Spencer 10x Objective Question

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J_WISC
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Spencer 10x Objective Question

#1 Post by J_WISC » Sun Feb 12, 2023 8:07 pm

Hello.

I've finally sorted through my small collection of objectives and have questions! Thank you for whatever information you can provide.

Here are two Spencer 10x objectives, side-by-side in each photo:

10x_A smaller.JPG
10x_A smaller.JPG (58.3 KiB) Viewed 1929 times
10x_B smaller.JPG
10x_B smaller.JPG (66.63 KiB) Viewed 1929 times

I think the one on the right has a shorter working distance than the one on the left. So much shorter, I must focus up before moving a 40x or 44x into position. Well, for the 40x and 44x objectives I have. Can someone provide an explanation? I learned by foolishly focusing down when I switched from the 10x on the right to a 40x, and crunching a coverslip. It's been my experience, until this moment, to usually use fine focus to move a little closer when switching to higher magnification. Probably a beginner problem. But the practical aspect is I'd like to watch for another 10x like the one on the left, compatible with other objectives I have.

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zzffnn
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Re: Spencer 10x Objective Question

#2 Post by zzffnn » Sun Feb 12, 2023 10:33 pm

It is more because their parfocal height is different. Even a difference by 0.3mm can resulting in crushing cover slips, if one is not very careful. I have crushed more than 3 cover slips, even though I am a trained microbiologist (I sometimes get sleepy, sloppy or plain lazy).

You can shim the shorter objective with tiny rubber bands at objective shoulder.

Older objectives of different generations may have that slight difference. Modern objectives are not always perfect in parfocal height either.

Try to make low power objective as reference point, then make high power objectives focus slightly upwards from that reference point.

Also make sure your correction collar setting is set at either end of the extremes, such as “0.14mm” or “0.20mm”.

apochronaut
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Re: Spencer 10x Objective Question

#3 Post by apochronaut » Sun Feb 12, 2023 10:43 pm

If the two objectives are both .25 N.A. , then they are the same cat. # achromat objectives. Spencer did not make very many .30 achromats but if the brass one is such, then that could be an answer.

Another possibility is that the parfocality of the left hand objective is deliberately different than the one on the right and requires matching higher magnification objectives.
Have you physically measured them?

J_WISC
Posts: 107
Joined: Sun Mar 06, 2022 7:28 pm
Location: Wisconsin, USA

Re: Spencer 10x Objective Question

#4 Post by J_WISC » Sun Mar 05, 2023 6:44 pm

Okay. I have physically measured the two 10x objectives with a caliper. Which was slightly difficult because one objective threads further than the other into the nosepiece. Anyway, after repeated measurements of the entire lengths and subtracting the thread portions ...

serial number 104038 is 29.2 mm shoulder to nose
serial number 341681 is 28.6 mm shoulder to nose

So, about a 0.6 mm difference.

N.A = 0.25 for both.

The lower serial number is the better objective, so despite the more pleasant experience of focusing up when moving from 10x to 40x, I must learn to be more careful. Or ... can I shim my collection of ~40x objectives so I'll always have to focus up when moving from 10x to 40x?

(I've not read about parfocality yet. I have such a hodgepodge of lenses, I didn't consider it important for me.)

apochronaut
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Re: Spencer 10x Objective Question

#5 Post by apochronaut » Tue Mar 07, 2023 1:58 am

Spencer and AO used a 34mm parfocal distance from about the mid. 1890's, right through until they trotted out the 400 series with a 45mm parfocal distance in 1985. With plan and highly corrected objectives, the number of glass elements required to effect corrections is ever more and thus a longer objective barrel is usually required. 45mm and longer barrels have become the standard , just as with a broader corrected condenser field, greater diameter glass is required. Brilliantly though, the most important objective patent up until 1975 was in a 34m parfocal barrel. From that point on, the stage was prepared for anything onwards and upwards in terms of objective design due to a critical design element and just about every advanced objective patent going forward for a considerable time, cited that seminal 34mm patent. If they didn't it was because they found a way to get away with not citing it or it was overlooked. That is just to point out how much can be accomplished in a short parfocal length in terms of objective quality.
Objective parfocal length with Spencer objectives can be variable with the old designs depending on whether the objective originally was a stocklist item or was manufactured to fit another parfocal system as a custom production item. For most of the 160mm period the objectives were 34mm parfocal but there were also objectives , particularly the fluorite and apochromat classes that were produced to 32mm or 36.65mm , in order to match Zeiss or B & L systems, likely by customer request, so assessing an objective's original role by measurement is important in understanding it's usefulness to an adapted system.
It might make sense in some cases to shim a lower magnification objective in order to allow clearance for a longer parfocal objective, when swinging it in. The amount of shimming required is minimal and unlikely to effect the image quality significantly. zzffnn's idea of using a small rubber band is workable. The rubber compresses to a certain point and the shim stays fairly permanent, although the rubber will often deteriorate over time. Cutting thin shims from foil or composite bottle cap liners is a little more permanent.

In your case, I am unclear as to whether the 4mm is hitting the slide when rotated in from the 16mm or whether you focus it into the slide by accident as a result of focusing down instead of up. I am reading the latter. Then it is likely a case of the production dates of the two objectives being quite distant and there being a mismatch solveable by shimming.
If the former, then it is likely a case of an actual parfocality difference and also may be solved by shimming.

apochronaut
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Re: Spencer 10x Objective Question

#6 Post by apochronaut » Tue Mar 07, 2023 10:48 pm

J_WISC wrote:
Sun Mar 05, 2023 6:44 pm
Okay. I have physically measured the two 10x objectives with a caliper. Which was slightly difficult because one objective threads further than the other into the nosepiece. Anyway, after repeated measurements of the entire lengths and subtracting the thread portions ...

serial number 104038 is 29.2 mm shoulder to nose
serial number 341681 is 28.6 mm shoulder to nose

So, about a 0.6 mm difference.

N.A = 0.25 for both.

The lower serial number is the better objective, so despite the more pleasant experience of focusing up when moving from 10x to 40x, I must learn to be more careful. Or ... can I shim my collection of ~40x objectives so I'll always have to focus up when moving from 10x to 40x?

(I've not read about parfocality yet. I have such a hodgepodge of lenses, I didn't consider it important for me.)
I did some measuring on 6 similar 10X objectives between serial#'s 157276 and 391551, so pretty much spanning the same production period as yours. 4 brass and 2 chrome. Four were right around 29.50mm, within .06 up or down from that. Another was .26 longer and another was .36 shorter( the oldest one). They did decrease the w.d. sometime between 1917 and 1930 from 5mm to 4.5mm, so most of them are quite close to the expected. The w.d. is usually in air, so the .18mm cover glass represents .10 in air.
So they all pretty much fall into place except the 28.6mm version that you have. 28.6 + 4.5 - .08 = 33. Likely it was parfocalized in the factory to go with Zeiss objectives. I had one fluorite in my hand once that was just a shade over 32mm long as a 4mm. Checking an older Zeiss homog. immersion objective just now gives me a measure if just a hair over 33mm for it.

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