Mixed FN objectives on same head?

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mrsonchus
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Mixed FN objectives on same head?

#1 Post by mrsonchus » Mon Oct 14, 2019 10:53 pm

Hi all,a question that intrigues me.

Microscope heads for the olympus BX infinity range come in a 'normal widefield' version, with 22mm FN eyepieces and 22mm FN objectives below.
There is also a 'super widefiled' head with 26.5 FN eyepieces (SWH10x-H for example) and 26.5mm FN objectives below.

Question, is there any image compromise when using a 26.5mm FN objective with a 'normal widefield' head designed for 22mm FN objectives, with 22mm FN 'normal widefield' eyepieces?

Any thoughts?

i.e. Can an objective with a FN of 26.5 be used on a head with FN of 22 without compromising image quality?
John B

PeteM
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Re: Mixed FN objectives on same head?

#2 Post by PeteM » Tue Oct 15, 2019 1:38 am

Seems to me the only significant loss of installing an objective that's plan to 26.5mm to a 22mm head will be to your wallet.

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mrsonchus
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Re: Mixed FN objectives on same head?

#3 Post by mrsonchus » Tue Oct 15, 2019 6:20 am

Thanks Pete.

There's a difference though when setting the condenser's n.a. to that of the objective.
When the rear focal plane of the objective is viewed through a phase telescope the condenser's iris is correctly the width of the view (i.e. the condenser's n.a. matches that of the objective at 0.9). However the view has a black periphery that seems to reflect the difference in FN. If I raise the condenser 'slightly' a point is reached where the imageing bright central area of the view of the objective's rear focal plane widens to fill the view from the phase telescope, the black periphery and condenser iris blades at objective's n.a. having enlarged out of view.
This approach does seem to improve the image's contrast but the field-iris is no longer at focus as the condenser has been raised beyond it's Kohler ideal height.....

Alternatively, in order to stop-down the condenser to bring the aperture-iris in to match the edge of the visible image area the n.a. (as set on the condenser's index markings, which are entirely accurate) is severely reduced from nominal 0.9 to about 0.55/0.60. This does remove the glare and raises the contrast (as seen through an eyepiece) as expected, but reduces it seems? the working n.a. of the objective (a dry 60x 6.5 FN UPlanApo) from as above, 0.9 to 0.55/0.60 - it seems to me but perhaps I'm wrong - I know very, very little about this subject beyond observable phenomena. The reduction in working n.a. presumably reduces the potential resolution of the objective in use?

I obviously need to investigate the literature further on this, and maybe re-state this 'problem' more clearly and with some images of the referred-to views above...

Any thoughts all?
John B

MichaelG.
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Re: Mixed FN objectives on same head?

#4 Post by MichaelG. » Tue Oct 15, 2019 8:02 am

John,

Forgive my ignorance of the system, but ... is the body tube larger diameter on the ‘widefield’ instruments :?:

My suspicion is that you may get flare from internal reflections when using excessively large FN objectives.

MichaelG.
Too many 'projects'

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mrsonchus
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Re: Mixed FN objectives on same head?

#5 Post by mrsonchus » Tue Oct 15, 2019 8:37 am

Hi Michael,
I don't know but the super and norma widefield eyepieces look the same size..

What would be the reasoning for this?

It's an interesting question though. one I may contact Olympus about?

The cheapo Chinese 60x 0.8 n.a. FN22 objective I have has exctly the same image in terms of brightness and contrast however.

That said I wonder also if the simple Abbe condenser although in n.a. terms rated as 1.25 may have it's limitations especially at n.a. 0.9 dry, being uncorrected of course ?

The 60x objectives (dry) are notoriously sensitive I know...
John B

MichaelG.
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Re: Mixed FN objectives on same head?

#6 Post by MichaelG. » Tue Oct 15, 2019 9:06 am

A quick look through some Olympus literature suggests that any differences must be in the head [observation tube in Olympus parlance] ... so, if the tube diameters are the same, perhaps the prisms are larger in the widefield heads.

MichaelG.
.

See the list of Observation Tubes in this BX3, where there are only two 26.5 variants available:
https://www.olympus-lifescience.com/en/ ... 8&fl=en_US
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PeteM
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Re: Mixed FN objectives on same head?

#7 Post by PeteM » Tue Oct 15, 2019 4:25 pm

Interesting point, John, about the condenser and properly filling the field of view. I hadn't noticed that before, but I also don't pay much attention to the condenser markings (using visual adjustment).

Have seen darkening of the edges in the reverse situation - when using a 22mm FN objective and looking through a larger field # head.

It could be that the N.A. index on your condenser is meant for a system that's 22mm FN throughout. Obviously the iris has to go wider (marked lower NA) to fill a 26.5mm objective. So it could be that you're not actually losing numerical aperture, but rather that the condenser isn't properly marked for the wide field objectives and head?

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Re: Mixed FN objectives on same head?

#8 Post by mrsonchus » Tue Oct 15, 2019 11:07 pm

Hi Pete, I see what you mean..

I also have (it arrived today) an aplanat/achromat 1.4 n.a. condenser that doesn't show this effect with either of the 60x objectives of n.a. 0.8 and 0.9.

What I have subsequently found after some testing and comparisons this afternoon is that the apparent resolution and sharpness of the image viewed and photographed is completely unaltered by this.

What I have found though is that slide quality is an extremely important factor with the 0.9 n.a. 60x (and 0.8 n.a. Chinese achromat 60x).
I use the term 'slide quality' to mean factors comprising the following:
section thickness
section thickness uniformity
section flatness as mounted
staining contrast
proximity of section to coverglass underside
thickness of slide

Switching to a different slide of better quality as defined in part above, the situation improved enormously, regardless of the original observed border phenomenon.

With a 'better' slide both 60x objectives performed very nicely with very good contrast and resolution....
The non-planar Chinese 0.8 n.a. objective had as expected an abscence of sharp focus around the center when center is focused.
The same objective, again as expected, has slightly lower resolution than the UPlanApo 0.9 n.a. objective.
Other differences, none deal-breaking it must be said, were apparent with a 'good' slide.

The correction (coverslip thickness) collar of the 0.9 objective, when set to 0.17 and parfocal to the other objectives as a result, as presumably the 0.8 objective is (fixed at), lowered the image quality enormously in terms of contrast, resolution and DOF. When optimised (actually at 0.12 although the coverslip is nominally 0.13-0.15 but not measured) the 0.9 objective definitely outperformed the 0.8 cheapo Chinese, but not even close to the order of magnitude difference in their purchase prices....

So, the original 'problem' although not yet understood or explained (by me at least) seems to be essentially irrelevant to image quality with either a simple Abbe 1.25 condenser or an aplanatic/aspheric 1.4 condenser....

In a nutshell it seems that my 'problem' is in practical terms a non-problem, and that differences in perceived and photographed image quality are not a result of this observation as originally stated, more a result of what every piece of literature will always state, slide quality and system setup, when using these relatively high n.a. (say 0.8 and above dry) 'high-dry' objectives. My experience tells me that for these objectives a correction collar is very desireable for optimal results, but by no-means essential for acceptable and useable results in the amateur context....

It's a bit like chasing shadows, catching-up with one of the blighters, thoroughly investigating it from many angles and with a large investment of time and effort, only to conclude -"hmmm, it's a shadow"!! :D :D :shock:
John B

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