Water v. Oil immersion

[jwsmith]

The following link leads to an interesting discussion of water- and oil-immersion objective lenses
http://microscopyu.com/articles/optics/ ... tives.html

My analysis is, that we amateurs would benefit from using water-immersion objective lenses....for the following reasons:

1) The all-four-corners "resolution degradation" of water v. oil......is on the order of 10 percent
2) The all-for-corners c o n v e n i e n c e of NOT using oil........is truly: m a g n u m ...!!..
3) For a specimen in water under coverslip and oil immersion above: an oil-immersion lens does well if coverslip and specimen are in contact.
4) However where the specimen is 20-microns below-coverslip, an oil-immersion lens loses nearly 50% resolution
5) Water immersion performs uniformly in the conditions of sentences (3) and (4) above.

C o n v e n i e n c e plays a large part in whether an amateur like myself BOTHERS to use his High Resolution objective lens...
Therefore perhaps one's decision whether to invest in a water-immersion objective, should turn on this kind of self-assessment.

Judd

[apochronaut]

In former times, the term oil immersion was much less used. The term homogeneous immersion was more often seen, this being an acknowledgement that the system worked best with a complete immersion, from the condenser through to the objective front, that maintained as close to homogeneity of the refractive index as was possible. Cedar oil was the immersion medium of choice.The consideration that the mounting medium itself was a hazard to the homogeneity was somewhat discounted because there was an assumption made that A) prepared slides would be done in such a way, that they preserved the homogeneity and that B) living samples would be temporarily mounted in such a way , so as to preserve homogeneity as much as possible. To this end , glycerin has been often used in mounting, if not only to slow down the physical processes of highly motile organisms.

Water Immersion objectives also arrived early on as an alternative but although they remained in the stable of most companies, they never really replaced or could even dent the domination of homogeneous immersion objectives.

Recently they are back in vogue, primarily in use for very specific techniques, where the sample is aqueous and very precise illumination procedures are maintained. Confocal microscopy is one. It also seems that Lomo, the Russian maker is partial to them. I'm not sure why this is but many have correction collars, a feature that was not evident on the earlier versions, so there is the possibility to stretch the wings a little with those..

The defects of oil immersion are many but it is a constant battle to achieve resolution that is equivalent to that of an oil immersion objective from a dry objective, or less messy objective. For this we turn to water immersion, a much simpler and less messy technique. However, the reasons that oil immersion exists is to get the N.A. up to a level where high resolution is possible. The maximum limitation of water immersion is N.A. 1.32 but in practice, it is limited to about 1.20. For oil , 1.52 or so, but the practical limitation is about 1.4. So, despite the limitations that sample design place on an oil immersion system , if you want maximum resolution , and are a little hesitant and restricted from using methyl iodide as a high resolution alternative, water although nice and convenient and non-messy and non-toxic can't really get up there. There are also a severe shortage of widely available and inexpensive W.I. objectives out there, most seem to be Lomo or older designs. Modern W.I. objectives from the major makers are prohibitively expensive.

[jwsmith]

Apochronaut....

Yes.....water immersion lenses.....prohibitively expensive....essentially non-available....

I apolologize to all.
I undertook "looking into" water-immersion objective lenses without first checking availability and price.
I just assumed them to be both available and economical.

Entry pricing seems to be northward of $4,000.....
Holy Toledo.

I apologize to everyone.

Judd

[lorez]

Judd,

Don't be too quick to toss the towel. The LOMO water immersion objectives are surprisingly good and until the word gets out they are reasonable and appear fairly often.

lorez

[gekko]

Judd, this might also be of interest (if you've not already seen it):
http://www.microscopy-uk.org.uk/mag/ind ... vwimm.html

[apochronaut]

Lomo to be fair, are fairly good and available at the right price. They are an excellent way of getting into water immersion, if you have a 160mm microscope. They also can be used on some infinity scopes with short barrels( AO 10/20,110/120, 60,160), for dipping and seem to be older Zeiss 160mm designs. They do seem to be prone to inter-element cement failure, just like the Zeiss lenses they are modeled after. A full disclosure and good photography would be the most important aspect of buying an older Lomo lens.Perhaps they should have left the cement specifications behind when they departed from the Jena factory.
Others to look for are Spencer 3mm-60X N.A. 1.15, Koristka-Milano 1/12 inch N.A. 1.20, Bausch & Lomb had several; a 20x and a 40X at least, Baker 50X N.A. 1.0, Leitz and Zeiss. I have seen a Nikon, I think too. Those are all 160mm( except for the leitz), have short barrels and usually are quite cheap, and the 5 of those I have used are excellent objectives. There is no reason to doubt the others.

