50x Oil objective with iris diaphragm intended use?

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Fen
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50x Oil objective with iris diaphragm intended use?

#1 Post by Fen » Fri Oct 09, 2020 10:51 am

I happen to have an Olympus DPlan 50 oil ojective that doesn't really get any use here, because.. well, oil.
What is this actually used for in a professional setting and why the iris diaphragm? Why use this over a 40x or 60x dry objective?
DPlan50xOil.jpg
DPlan50xOil.jpg (9.98 KiB) Viewed 787 times

apochronaut
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Re: 50x Oil objective with iris diaphragm intended use?

#2 Post by apochronaut » Fri Oct 09, 2020 11:16 am

Within any series of objective lenses , there are some that are designed for higher resolution than others. A 50X .90 objective like this one pictured , can be expected to provide resolution of finer details than one that might be marked 50X .70 for instance.
It would be possible to make a 50X .90 objective to work without oil and there are many such objectives but if you are also using a higher power objective , a 100X for instance that does require oil, having a partner objective that also requires oil makes detailed observations more efficient and practical.
Any objective with an N.A. higher than 1 must have an immersion medium of some sort, so most high resolution objectives are immersed types.

Your objective in question is also marked that it will work with or without a cover slip, which makes it quite versatile. If it were to have been designed to be used dry, it would most likely have been designed with a correction collar in order to control the working distance, something a high degree of precision is needed for with higher N.As. As an oil objective , the oil controls the working distance. It is probably less expensive to make an oil objective than one with a correction collar.

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Fen
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Location: Netherlands

Re: 50x Oil objective with iris diaphragm intended use?

#3 Post by Fen » Fri Oct 09, 2020 12:30 pm

That's interesting, thanks! I'll have to experiment with that sometime then and see how it compares. Maybe it'll produce some nice pictures once I get my camera adapter delivered.

PeteM
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Re: 50x Oil objective with iris diaphragm intended use?

#4 Post by PeteM » Fri Oct 09, 2020 6:11 pm

A very useful objective. Worth the minor aggravation of applying and removing immersion oil.

Stopping down the iris can be helpful in darkfield. The longer working distance than a 100x oil immersion objective is handy to see deeper into some specimens - and avoid crashing into others. The high numerical aperture means significantly sharper images than a 40x with a .65 na.

wabutter
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Re: 50x Oil objective with iris diaphragm intended use?

#5 Post by wabutter » Thu Oct 22, 2020 3:07 am

As far as intended use: The most common application of the 50x oil objective was for scanning differentials in hematology. If a suspect cells was found, switching to the 100x oil could be done and then going back to the 50x scanning lens. Of course a 40x dry could have been used, but usually was contaminated by the oil upon switching go back. Interestingly, the 0.8 to 0.9na generally provided enough resolution that switching to 100x was a rarity. Ultimately, the 50x was desired because it effectively double the speed of doing a diff.
In today’s world, the diff is handled by an automated blood counting system. Manual diffs are done as a backup or in smaller labs.
In the clinical world, the iris diaphragm added versatility by allowing the 50x to be used with a darkfield condenser for detection of the spirochete Treponema.

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