Better objectives for a Chinese microscope

[kit1980]

I have this microscope - OMAX 40X-2500X Digital Trinocular Compound Siedentopf LED Microscope with Kohler Illuminator and 14MP Camera (https://www.amazon.com/gp/product/B00UCEPH70) + plus a dry dark field condenser for a couple of weeks now. I like the microscope and have a lot of fun with it, but I really want to try better objectives (brand name + better correction).

I've done some research, and here is my findings/questions, please confirm/answer:

1. A compatible objective should have 160/- or 160/0.17 markings, anything with the infinity symbol is not compatible with the OMAX
2. A compatible objective should be DIN, not JIS (and JIS is more likely to be used in Japanese scopes - Nikon and Olympus)
3. Leitz/Leica objectives require corrective eye pieces, and use of the OMAX's USB camera can be problematic
4. Because of 2) and 3), the safest choice in probably Zeiss
5. Used apochromat lenses are more likely to have delamination problems
6. Will a phase contrast (Ph1, Ph2, etc.) or EPI objective work for bright/dark field?

So far I preliminary decided to buy a used Zeiss Neofluar 40/0.75 160/0.17 40x objective. Please comment on this choice and the whole upgrade idea.

[zzffnn]

Yes to all, but no to 4). Zeiss DIN objectives need eyepiece compensation too. And to 6), note that EPI objectives do not work well for biological (cover slip) subjects.

As far as I know, the only DIN objectives that do not need eyepiece compensation is Nikon CFN 160 mm series.

If you don't have to use that USB camera, you can couple compensating eyepiece to DSLR/mirroless camera lens of "normal" strength (50 mm focal streng in full frame).

[kit1980]

As far as I know, the only DIN objectives that do not need eyepiece compensation is Nikon CFN 160 mm series.

It looks like it's hard to make an objective that doesn't require compensating eyepieces.
So I'm starting to wonder if my OMAX also has some kind of compensating eyepieces. But how to understand if it's true, and if it's Zeiss, Leica, or Olympus-compatible correction?

[kit1980]

As far as I know, the only DIN objectives that do not need eyepiece compensation is Nikon CFN 160 mm series.

It looks like it's hard to make an objective that doesn't require compensating eyepieces.
So I'm starting to wonder if my OMAX also has some kind of compensating eyepieces. But how to understand if it's true, and if it's Zeiss, Leica, or Olympus-compatible correction?

OK, I've done some tests according to http://www.olympusmicro.com/primer/anatomy/oculars.html (held up to a light source and look for a orange hue at the periphery) and decided that OMAX's eyepieces and photo adapter are not compensating.

[75RR]

OK, I've done some tests according to http://www.olympusmicro.com/primer/anatomy/oculars.html (held up to a light source and look for a orange hue at the periphery) and decided that OMAX's eyepieces and photo adapter are not compensating.

Even if they were compensating they would only compensate (i.e. correct the image) for the objectives they were designed for.
Compensating eyepieces are not universal. It is best to think of them as the final stage in correcting the objective image, in a sense they are part of the objective. That is why one should always use eyepieces and objectives from the same manufacturer.

[lorez]

One question that is not is addressed is whether the frame and other elements of the microscope will adequately support the new objectives. I would suggest that you enjoy your microscope as it is while looking for a complete bigger and better system.

lorez

[vasselle]

Et si vous voulais faire de la photo je vous conseille des objectif plan.
Cordialement seb

[75RR]

One question that is not is addressed is whether the frame and other elements of the microscope will adequately support the new objectives. I would suggest that you enjoy your microscope as it is while looking for a complete bigger and better system.
lorez

Lorez makes a good point. If you are going to put another make of objectives and eyepieces on your microscope (from one of the big 4) you might as well go whole hog and get the same makers frame to go with it. In the meantime enjoy the microscope you have and try to get the best from it by improving your skills. A good 70% of those amazing photos we are privileged to see are due to the microscopists skill, that leaves just 30% for the equipment.

[kit1980]

One question that is not is addressed is whether the frame and other elements of the microscope will adequately support the new objectives. I would suggest that you enjoy your microscope as it is while looking for a complete bigger and better system.
lorez

Lorez makes a good point. If you are going to put another make of objectives and eyepieces on your microscope (from one of the big 4) you might as well go whole hog and get the same makers frame to go with it. In the meantime enjoy the microscope you have and try to get the best from it by improving your skills. A good 70% of those amazing photos we are privileged to see are due to the microscopists skill, that leaves just 30% for the equipment.