Many modern ones are plan fluorite or plan apo types and configured for many specific applications and many are also multi-immersion objectives; hence the extreme cost.
The revolution in microscopy towards infinity correction and the recognition that for fine work , nothing but apochromats will do , has unfortunately placed a lot of modern equipment far above the budget of most non-professional microscopists.

[zzffnn]

Very good summary Judd.
As Phil said, LOMO water immersion objectives are optically very good and can be bought for between $70-$150. They are not perfectly plan/flat to edge, but very sharp in the center.

[charlie g]

I low cost purchased LOMO 70X WI objective (no correction collar on this gem) a few years ago..but it's 34 mm RMS short barrel objective...so my work horse Labophot DIN 45 mm stand can't 'focus down enough' to use this gem...mind you...this LOMO functions excellent on my vintage 34mm RMS stands...sigh.

I always catches my atten tion how a 'younger' Spencer optical/Buffalo shiny silver metal : '1.8 mm, 95X objective' is finly engraved:Hom. Immer. ...yet my older brass (?circa 1930's?) B&L Fluorite system 1.8mm, 100X objective is engraved: Oil Immersion. And I have other vintage german objectives (circa 1930's) which are engraved: Oel.

All I'm thinking is that Hom.Immer may be a non-specific spec...? glycerin, cedar oil, immersion oil,...?water+glycerin?!!...where as: Oel, or oil spec is silent on air between substage condenser and bottom of object-slide.

So is my: 'Hom Immer. objective' merely wagging it's finger at me to remember to oil the substage condenser to the slide?

A lot of my 'over thinking' all the labor and fine detailed engraved objective-barrel specs swirls when I go to my cigar-box of vintage, and some 1980's objectives. thanks for a great thread...charlie guevara

[zzffnn]

Charlie,

Your LOMO 70x water immersion objective has a parfocal distance of about 33 mm. You can use a 12 mm extension ring to make it parfocal with other DIN 45 mm objective on your Labophot. I bought such an extension ring from RAF Camera before and used my LOMO 65x 1.1 water objective on my Labophot 2 with other DIN objectives (such as that Labomed 60x). It works reasonably well that way, though the LOMO objective performs better at its stock 33 mm parfocal distance on my LOMO Biolam.

Eventually I modified my Labophot 2 and raised its stage and condenser 12 mm higher to use all LOMO objectives without extension. The LOMOs do not provide a true plan/flat view, but their central resolution is very good. I have high aperture LOMO objectives (10x 0.4, 20x 0.65 and 30x 0.9 WI) on my Labophot now. I personally like it that way.

Hom. immer. requires oiling condenser to slide bottom. Otherwise it is not really homogenous optically. Glycerine+water or just water may be push it too far (not really homogeneous, but still better than dry condenser).

Condenser immersion (with oil, glycerine or water) is worth the trouble to my eyes. At the very least, images are brighter and sharper looking.

[apochronaut]

I did a little test about midway down this thread, viewtopic.php?f=5&t=1182, covering as many condensers on a single stand as I could . I was impressed at just how good a dry .90 achromat can be, even when compared to an oiled 1.3 achromat. It is better for sure , than an immersed abbe 1.25. For illuminating an oil immersion objective, those condensers that are rated with higher N.A.s than 1, really do benefit from oil immersion but if you can at all find a low cost .90 achromat and make it work on your scope, you will never immerse another abbe condenser. An immersed abbe aspheric is a whole other thing and are in a different league than a standard abbe and of course an immersed 1.3 or 1.4 achromat is close to the ultimate.
The differences aren't that great though, between the better 3 types and for sheer convenience the .90 achromat wins hands down.

[charlie g]

Great specific experiences you both just gave me...thank you zzffnn and apochronaut.

