides out of place when you switch to trinoc view). It is attached with a dovetail to the zoom body, which goes from 1-2x and of course does have lenses in.
As far as field of view, it does take 30mm eyepieces which may not have been very correcting--mine came with olympus stereo eyepieces that seem to work just fine(?).
It definitely has no way of telling which objective is rotated into place, and they can be freely mixed and matched on the nosepiece (they made at least 8 different objectives for it).
As a side-note, does the balplan produce an erect image? The microzoom does.

phil m · Post by **phil m** » Mon Nov 19, 2018 2:32 pm

I was hoping you would chime in on this, since you have one.
No the Balplan does not have an erect image. It has the same features in that regard as any other diascopic microscope. The reversal no doubt ytakes place in the zoom head.

If you were to take the head apart, you would probably find the telon lens inside, in the same place as the one used for the Balplan.

Could you look just above the nosepiece and see if there is a lens pack in there? It might be a little difficult on the microzoom because of the zoom optics.

Scarodactyl · Post by **Scarodactyl** » Mon Nov 19, 2018 2:47 pm

I can take the head and optional mag changer off easily with the set screw/dovetail arrangement, but the zoom body is bolted to the base, and the nose also looks to be held on with bolts or screws. I'm not sure how easy it would be to disassemble, and I've been trying to sell it so I want to be sure I don't do anything difficult to undo.

phil m · Post by **phil m** » Mon Nov 19, 2018 2:54 pm

On the Balplan it is positioned right above the nosepiece, as the back lens of the objective, only about 15mm above. That is really where it has to be. You might be able to see it right there, if you look up through the nosepiece from below. Maybe with a mirror?

Scarodactyl · Post by **Scarodactyl** » Mon Nov 19, 2018 3:10 pm

Oh, I see what you mean. Unfortunately I can't see very far into it, but there isn't one immediately behind the thread of the objective--I think I can see 15mm in but I'm not completely sure. I'll let my phone charge a bit and try to get some pictures more straight-on.

Scarodactyl · Post by **Scarodactyl** » Tue Nov 20, 2018 9:39 pm

Sorry it took a while. The scope finally sold, but I dod get a shot before packing it. Yuo, there is a lens up in there:

ray_parkhurst · Post by **ray_parkhurst** » Tue Nov 20, 2018 9:55 pm

Oh no! More tube lenses!

Will be interesting to see what this thing requires for focal length to get to rated mag.

phil m · Post by **phil m** » Wed Nov 21, 2018 7:59 am

I have a diagram from about 1964 that illustrates the rays coming from a B & L semi-objective, it's native focal distance and the diversion caused by the telescope lens to the eyepiece. There are no measurements but the drawing is to scale by one of the original engineers of the B & L development program.

If one assumes that the nominal distance of the complete system was 160mm: at least that's what b&L had been working with up until about then, then the semi-objective would have around a 55mm focal length. You might try that as a starting point.

The other thing is that the telescope lens probably has corrective properties. It certainly has field flattening properties because that is in the literature but there may be small amounts of other corrections.

I. MIller microscope ended up with a substantial amount of the B & L inventory at the close of their operations. They would have that telescope lens.

ray_parkhurst · Post by **ray_parkhurst** » Wed Nov 21, 2018 8:05 am

phil m wrote:I have a diagram from about 1964 that illustrates the rays coming from a B & L semi-objective, it's native focal distance and the diversion caused by the telescope lens to the eyepiece. There are no measurements but the drawing is to scale by one of the original engineers of the B & L development program.

If one assumes that the nominal distance of the complete system was 160mm: at least that's what b&L had been working with up until about then, then the semi-objective would have around a 55mm focal length. You might try that as a starting point.

The other thing is that the telescope lens probably has corrective properties. It certainly has field flattening properties because that is in the literature but their may be small amounts of other corrections.

I. MIller microscope ended up with a substantial amount of the B & L inventory at the close of their operations. They would have that telescope lens.

Thanks Phil, I will check with them...Ray