I cannot justify buying another microscope. Also I want to do it for education purposes - compare the different objectives on the same scope.
As zzffnn pointed out, Nikon CF (that I found stands for chromatic-aberration-free) objectives don't really require compensating eyepieces, and I hope the frame itself should not be a problem.

[zzffnn]

Nikon CF 160mm objectives are not cheap though - 1-2 of those may cost the same as an entire scope.

Nikon CFI are infinity line with oo symbols - those won't work on 160 mm scopes. Just a note.

If you don't have to use DIN 45 mm objectives, I heard short LOMO 33 mm achromats <= 20x are also free of CDM (lateral color). But those do not have any coating (reflection could be a problem) and most of them are NOT plan. LOMO apos and DINs are not CA free.

[c-krebs]

My first, second , and third choice for your situation is the same .... Nikon CF objectives. They are excellent and can be found in (very good) Plan Achromat types as well as Plan Apo types (not cheap!). These are finite type, 160mm tube length, and 45mm parfocal (DIN). They do not require chromatically corrective eyepieces.

You can find a brochure of what I believe was the last incarnation (before the change to "infinity") of this type objective here. It is dated February 1989:

http://www.krebsmicro.com/Nikon_CF.pdf

Starting on page 10 they start breaking down the different series that were offered. They range from spectacular Plan Apos to more pedestrian ("student grade") E Achromats.

CF/CF N Plan Apochomat (superb!)
CF/CF N Plan Achromat (excellent quality plan achromats. Not all achromats are "created equal)
E Plan Achromat
Achromat
E Achromat

You can see that I have only made comments for the first two "levels". I personally have some experience with some of the ones in these categories (not all), so I am pretty confident in their performance.

The others I just don't know and won't comment because I have only used 1 or 2 over the years (E Plan Achromats). I also have absolutely no idea of the quality of the objectives you are now using, so I can't offer any solid comparison. The only thing I feel safe saying is that that the Plan Apos will be superior. The CF/CF N Plan Achromats should be at least as good, and a good chance they will be somewhat higher quality. For the other three level I have no idea. Many of the Chinese achromats have improved a great deal, and many look good. I've seen many excellent images made with them. It does however seem pretty clear that the quality can vary considerably.

To muddy the waters a little...

Nikon came out with the CF 160mm finite series in 1976. There have been a few iterations since then. Unfortunately they did not mark the vast majority of these objectives with the "CF" designation. And the barrel style has changed at they updated the series. So the only way to recognize these objectives is via the barrel style. The brochure I linked above shows the last version. The barrel style is pretty distinctive. Previous versions are worth consideration as well, but I have never been able to tack down a brochure for the earlier "CF" objectives. If you look for brochures and manuals for the microscopes of that time (Labophot, Optiphot) you can get an idea of what some look like. (Prior to 1976 the Nikon objectives were for the S-series microscopes, and they did requite corrective eyepiece and photoeyepieces). For example look through this brochure:
http://earth2geologists.net/Microscopes ... ochure.pdf

Be forewarned... it you go searching on Ebay you will be stunned and confused by the number of Nikon objectives that are on there, and most are not what you want! I did a quick Ebay search for the 20X. As expected prices are all over the place, but found some nice looking (latest versions) CF Plan Apos in the $550-$600 range and CF Plan Achromats in the $165 to $200 range). That should give a little idea of what the expense involved would be.

BTW, Zeiss 160mm finites should be used with their corrective eyepieces.

[kit1980]

Thanks for the answer, Charles!
I've already downloaded the Nikon_CF.pdf brochure earlier today.

For now I've decided to take a chance with a Nikon CF objective. That's what I've found on ebay, and I would really appreciate comments/suggestions:

1. Nikon Plan Fluor CFN 40x /0.75 160mm TL Microscope Objective $449.99 http://www.ebay.com/itm/Nikon-Plan-Fluo ... 1631052426 - looks very good, but pretty pricey
2. Nikon Fl Plan 40x 0.75 160 0.17 Microscope Objective Fluor $64.99 http://www.ebay.com/itm/Nikon-Fl-Plan-4 ... 2081194426 cheap, but very rusty, and probably not even CF
3. Nikon CFN Plan 40x Microscope Objective $175.00 http://www.ebay.com/itm/Nikon-CFN-Plan- ... 1621832440 - only Plan, but looks good and not too expensive
4. Nikon FLUOR 40x /.85 Dry 160 TL Microscope Objective - Coverslip Collar $329.99 http://www.ebay.com/itm/Nikon-FLUOR-40x ... 1321815426 - is it even CF? still too pricey
5. Nikon Plan 100/0.90 dry objective with adjustable collar $100.00 (starting bid) http://www.ebay.com/itm/Nikon-Plan-100- ... 2021042323 - dry x100, is it even CF?