A kind microscopist from forum: "Microscope", Brian Matsumoto ( he has an excellent book out:"Practical Digital Photomicrography"...as much about microscopes as about image captures in life sciences...yes I purchased his excellent book)...well Brian suggested I low cost purchase a: "Plezy microscope objective adapter..and thats what I did for my LOMO 70X WI objective. This: Leitz Wetzlar Germany engraved "Plezy" has a glass element in it ...wether that glass is a lens...or a glass barrier...it is more than getting a brass adapter (which I first purchased)...I sort of lost intrest in the forced marriage of this LOMO RMS short barrel objective to my circa 1980's Labophot...only because I purchased a Nikon Plan 100X/ 0.90 dry objective..and I have 60X objective with correction collar.

But the specific use of a simple adapter (mine is brass) which zzffnn states does permit performance on a 45mm DIN stand..thank you for that. I note zzffnn senses the performance by use of an adapter...'takes away some of the performance of the LOMO 70X WI objective'.

And what an intresting combination of objectives you configured on your microscope, zzffnn, very intresting.

Mostly I observe live wetmount slide preps of protists and meiofaun...so for use of my 60X, or 100X dry objectives..a very thin sweet-spot of fluid thickness/fluid volume under the coverslip!

So I guess I glean from this thread that 0.90 substage condensers can feed a light train for a lot of high NA objectives which are not oiled/ or which are oiled to the microscope slides lower surface...and I guess to have made the effort of actually engraving:"Hom. Immer." on an objective barrel implies you should oil the condenser to the slide. (?) all the best, charlie guevara

[zzffnn]

Charlie,

If I remember correctly, that Leitz Plezy adapter was designed to adapt 37 mm parfocal objectives to 45 mm parfocal objectives. So it is better than plain extension adapter of 12 mm, but it is not a perfect fix either. From LOMO WI objectives' 33 mm to 37 mm, there is still 4 mm of difference.

The reason I did not go with that Leitz Plezy was because it is rare and not cheap. At least I could not find one when I was looking.

Also if our subjects are transparent water protists that move around, our eyes may not be able to tell significant different between 12 mm mechanical extension, Leitz Plezy or the perfect 33 mm parfocal. If we photograph a Klaus Kemp test diatom slide or use flash to capture/freeze motion of protists, we may be able to see some slight difference.

It was (different but) not a whole lot of difference to my eyes. I converted my Labophot 2 anyway, because LOMO objectives are cheaper than Nikons and I am more into videotaping than photographing (LOMO objectives' higher aperture value is more important to me, as in videos I cannot use flash to add light - for photography with flash, plan objectives from Nikon may be a better choice).

[apochronaut]

Great specific experiences you both just gave me...thank you zzffnn and apochronaut.

A kind microscopist from forum: "Microscope", Brian Matsumoto ( he has an excellent book out:"Practical Digital Photomicrography"...as much about microscopes as about image captures in life sciences...yes I purchased his excellent book)...well Brian suggested I low cost purchase a: "Plezy microscope objective adapter..and thats what I did for my LOMO 70X WI objective. This: Leitz Wetzlar Germany engraved "Plezy" has a glass element in it ...wether that glass is a lens...or a glass barrier...it is more than getting a brass adapter (which I first purchased)...I sort of lost intrest in the forced marriage of this LOMO RMS short barrel objective to my circa 1980's Labophot...only because I purchased a Nikon Plan 100X/ 0.90 dry objective..and I have 60X objective with correction collar.

But the specific use of a simple adapter (mine is brass) which zzffnn states does permit performance on a 45mm DIN stand..thank you for that. I note zzffnn senses the performance by use of an adapter...'takes away some of the performance of the LOMO 70X WI objective'.

And what an intresting combination of objectives you configured on your microscope, zzffnn, very intresting.

Mostly I observe live wetmount slide preps of protists and meiofaun...so for use of my 60X, or 100X dry objectives..a very thin sweet-spot of fluid thickness/fluid volume under the coverslip!

So I guess I glean from this thread that 0.90 substage condensers can feed a light train for a lot of high NA objectives which are not oiled/ or which are oiled to the microscope slides lower surface...and I guess to have made the effort of actually engraving:"Hom. Immer." on an objective barrel implies you should oil the condenser to the slide. (?) all the best, charlie guevara

When objectives were being labelled , " Hom. Immer.", it was more a designation that they were not water immersion. The fact that they had an N.A. of greater than 1.0 made it evident that they were not dry objectives. The common practice of oiling the condenser to the slide gave enough of an N.A. lift to the condenser that objectives could more approximate their potential. The use of aspheric lenses, coatings and the realization that the objectives of 1.25 N.A. seldom achieved that specification anyway, gave way to the realization that an unoiled better quality condenser, could provide better results than an oiled poorer quality condenser. In other words , there are other factors involved in the quality of a condenser, than simply aperture. So, yes a Hom. Immer. objective needs to be oiled to the slide but only if an oil immersion condenser is used, should the condenser be oiled to the slide and condensers labeled with apertures below 1.0 provide poorer performance ,when oiled to a slide, even with Hom. Immer. objectives.

