How to find the tube length of an infinity ;scope or objective?

[mrsonchus]

Hi all, I've a hankering to look for some different (higher-quality) objectives for my infinity-corrected 'scope and know that they too have a 'tube-length' that varies and is often manufacturer-dependent (although not uniquely so)..
So, I need to find out 'what spec' (tube-length and thread) my 'scope is using - anyone know how this may be achieved?

Also, how do I make a meaningful and accurate assessment or preferably measurement of the quality of my objectives? They all seem very good to me but I'd like to get the highest resolution that I can without buying a new 'scope, basically I'm concerned with x60 and above objectives, it seems to me that below this they are not as critical....
Any advice or pointers would be great!

[lorez]

Hello John B,

You are asking some interesting and challenging questions. I have some thoughts, but probably not good answers.

The microscope optical tube length is the distance between the tube lens rear principal plane and the intermediate image plane (Zeiss)
http://zeiss-campus.magnet.fsu.edu/tuto ... flash.html

I have no idea of how to measure this, other than to rely on the available literature from the manufacturer (which is sometimes difficult to get).

I can tell you that it does make a difference. Using the wrong objectives will give you a diminished image.

The numerical aperture of the objective is the indication of the resolution, but there are other factors that are rarely/never mentioned in these discussions.

The quality of the materials used in the objective is very important as is the quality of assembly. Also, the availability and proper use of the microscope condensers and apertures. Of course, the microscope must be clean and in good condition.

Back in the day I had a "star slide" from Bausch & Lomb which was used in testing the resolution of their objectives. The slide had a mirror coating with sputtered microscopic holes in the coating. The light would shine through the holes and the correctly set objective would see a star shape. Time and crummy habits have hidden the slide... I've looked for it, but with no luck.

Now I do empirical testing. I have several slides that have significant points of reference that I have marked and I use them to test the objectives. My base line is the image created by a high end objective, some very specific notes, and a lot of practice. The slides are of a couple of diatoms and some bacteria. Knowing that an objective is able to resolve the features helps me judge its quality and condition. For the most part I do nothing to change anything. I just know where I am.

This probably clouds the issue more than it resolves it...

lorez

[mrsonchus]

Hello John B,

edited for brevity.....

I can tell you that it does make a difference. Using the wrong objectives will give you a diminished image.

The numerical aperture of the objective is the indication of the resolution, but there are other factors that are rarely/never mentioned in these discussions.

The quality of the materials used in the objective is very important as is the quality of assembly. Also, the availability and proper use of the microscope condensers and apertures. Of course, the microscope must be clean and in good condition.

edited for brevity.....

lorez

On the contrary Lorez, that's a big help to know I may have a correct understanding of the situation - yes I suppose the direct-observation is the best I can do at this time, but how can I know if I can do better? A dilemma for sure.... Also my condenser is embarrassingly basic although I do know how to use it properly (I think) and always use Köhler illumination above x4 of course.

I'll have to look into it! Thanks for your help.

[lorez]

but how can I know if I can do better?

I can't remember what microscopes you are using at the moment, but if you know your condenser is "embarrassingly basic", you may want to start there.

I think you are getting very good results with what you are using.

The biggest challenge is finding out what the tube lens focal length is.

Up to a point we can always do better. It's just a matter of a few $$. Having something more than the empirical test is beyond what I have ever needed. I see what the pros are using and see what they are seeing and go from there. Do you know a pro ?

We haven't even mentioned the various degrees of lens construction, i.e. fluorites and planapos, but they are also subject to quality issues.

lorez

[mrsonchus]

I think similarly Lorez that the condenser must be my first optimization - it's clearly a weak-link. My 'scope's just a branded Chinese affair but does perform very well, especially the x100 HI objective I find.

Thanks for the help.

[apochronaut]

John; There are several infinity systems calculated by the various major makers. Some of the Chinese microscopes fall into place with some of them because they are made within a program, that also makes objectives for one of those makers.

The infinity systems sometimes have fully corrected images leaving the objective, just needing conversion from infinity to a convergent beam, in a telon lens.This is usually located in the base of the viewing body or head. Others, produce an infinity image that needs further correction in the telon lens.

AO/Reichert/Leica objectives are fully corrected and have a tube length of 200mm to the telon lens.
Olympus has fully corrected objectives and a 180mm tube length to the telon lens.
Nikon uses 200mm but there are further corrections in the telon lens
Zeiss uses 164.5mm and there are further corrections in the telon lens.
This is why the Zeiss Primostar with a 180mm tube length is left out in the cold, when it comes to using "real" Zeiss infinity corrected components . There is a very good chance that it's maker originally was contracted to make Olympus optics and Zeiss stepped in for a bit of the action at some point.
I have used an Olympus configured objective on an AO microscope and they are perfectly corrected. There is probably a 10% increase in the magnification but that's o.k.
My impression is that a lot of the Chinese infinity stuff are Olympus pattern, which is good because it allows for the possibility of upgrading one with genuine Olympus optics .
Is there a dealer you know, who would let you try out some objectives, to see how they work? You will get an image nontheless but if unmatched there will likely be lateral chromatic aberration to varying degrees and or lack of planarity.