[lorez]

[quote]- compare the different objectives on the same scope... and I hope the frame itself should not be a problem.[/quote

In my original post I suggested that the stand may have some bearing on the performance of the objectives. I have the good fortune to be familiar with all of the objective grades Charles has detailed (all of which are better that what are on your microscope) as well as the microscope you currently own. I don't think you are going to see the full potential of the objectives when used on your current stand. If I were to do what you are going to do I would get a 40X/0.65 objective from either Nikon E achromat or E planachromat series or the Olympus CH2 or BH2 series. Any of these should be available and not too expensive. You should see some improvement in your image, but will not see what the objective is capable of in its designed environment.

lorez

[c-krebs]

Keep in mind I can't give you any definitive answers... never used your current optics, and I have only used one of the ones you have listed.

#2) is out. Not "CF" and not 45mm parfocal.

#3) I have, and have used. This the best "achromat" version of the Nikon CF 160mm finites. As I mentioned earlier it is a very good achromat. It has an NA of 0.70. This might be important to you... if your darkfield condenser is the OMAX Dry Darkfield Condenser NA 0.7-0.9 you will not be able to get darkfield with this (and the others on your list). The lower number on the darkfield condenser NA "rating" must be larger than the NA of the objectives. I believe Omax does offer an oil darkfield condenser. If so that would work. The price on this objective seems quite reasonable.

The other 3 are "CF". #5 it maybe a little too "specialized" in that I think you would get far more use from a 40X or 20X (but I don't know your preferred subject matter). Both #4 and #5 have correction collars. This is really needed when the NA goes above 0.75 unless you are very attentive to coverslip thickness (actually measure it) and have your subject smack up against the coverslip. So they have the collars. This adds another layer of complexity to shooting as you really should visually adjust the collar for each scenario. Also, even if you have or get an oiled darkfield condenser you should be aware that such a high NA, (while desirable for their potentially greater resolution) are more difficult to work with in darkfield. Also, I am not familiar with your condenser. If it is a basic Abbe condenser you may not be able to take full advantage of the larger numerical apertures. It is pretty common that a dry Abbe condenser may not provide lighting that can make full use of an objective NA over about 0.70 or so.

[75RR]

I cannot justify buying another microscope.

It is sometimes cheaper to buy a stand with objectives than to buy them individually.

[Charles]

A less expensive option. If you already bought and have the Zeiss Neofluar objective coming, why not just get a pair of Zeiss KPL eyepieces?

[kit1980]

A less expensive option. If you already bought and have the Zeiss Neofluar objective coming, why not just get a pair of Zeiss KPL eyepieces?

I didn't buy the Zeiss - it was just an option I considered before I consulted this forum.

[kit1980]

Thanks everybody in this thread for suggestions, a lot of useful information!

After reading all opinions, I decided to buy a used Nikon CFN Plan 40x objective and I just have ordered one.
I'll post some comparison pictures when I have them.

[apochronaut]