[zzffnn]

Charlie,

Re "homogeneous immersion":

I guess I was talking about the theory, while Phil was talking about the reality. Hom. Immer. on an objective means that it requires immersion oil between obj and slide to realize NA 1.25 or higher. In theory, a perfect homogeneous optical system should be condenser lens glass-oil-slide glass-mountant oil (Canada balsam)-objective lens glass.

But, that perfect system is rarely realized in reality.

In practice, there are many things that can reduce aperture thus easily voiding true homogeneity.

For example, if you do not have canada balsam as mountant (say you use water, whose refractive index is less than oil or glass), then your system is not truly Hom. Immer.. Or if you use a filter that blocks off part of your condenser, then your actual condenser NA would be less than that defined/required by Hom. immer.

Often time, you do not need perfect optical homogeneity to get good image. And sometimes (for example, with plain bright field without stacking), slight reduction of aperture (voiding true homogeneity) can actually enhance contrast and depth and produce a better image. So don't worry too much about it.

[gekko]

For what it is worth, my understanding of homogeneous immersion is where immersion oil having an index of refraction equal to that of both the cover glass and the the first element of the objective fills the space between objective and cover glass (as well as any space between the object and the bottom of the cover glass). Condenser immersion, filters, and aperture used are irrelevant. In other words, HI deals only with the medium between the object and the first element of the objective. But it may be that my understanding of what HI means is archaic and its meaning may have changed over time, even though the term is no longer used.

it requires immersion oil between obj and slide to realize NA 1.25 or higher.

I think you meant "to realize NA 1.0 or higher" (just getting you back ). (Practically, the limit would be closer to NA of 0.95)

I would like to add to what apochronaut said regarding the use of an un-oiled 0.9 NA achromatic condenser vs, an oiled 1.25 NA Abbe: One usually sets the condenser aperture to something between 80% and 60% of the objective aperture in order to get good contrast (with nearly transparent critters, 50% or even 40% may sometimes be needed to see them). For a 100x oil-immersion, NA 1.25 objective, a dry 0.9 NA condenser will have an aperture of roughly 70% of that of the objective. So, as apochronaut said above, even with a 100x oil-immersion objective, it would be better to use a (dry) 0.9 NA achromatic condenser than an oiled-to-the slide (uncorrected) Abbe condenser.

[zzffnn]

Sorry gents, I was typing too fast on my phone last evening and meanings got lost.

I was talking about Hom.Immer. system (thus including condenser) and its theoretical benefit thereof. If it is just Hom. immer. for an objective, then sure condenser is not relevant.

I had a brain fart when I typed:
"slight reduction of aperture (voiding true homogeneity) can actually enhance contrast and depth and produce a better image".

It should be "voiding the THEORECTICAL BENEFIT OF true homogeneity" - for example when you use filters you reduce the maximum possible resolution according to theory, but in reality you gain contrast and focus depth.

gekko:

I did NOT mean using oil immersion "to realize NA 1.0 or higher". "or higher" there means " > = " (EQUAL or more). You don't really need oil immersion to get close to NA 1.0, as a good dry objective can already get to close to 0.95. Yes, there are 50x-100x oil objectives with NA anywhere between 0.9 to 1.4. But at equal or below 1.0, the benefit of using oil is less about realizing NA, but more about matching optical properties, tolerance of cover slip thickness and tube length, cost, ect.

Homogeneous immersion system is the ideal situation to achieve maximum numerical aperture and resolution in an optical microscope. Note the terms "ideal" and "maximum" there. Good oiled apo objectives get much closer to that ideal maximum, which is >= NA 1.25 and much more than NA 1.0.