The cheapest upgrade, condenser -wise is to go from an abbe to an abbe aspheric , sometimes called an aplanatic. If you are not going to up to an N.A. higher than 1.25, and this would imply fluorite or apo objectives, due to the D.I.N. specification, then there is no point in upping the condenser. If you check your dovetail or sleeve size, you may easily be able to convert one of the cheaper neglected abbe aspherics on ebay, quite easily. An achromat, with a low 1.25 N.A. is not that popular either, although they are rarer. I have seen them sitting unwanted, while people search for 1.4's. Another option is a .90 or .95 achromat, which are usually better than an oiled 1.25 abbe. The little bit of N.A. loss is more than compensated for by the corrected light and freedom from astigmatism ....and they can be used dry.

[mrsonchus]

Hi Apo and thanks for this very helpful reply, a lot of very useful information there for me. The head of my 'scope does indeed have a lens right at it's base, not inset into the head at all it sits directly above the join between the head and the stand. Looking down into the stand of course I can see the back of whatever objective is in the viewing-position of the nosepiece.
Is a measurement between the (correct point on the) objective and this lens in the head of any value, as the differences between the systems is quite large there would be plenty of margin for error while distinguishing between say a 200mm and a 180mm?

The condenser - is indeed a simple (and rubbishy if it's build-quality is indicative) abbe 1.25 type. The width (dia) of my condenser into the holder is I think 38mm. The idea of an abbe aspheric/aplanatic condenser appeals to me - would I still need as now a swing-in bull's-eye lens to fill the x4 objective do you know. Is it possible to convert up, down or either way for condenser diameter with an adapter maybe?

If I knew which objectives would work perhaps I may look for a x60 or thereabouts with a correction-collar to squeeze a little more resolution from my system?

[apochronaut]

It shouldn't be too hard to measure but then you also don't know if the system has further corrections required. I'd try to get an Olympus infinity objective somewhere and try it out.
You will still need the aux. lens to fill the field for the 4X . You can probably find an older Japanese condenser , many of which were 37mm and make it work in there. I'll check what I have here and see if there are any that might be able to be adapted. The U.K., isn't exactly as buoyant a supply as the U.S., either; kind of like Canada.

[mrsonchus]

Thanks Apo' - you're right about the UK and microscopy supplies - very limited and very pricey as a result I think - it makes me green to see how cheaply and easily microtomes (rotary) may be bought for in the US, not to mentions all-things microscope!

[zzffnn]

Phil,
I am under the impression that Nikon's CFI line of objectives are self-corrected. Many photographers at the other forum use those Nikon CFIs with a Raynox camera lens as tube lens, along with macro camera rigs (they even push the objectives slightly outside of their magnification range). Those macro people use full field of a sensor and always peek into individual pixels (they are more sensitive to CA and SA, if there is any).

[apochronaut]

That's entirely possible , Fan. My information on the Nikon is the haziest of them all. It's primarily based on what Charles Krebs said. Using a Raynox lens, however, doesn't necessarily imply complete correction because most micrographers are looking for complete frame coverage, thus cropping some undesirable peripheral sections out of the image. Corrections can also be applied after the fact. There may be other reasons why certain pieces of equipment are popular, such as brand adherence, having one already, access to published performance data, peer evaluation, inheritance of lemming genes, etc.
The way to tell, is try one out and see.

[apochronaut]

I had a quick look on your behalf, John. I have no idea , how much you intend to spend but the Lomo 1.4 oblique condensers seem to be fairly inexpensive, for what you are getting. There are several and the seller, who I bought from once, some time ago endeavours to keep his shipping fee down. It would be hard for you to get anything better , for anywhere near that price. There is a mm difference but that shouldn't be hard to plug.

[mrsonchus]

I had a quick look on your behalf, John. I have no idea , how much you intend to spend but the Lomo 1.4 oblique condensers seem to be fairly inexpensive, for what you are getting. There are several and the seller, who I bought from once, some time ago endeavours to keep his shipping fee down. It would be hard for you to get anything better , for anywhere near that price. There is a mm difference but that shouldn't be hard to plug.

That sound good Apo' will the 'oblique' condenser be fine for 'normal' bright-field also? (sorry if that's a daft question..)
Would you please pm me the details Apo' and I'll have a look, it sounds very interesting to me - I can easily take care of a 1mm gap.
Thanks for your help.

[apochronaut]

The 1.4 Lomo is an aplanatic condenser. It therefore has the peripheral corrections for astigmatism but not yet the full corrections for chromatic aberration that an achromat condenser would have. Since it is a 1.4, it will easily let any future objectives you may purchase, fully flower and function at peak performance. The only step up from there would be an achromat but the price differential to that stage , could be severe. I have occasionally seen achromats sold on ebay, at ridiculously low prices but those episodes are rare and the result of the seller, not having a clue, what they are selling. I try not to take advantage of those situations, and have notified some sellers as such. I looked for years for a 1.4 N.A. achromat aplanat for a Spencer # 5 microscope. All of a sudden, one showed up for 30.00 in Toronto, a place I visit once a week. The seller turned out to be one of a coop of dumpster divers( what do you call them in the U.K? skip skulkers? skip skuba divers? skip skroungers? who all used the same username. This guy looked like Keith Moon and for good reason; He played drums in the biggest Who tribute band in th world. They even toured the U.K. but he made his living, dumpster diving. I bought a couple of Sonor drums from him a year or so later. Very strange, I must say and he didn't mind selling that condenser for 30.00. I'd really like to know what dumpster that came out of?

The oblique feature has no bearing on BF performance. It is simply, a high N.A., aplanatic condenser, with the added oblique illumination feature.

[Crater Eddie]

Correct. Normal bright field operation is the default mode, if you want oblique you turn a knob one way or the other to achieve the offset. My LOMO L-2-2 has one of these condensers.
CE