Huygens and Kellner eyepiece designs are fairly universal and are well corrected to provide excellent performance with standard achromat designs. These are really the only eyepieces in general use that are not compensating or in some way complementary. Compensating, became a term used to denote a level of eyepiece performance, that overcame defects in an objective's ability to achieve perfection. It was a given that an achromat , was not perfect, so when an objective design attempted to raise the performance bar, such as in providing enhanced planarity or colour correction, often matched or compensating eyepieces were part of the system, with each makers eyepieces having slightly or greatly differing engineering, depending on the requirements. Such eyepieces, were marked with a designation, indicating it's intended usage. There were various of these, such as com.,compens, kompensating,k,plan kompensating etc. but each one had a unique application. The protocol, whereby compensating eyepieces have a yellow refraction at the field stop and non compensating eyepieces a blue refraction, only applies to eyepieces that are intended to over compensate for lateral chromatic aberration. Eyepieces intended to compensate for other aberrations or distortions, such as planarity, would not necessarily have that feature.
Eyepieces of universal correction are in decreasing use, primarily due to the modern need for wider fields, so standard well corrected eyepiece designs are almost a thing of the past, except on cheaper scopes. Manufacturers have their eyepieces tuned to their objective's unique requirements in a number of ways now, without calling them compensating. They usually have proprietary codes on the barrel. Sometimes eyepieces from two manufacturers, are similar enough to be interchangeable but that can only be determined by trial and error and the chance of basic grade eyepiece designs being close is greater than that of very sophisticated ones intended to provide enhanced performance.
The fact that Nikon CF objectives are supposed to be chromatic aberration free and therfore not in need of compensation is pretty irrelevant because, first of all, they are not chromatic aberration free, otherwise they wouldn't be called achromats and secondly,the eyepieces for them are still unique to them, so are unlikely to have any necessary universality.
If you are upgrading to better objectives, I would choose them based on value for the dollar, rather than on brand loyalty, compensation freedom, or some illusion that one brand is better than another. You aren't going to notice the difference between the better name brands, in enough of a way, that it matters that much and, you will likely need matching eyepieces anyway, irregardless of which objectives you gravitate towards.The principle defects of Chinese optics, are inherent to their eyepieces as well, so they will also benefit from an upgrade but at least temporarily, they might be compatible, enough.
Always use eyepieces, designed for the objectives employed, unless you are 100% sure that they are a match and will not inhibit performance.

[zzffnn]

I would think Mr. Charles Krebs, as one of the best microphotographers out there, went through lots of trial and error and pixel-peeping, before he felt comfortable recommending using Nikon CFN plan achromats without eyepiece compensation? I am not using "pixel peeping" as a negative term, by the way, I only meant carefully examination of individual pixels.

I know CFN plan apos are nice and Mr. Krebs also uses those, but those can be too expensive for many beginners. The CFN achromats are more attractive to many due to their lower cost - so it would be nice to hear confirmation from an expert, if those achromats do perform well without eyepiece compensation.

[c-krebs]

The fact that Nikon CF objectives are supposed to be chromatic aberration free and therfore not in need of compensation is pretty irrelevant because, first of all, they are not chromatic aberration free, otherwise they wouldn't be called achromats and secondly,the eyepieces for them are still unique to them, so are unlikely to have any necessary universality.

Certainly you sometimes need to distinguish between the marketing and the actual technology. When Nikon introduced their "CF" ("chrome-free) objectives in 1976, I would think that they certainly were not stating that all their objectives were totally free of chromatic aberrations. As you say, the very definition of an "achromat" (of which they offered many) would make that obvious. What they meant was that all of the chromatic corrections, to the degree intended in a particular objective design would be performed in the objective, and no color correction would be done in the eyepieces. Certainly no one would expect a <$100 objective to be as well corrected as a >$3000 objective.

The least expensive (and most common) objectives, employed on a majority of laboratory microscopes, are the achromatic objectives. These objectives are corrected for axial chromatic aberration in two wavelengths (blue and red; about 486 and 656 nanometers, respectively), which are brought into a single common focal point.

Apochromats are corrected chromatically for three colors (red, green, and blue), almost eliminating chromatic aberration, and are corrected spherically for either two or three wavelengths

http://www.microscopyu.com/articles/opt ... intro.html


As an example, older Nikon and Olympus compensating eyepieces were used with high numerical aperture fluorite and apochromatic objectives to eliminate lateral chromatic aberration and improve flatness of field. Newer microscopes (from Nikon and Olympus) have objectives that are fully corrected and do not require additional corrections from the eyepieces or tube lenses.

http://www.microscopyu.com/articles/opt ... specs.html

As you also mention there is no reference to any other corrections (non-color related) that might have been accomplished with the eyepieces. This has historically been something of a difficulty with very little info available from most manufacturers. In some cases you were told to use corrective eyepieces with certain objectives but not with others... in the same manufacturers line! Some manufacturers tried to simplify things by designing in the same amount of needed correction into all their objectives so there was no need to swap out eyepieces.