[charlie g]

At this point in the conversation, I sense I (we all perhaps) should trot off to easy online 'search of the literature' for why some objectives are with etched spec: oil, others glycerin, others WI, and then here I have a circa 1940's (?) : "Spencer Lens Co. Buffalo,N.Y" " Hom.Imm.-1.8mm-N.A. 1.25-95X" serial # 643757..? It took work to etch that spec on the shiny silver metal objective barrel...why the choice of 'Hom. Imm.'..is it just to say closer RI to the glass slide than air? sense the total lack of specifying what the 'bridge media is'...I sense this implies 'bridge media better than air space'..in the light train up to the labeled:'Hom. Imm. objective'. So..err..I disagree with gekko that 'Hom. Imm.' does not imply 'adding bridge media' only between coverslip and said 'Hom. Imm. objective'.

I have microscope booklets which state things like 'many microscopists use the oil-objective without oiling the condenser to the bottom of the slide'...'for less demanding microscopy this is acceptable.'.

I obviously find it easier to share disagreement with points of this excellent thread..well than for me to get off my..err...chair..and do a bit of lit. searching! all the best, charlie guevara

[gekko]

Charlie, homogeneous immersion does not mean glycerin, water, or other stuff. It means immersion oil of the same refractive index as first element of the objective (and, if a cover glass is used, the cover glass is assumed to have the same refractive index too). You can use a condenser oiled to the slide, dry, or no condenser at all. That,as far as I know, was the meaning of the term when those old objectives were made. It dealt only with the medium between object and the objective's first element (lens). Nowadays, those objectives are marked as oil-immersion (Oil, Oel, or a black band). But, in the final analysis, and definitions aside, an objective marked as HI can be thought of simply as an oil-immersion lens.

zzffnn, as I said, your understanding of HI is quite different from mine, and I think the meaning may have changed over time, even though the term is no longer used, as far as I know, to designate objectives. By the way, you did write, "Hom. Immer. on an objective means that it requires immersion oil between obj and slide to realize NA 1.25 or higher." .

[charlie g]

Hi again, gekko...again we are tap-dancing around 'the media used in the Hom. Imm.'...I trotted out numerous 'higher than air RI media'...simply to draw attention to in the same time period as my Hom. Imm. etched objective...other objectives were being etched with the spec:'oil imm.'. This leads me to think attention is being directed at 'a homogenious light path' from condenser on up to the Hom.Immer. objective lens face.

Obvious to me the glass of the slide might differ in RI from RI of cedar oil, glycerin, water, or immersion oils of other chemistries...obvious to me the face lens element of a Hom.Imm. objective might differ in RI from that of the coverslip, and the fluid a specimen is immersed in, and the RI of the microscope slide...so it's all a matter of best matchup better than an air interface.

I sense it just as logical to assume there was a very good reason why only certain objectives were with the carefully etched spec: Hom. Immer....rath than etched: WI, or oil immer. My sense is that an objective not being etched:oil immersion...but rather Hom. Imm....well the entire light path is being alluded to...above the slide, and below the slide.

Of course it's just my sense of the matter...just as the substage condenser is crucial part of the function of high NA objectives...Hom.Imm. to my sense involves that conderser...but it's all requireing me to 'look it up'...sigh. all the best, charlie guevara

[gekko]

Sure, you can define HI any way you please. But remember the HI designation is applied to the objective and not to the microscope, nor to the entire light path. Here is something that might interest you in this regard:
http://www.smecc.org/history_of_oil_imm ... lenses.htm

[charlie g]

http://www.olympusmicro.com/primer/anat ... rsion.html

I just visited and read the Olympus Resources tutorial on homogeneous immersion microscopy...everyone in this thread is correct according to Olympus tutorial!

Too bad my links never work...perhaps someone can put up the funtioning link?! all the best, charlie guevara

[charlie g]

ooops...it seems my link works....err...please visit it and all can see what Olympus defines homogenious immersion as...and all can see every one of you kind posters were correct in what you stated...and you all were very specific in your coments.

all the best, charlie guevara

[charlie g]

Very intresting link to Jim Solliday's article on history of oil immersion objectives, thank you gekko.

I find it (that article) totally silent on what is going on with substage condensers...in all that history.

Homogeneous immersion is mentioned , but the article is silent on the condenser types, or their setups.

