Objective FN versus Ocular FN - Compound Microscope

[mrsonchus]

Hi all, just a quick question I've been pondering during the few idle moments I've had over the last couple of days....

With an objective of FN 26.5 and FN 22 oculars in a head also of FN 22(mm), should the field-iris be set to the objective's FN or that of the ocular/head combination.

[PeteM]

Interesting question; and someone else may have a definitive answer.

It seems to me that as long as you don't illuminate the objective beyond its field, that you'll minimize most reflections, preserve contrast, and get the best resolution of the objective.

On the other hand, there may well be a stop or baffle in your head that will clip that max 26.5mm field to something closer to that head's field of 22mm or so (depending on what head you have)? And yet another stop in your eyepiece. All perhaps suggesting that as a practical matter you might was well use a phase telescope and adjust the aperture to fill the view through your head?

[abednego1995]

The field iris should be set to the smallest FN in the system. If you use a FN22 ocular on a FN26.5 head, it would be FN22. Many heads (and objectives) will allow fields up to nearly 30mm only limited by the ocular tube diameter themselves but whether that periphery would be pretty is a different matter.

Cheers,
John

[Scarodactyl]

D
Not to go too far afield here, but does anyone make an eyepiece with a fn over 26.5? I assume it's really hard to get wider FoVs since the Chinese eyepieces never seem to go that high.

[mrsonchus]

Hmm, a few points to ponder, thanks all.

So, the head and oculars are the 'ordinary' widefield Olympus 22mm FN, the objectives with matching 22mm FN are the Plan N UIS2 line.
One objective (a high-dry 60x) is of the UPlanApo variety with the 'super widefield' 26.5mm FN....

Is it 'better' (theoretically at least - I find that theory and experience/perception with microscopy always overlap but almost never coincide!) to set the diameter of the field-iris to 22mm or 26.5mm? In other word maybe - do I set the Kohler-condition of the field-iris to 'my view' (22mm) or that of the objective in question (26.5mm)?

I'll try the two options later and take a few images to post back here.

[MicroBob]

Hi John,
you can't see the field beyond these 22mm - so why illuminate it? The illuminated border between 22 an 26,5mm will only produce stray light. Whether this is important with modern high quality optics and their good coatings is hard to predict.
A way to check this objectively ist to take pictures with fixed exposure and mic lighting and compare the histograms.

Bob

[mrsonchus]

Hi Bob, that makes perfect sense to me too; also perhaps one may say that the first to receive light is the objective, designed to be illuminated to 26.5mm.

That suggests another related question, namely, 'does illuminating an objective below the diameter of it's FN compromise or alter, either-way, the image 'passed-on' by the objective to the rest of the optical path of the 'scope?' - in a brightfield setup in this particular context.

A few images coming right-up.

[mrsonchus]

Here's a link to my G-Photos shared folder with a set of images both edited and as-shot of the 22mm and 26.5mm settings of the field-iris.
The 26.5mm are as expected a little brighter than the 22mm images, but I can't see any difference worth mentioning in the two versions. I've included sets of 'enhanced' versions of both the 22mm and the 26.5mm images, the adjustments of PSEv9 'autofix' and levels - RH side of histogram set to 215 to bring up the brightness to the same threshold.

These Botanical slide images are not the best with which to perform such tests as few sharp edges exist with this type of section, but interesting anyway.

Personally my conclusion from this simple comparison is that there's essentially no difference....

Here are a few image-pairs from the enhanced 22mm and 25.5mm sets, the whole sets is available via my above link....



and




and




and


Finally a xylem TS at 22mm and 26.5mm FNs,


and


Not very informative I'm afraid, but I had the spare time this afternoon!

[PeteM]

Thanks, John, for running the comparison.

[Hobbyst46]

Hi John B.,
From the images, especially the non-enhanced ones, it seems that the 26.5mm aperture setting were better illuminated than the 22mm. So I think, that the field width imposed by the photo tube and any components within it is wider than 22mm, irrespective of the eyepieces (I assume the camera is attached for direct projection, not afocal).

[mrsonchus]

As far as I know the phototube and PE 2.5x photo-eyepiece combination is the same for the SW (26.5mm FN) head as for the 22mm FN head. I wonder what the FN of the PE 2.5x is? I'll have a peer down it with the 60x objective in place and see where the field-iris sits when set at 22mm and 26.5mm.

[Roldorf]

Hi John.
I guess it is more of a personal preference and on what one expects to see, as I find the 22mm images to have more detail especially on the un-enhanced images. Still an interesting question and experiment. (I looked at the full set on GP)

Alan