But there are reasons I am willing to suggest the finite Nikon CF objectives. First, I have used a fair number on several brands of scopes. Another is because of their proven performance when used as a lens on a bellows or extension tubes, with the image projected directly onto the camera sensor. There are no other optics involved. Over the past 9 years or so countless images have been made using Nikon CF finite objectives in this manner. My personal favorite is (was...since I moved to using infinity style optics for this) the Nikon CFN 10/0.30. Another real gem is the CFN 4/0.20 Plan Apo. (It is the only Apo that had sufficient working distance to be used effectively in this manner. It is so highly regarded for this use that the used prices are generally much higher than other name brand Apo 4X objectives). The various 5X, 10X, 20X, and 40X M Plan Achromats (210mm finite) were, and are, used in this manner as well. With all those eyeballs scrutinizing those image files certain problems would have been pretty obvious. When used in this manner the image is projected onto camera sensors that have diagonals far greater than the 18-20mm field numbers utilized on most microscopes. (In my case Canon APS-C with diagonal of 27.3mm. More than the 25-26mm field size of a designated "super wide-field" from Nikon). Any field curvature would have been clearly seen... it was not an issue. A few of the M Plan Achromats showed a little more longitudinal CA than would be liked (but quite good in regard to CDM), but they are, after all, achromats. So while I am aware and leery of eyepiece compensation, I feel the Nikon CF 160mm finite have a track record that allows them to be recommended as an upgrade on certain microscopes.

[zzffnn]

Mr. Krebs,

I don't doubt your authority and experiences - I would appreciate another confirmation on an assumption that I made though.

Unless a person stares at edges of a single frame of a perfectly flat subject, (s)he won't notice field flatness issues with focus stacking and direct projection (objective directly to sensor). I assume you have done such examination? I am only asking for my learning purpose and not to doubt your words.

And sorry for calling you "Mr. Krebs" instead of Charles, as we have many Charles in this forum and another in this thread.

[c-krebs]

Unless a person stares at edges of a single frame of a perfectly flat subject, (s)he won't notice field flatness issues with focus stacking and direct projection (objective directly to sensor).

Absolutely... as long as it is solely a field flatness issue. I've used objectives that required some focus change for the edges/corners when viewed through 20mm field number eyepieces on a microscope. With some, the slight re-focus brings the periphery into sharp focus and a quality image. With others, while the "focus" improves somewhat, the image quality itself is pretty sad looking. While they do have field curvature they also produce a rather small image circle (that is of good quality). So if you think you are getting "soft" corners on photographs and you assume it might be a field flatness problem, you should check to see if the bigger problem is that the objectives produces a rather small size "quality" image. (Stacking, since it uses the sharpest part of each frame is also pretty good at reducing (but not eliminating) the appearance of longitudinal chromatic aberration... where the out-of-focus details above and below the focus plane take on a greenish or magenta color).

While I don't recall ever finding field flatness to be an issue at all with the Nikon CF objectives used for direct projection, I'll admit I never did any exhaustive testing on a perfectly flat subject.
I just checked a few finite Nikon CFN objectives mounted on bellows (direct projection) in this regard. A CFN 4/0.10, CFN 10/0.30, and a 20/0.40 ELWD M Plan Achromat (210mm finite). They exhibited no field flatness problems. Next time I use a microscope with some of these objectives I'll take a look using the "official" Nikon CF eyepieces and also some non-Nikon ones.

I did notice that Charles is a popular name here! I'm not that crazy about being called Mr. Krebs, perhaps "Charles K" would provide sufficient differentiation.

[zzffnn]

Thank you very much, Charles K. I am finding your comments very helpful.

Yes, I am aware of image size limitation of some older objectives, including my LOMO short ones. I learned about image size issues from your web site actually. That was one of the reason why I have been using a micro 4/3 camera (afocally with compensating eyepieces).

[apochronaut]

I would think Mr. Charles Krebs, as one of the best microphotographers out there, went through lots of trial and error and pixel-peeping, before he felt comfortable recommending using Nikon CFN plan achromats without eyepiece compensation? I am not using "pixel peeping" as a negative term, by the way, I only meant carefully examination of individual pixels.

I know CFN plan apos are nice and Mr. Krebs also uses those, but those can be too expensive for many beginners. The CFN achromats are more attractive to many due to their lower cost - so it would be nice to hear confirmation from an expert, if those achromats do perform well without eyepiece compensation.

Microphotographs are miniature photographs.