I like the sense I glean from the Olympus link..that what all you posters specifically stated is correct...and I now have confidence that I am correct in understanding the spec: Hom. Imm. (according to Olympus) directing that a Hom.Imm. objective be utilized with an oiled substage condenser. all the best, charlie guevara

[gekko]

Homogeneous immersion is mentioned , but the article is silent on the condenser types, or their setups.

Exactly! And that is because the condenser has nothing whatever to do with the HI objective, Olympus's interpretation notwithstanding (I know I am being supremely arrogant... and I also know no one will believe me , but do check other sources, especially historical ones that describe what HI is near the time it was being used to designate objectives).

My apologies to jwsmith for steering this thread away from what it was intended to be.

[charlie g]

Bravo to jwsmith for this thread ( I early on thanked jwsmith for the thread ). Not so fast gekko! Your opine that what is etched on the barrel of an objective has no relation to the microscope, the substage condenser, how a Hom. Imm. objective is used???...err..DIC objectives which you energize our microscopy with...well that very specific spec: DIC entails oh so much more than that: 'DIC objective'...for proper use of that DIC objective.

I logically keep pointing out to you that during the time of objectives etched: 'Hom. Imm.'...the same firms had objectives etched: 'oil-immersion', WI, etc.. Why is that...well do not cite Jim Solidays excellent article on objectives developement...these objectives were/ are all used with properly suited substage condensers..it is no ommission of Jim Soliday...he simply was not dealing with the context of proper useage of those marvelous master optics craftspersons objectives....no where in that excellent article is the total setup discussed.

I doubt I would sense any arrogance on your posting, gekko...but please do sense that their is concern for total: light path from condenser to objective lens face of a: 'Hom. Imm. objective'...otherwise keep all etched specs: WI, oil, glycerin, etc..

And yes the Olympus link I posted does bring this fact as the reason for a spec: 'Hom. Imm.'...and yet like all of the specifics offered in this thread...that Olympus link notes..it often is too severe a setup...so in real world microscopy often enough...forget about oiling the condenser to a slides bottom...yet like oh so many of my microscope booklets..the Olympus resource notes: for critical work...oil bridge the condenser to the bottom of the slide. I sense you need to see that everyone who posted in this thread...gave very correct specifics...err..except for the part about: hom. imm. is engraved on the objective...not on the microscope/ not on the condenser...<(++)>...thank for your microscopy, all of you. charlie guevara

[zzffnn]

http://www.microscopy-uk.org.uk/mag/ind ... pjcol.html

In part 4 of the above article about COL (circular oblique light) contrast, Paul James mentioned that with COL, Abbé condenser and achromat objectives perform very well (though not so welll in plain bright field, when compared to aspheric condenser and fluorite objective). So what works best really depends on your very specific application.

I personally like using COL and UGF (universal gradient filter) with water immersion on LOMO Abbe condenser and LOMO objectives (some achromatic, some apo). Maybe one day I will upgrade my condenser, but it is not my priority right now (for example, I will rather some Zeiss 1.5H cover slips first to improve my image quality - as I found cover slip thickness makes a difference in my system). There is always something better to be had, if you are willing to shoulder the cost (money, time, effort).

What matters most, I think, is whatever that makes the user/hobbyist happy. However you roll it, do it or call it. If one wants to do real microscopy science and be precise, there are always Harvard and Rochester

[charlie g]

Hi zzffnn, what is: UGF in your microscopy? Ilike the LOMO nose piece setups you have on your stand! charlie guevara

(? UGF= a type of fluorescence?)

[zzffnn]

Hi zzffnn, what is: UGF in your microscopy? Ilike the LOMO nose piece setups you have on your stand! charlie guevara

(? UGF= a type of fluorescence?)

I have sent you a PM, Charlie. You may also Google "universal gradient filter microscope" to get to the same web pages.

[gekko]

Paul James mentioned that with COL, Abbé condenser and achromat objectives perform very well (though not so welll in plain bright field, when compared to aspheric condenser and fluorite objective).

The Abbe condenser also performs very well with phase contrast, oblique, and darkfield illumination. All of those methods do not require the condenser to have a large aplanatic aperture since they use only a small area or narrow annulus. If I am not mistaken, Abbe designed the "Abbe" condenser specifically for use with oblique illumination.

[zzffnn]

^ Exactly.
Interesting to know that Abbé condenser was designed for oblique illumination. Seems to make sense. Cheap and cheerful rules in reality