Infinity Optics and The Amateur

A Brief insight

By Paul James

 

If you screw in an infinity objective into your 160/170 mm TL microscope's turret and go through the motions of examining a specimen, you'll find that the image's quality is not quite what it should be. What has happened is that the objective ends up focussed a little away from its designated working distance so as to make the converging pencil beam of light focus at the plane of the eyepiece, which is about 160/170 mm up the tube. In its proper setting on an infinity 'scope, the objective, when carefully focussed at its correct working distance, will project its light as a parallel pencil beam which then passes through an optical assembly called the 'tube lens' which converges this parallel beam to focus appropriately in front of the eyepiece.

So in a nutshell the infinity system differs in one subtle way to the 160/170mm TL system in that the emerging rays from the objective are parallel and require the tube lens to focus it for the inspection of the eyepiece : In the standard 160/170mm 'scope the light emerging from the objective converges all the way to the eyepiece with no intermediary optics***. Ignoring some of the finer points the basic reason for the parallel beam from infinity objectives is to allow the use of intermediary optics within the space between the objective and tube lens, which if installed in a standard 160/170mm TL 'scope would cause anything from mild to severe spherical aberrations in the imagery. So theoretically the objective to tube lens distance in an infinity 'scope can be almost anything, BUT the tube lens to eyepiece distance is fixed, and designated as such : each manufacturer having slightly differing tube lens focal length, though they are around the 150 - 250mm mark. You can gather some idea of the system when you imagine that a refractor telescope's objective as being the 'tube' lens focussing the parallel light from a star onto the eyepiece. Moving the 'scope further away from the star has no effect of course, and so the designers of the infinity microscope have plenty of room for internal intermediary optical devices to install without any percievable effect on imaging performance.

Practicalities

Of course you'll either opt for a ready made infinity system or not, and even if you are interested and would like to change to the infinity system from the standard 160/170mmTL form there are some subtle details concerning optical performance and requirements that are inseparable. The main ones concern the manufacturers option of considering the tube lens's purpose as not only a focussing device for the parallel pencil beam but also an aberration correction device in the optical train. So basically their remit might be to share the corrective properties of image making between the objective and tube lens. The upshot of this is that not all infinity objectives will work to their best in all infinity stands. All infinity stands however will have their eyepiece to tube length distances correctly installed, but unless you know whether a particular manufacturer uses the tube lens for multi corrective purposes, you cannot expect to install any maker's objective and expect pristine results.

The Tube Lens Is Not New***

Thus the inherent advantage of the 'finite' or traditional 160/170mm system is that any objective from any maker will provide respectable imaging. But don't imagine that all these 'scopes have unhindered air space between the back of the objective and eyepiece, far from it. Leitz and Zeiss have invariably used intermediary correctors or tube lenses to either flatten the fields and or modify the TL . So the notion of a tube lens or intermediary optic is not a new one at all, as they are usually found inside many conventional 160/170mm TL binocular instruments, where the longer light pathway through the binocular head was compensated for by a tube lens making for continuous compatibility with the ubiquitous 160/170mm TL objective.

Some of the older Leitz 170mm TL stands have a tube lens ensemble of 170/223mm sited above the objective holder or an infinity/223mm optic in an infinity system. The latter figure, which here is 223mm is the intrinsic TL distance of that specific stand. The 170mm figure refers to the objective type of course, so substituting this optic for a tube lens for an infinity/223mm version, also converts the stand to an infinity microscope or visa versa.

Some internal tube lenses are not user removable. Those that have are usually devised as such to provide a multi purpose stand which the manufacturers or users can easily and speedily change. More lately infinity systems make up the vast majority of professional stands as there is no provision for the older 160/170mm optics anymore, or more accurately only for the amateur, without whom the ubiquitous 160/170mm objective would have disappeared from view.

Conclusive thoughts

So it's not quite as clear cut as first imagined, as both binocular systems require intermediary optics or tube lenses. You may chance upon an older stand and wonder if it is an infinity 'scope, but the chances of identifying its stabling is never easy in some cases. If you therefore find yourself inspecting a stand of an unknown system there are fundamentally 2 clues to its indentity. The obvious one is to look for the infinity symbol on the objectives : the figure eight lying horizontally : but if they are absent you might like to see if the tube lens, which should be housed above the objective holder, has any identifying figures on it. Age is not an entirely accurate way of identification either, since the earliest infinity 'scopes were made a long time back by Reichert.

Since the 160/170mm TL objective is still very much alive and kicking as it were, the 'finite' system for the majority of amateurs heralds a few more years of use before many of their stands are consigned to the curio category or book end division. Until that era dawns the infinity system will itself remain a curiosity to many. I do think however that the finite system will continue to be produced, but frankly I can't imagine that the quality will ever be top drawer. That category remains in the professional instrument making bracket, and eventually many of these infinity 'scopes will gravitate to the amateur market as did the top end finite 'scopes in the latter end of the 1900's.

Bottom line is that either system can produce first rate imaging all things being equal, with the advantage going strongly towards the infinity system when internal accessories like splitting prisms, DIC components etc. immediately between the objective and tube lens comes into every day useage.


All comments welcome by the author Paul James

 

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Published in the July 2006 edition of Micscape.

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