The optical conditions required for direct optical observation, which require eyepieces, are somewhat different than the conditions required for photomicrography. It's hard to put a figure on it but I would guess that somewhere in excess of 99% of the total use of microscopes is by virtue of looking through the eyepieces. That is in fact, what most microscopists do. Even the ones that do photograph, only take a fraction of time away from optical viewing.
I am getting the impression that I have missed something in this thread: that the thread is about using objectives as photographic lenses. I missed that in the original question. My part of the discussion was about using objectives as microscope lenses. Since the Omax microscope, presumably has either Korean or Chinese eyepieces, then, is the suggestion that using a Nikon CF objective would work perfectly with his existing eyepieces?

[75RR]

Original poster wanted better quality objectives and to still be able to use his USB Camera

I have this microscope - OMAX 40X-2500X Digital Trinocular Compound Siedentopf LED Microscope with Kohler Illuminator and 14MP Camera (https://www.amazon.com/gp/product/B00UCEPH70) + plus a dry dark field condenser for a couple of weeks now. I like the microscope and have a lot of fun with it, but I really want to try better objectives (brand name + better correction).

I've done some research, and here is my findings/questions, please confirm/answer:

1. A compatible objective should have 160/- or 160/0.17 markings, anything with the infinity symbol is not compatible with the OMAX
2. A compatible objective should be DIN, not JIS (and JIS is more likely to be used in Japanese scopes - Nikon and Olympus)
3. Leitz/Leica objectives require corrective eye pieces, and use of the OMAX's USB camera can be problematic
4. Because of 2) and 3), the safest choice in probably Zeiss
5. Used apochromat lenses are more likely to have delamination problems
6. Will a phase contrast (Ph1, Ph2, etc.) or EPI objective work for bright/dark field?

So far I preliminary decided to buy a used Zeiss Neofluar 40/0.75 160/0.17 40x objective. Please comment on this choice and the whole upgrade idea.

zzffnn suggested using Nikon CFN to get around the compensating eyepiece requirement.

Yes to all, but no to 4). Zeiss DIN objectives need eyepiece compensation too. And to 6), note that EPI objectives do not work well for biological (cover slip) subjects.

As far as I know, the only DIN objectives that do not need eyepiece compensation is Nikon CFN 160 mm series.

If you don't have to use that USB camera, you can couple compensating eyepiece to DSLR/mirroless camera lens of "normal" strength (50 mm focal streng in full frame).

The rest of the discussion was basically about the quality of these objectives.

[c-krebs]

I am getting the impression that I have missed something in this thread: that the thread is about using objectives as photographic lenses....
... Since the Omax microscope, presumably has either Korean or Chinese eyepieces, then, is the suggestion that using a Nikon CF objective would work perfectly with his existing eyepieces?

No, there is really no way to know except to try. Sorry if I got a little "long-winded" in replies. But if the original poster wants to try "upgraded" objectives with his existing microscope he will need to go with another brand. I am nearly certain that the common Chinese or Korean eyepieces are fairly basic and not chromatically corrective. (I am certain that the reduction lens used with his camera is "neutral"... or non-corrective). It seemed logical to suggest objectives that were designed to not need chromatic correction via the eyepiece... why "upgrade" to an objective that will likely start off with uncorrected chromatic aberration? (Many, if not most, of the finite 160mm objectives he could consider were designed to use eyepiece chromatic correction... not the Nikon CF). When it was (accurately) brought up that chromatic correction is not the only correction that can be applied in eyepieces, I perhaps went a little "off-topic" to explain how my long experience with these objectives "used as photographic lenses" showed to me that there was no noticeable optical deficiency that indicated a need for a field flatness correction. In other respects as well the images were excellent. In any event, the Nikon CFN's are very good, readily available, and reasonably priced used. If concerned or desired, Nikon CF eyepieces are also available quite inexpensively with a little patience (I just picked up a pair for $49!).

[kit1980]

So, I received my Nikon CFN Plan 40x today.

Some initial observations:

1. Nikon focuses on slightly different plane, but also the image is centered slightly differently
2. Nikon gives much brighter picture with the same amount of light
3. Dark field works just fine with my current dry 0.7-0.9 condenser
4. Nikon seems to perform roughly the same in the center of the field, and much better on the periphery

Because of the difference, it's hard to compare my existing 40x objective and the Nikon: I cannot just switch objectives, but need to adjust focus, stage, recenter field diaphragm/condenser, and adjust brightness.

[kit1980]

Here are two picture of some debris with two objectives (guess the order). I tried to focus on a diatom just below the center. Do you see the difference?
The images are straight from my USB camera except resizing to 1024 px.

[c-krebs]

Second image is clearly better. (Hope that's the new objective )