Tube length question

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Garry DeLong
Posts: 38
Joined: Mon Apr 04, 2005 4:56 pm
Location: Portland, Oregon USA

Tube length question

Post by Garry DeLong »

In 1999, when I knew nothing about the relative monitary values of microscopes, I purchased a new Leica BF200 student scope from the company that supplied equipment to the lab I worked in. Believe it or not, I paid almost $1000 for it because I did not know better. Back then I'm not sure if Lomo was available and the Chinese scopes were even worse than they are now, so these scientific distributor types had much less serious competition. I am now fiddling with new eyepieces, eyepiece cameras, etc and I realize that when I remove the Leica eypiece (WF 10x) and insert anything else, the tube length is shortened by about 20 mm because most of the length of the Leica eyepiece is simply empty tube, a spacer, if you will. Not knowing much about tube length, I purchased a 20x WF eyepiece and realized the appearence of the microscope was much stubbier. The shorter eyepiece doesn't seem to increase spherical or chromatic abberation even though the objectives are marked "160.". I have only seen discussions of problems when the tube length is to long --170mm vs 160mm--but still, shouldn't my image be really messed up? By the way, people in medical labs that use microscopes all the time often know very little about the instruments they are using or their relative value. A discussion of tube length would utterly perplex them and I doubt that they have ever heard of Lomo, but that's the subject of another topic.
Garry DeLong

Planapo
Posts: 15
Joined: Tue Mar 22, 2005 2:11 pm

Clear as mud?

Post by Planapo »

Hello Gary, I don't think you are the only one to be confused by tube lengths.

There are two kinds of tube length: the optical tube length (OTL) which is the length of path the light thinks it is travelling, and the mechanical tube length (MTL) which you can measure as the physical distance between the objective's and the eyepice's focal planes (which may be internal or external according to design).

Changing the MTL by moving an eyepiece out of its hole, you also change the OTL.

Changing the OTL by inserting a lens or even a filter in the path, you don't change the MTL.

The OTL is the important one and because some eyepiece designs have their focal plane inside them, and others have it some distance below, changing eyepieces from one maker to another, or from one style (e.g. Ramsden) to another (e.g. Kellner) should not have any effect on the OTL of the system.

It is as well to remember that the whole point of specifying tube length is to get the objective working at the optimum distance from the specimen, at which the corrections are at their best... deviating up or down from this distance will deteriorate the image. This optimum position is called the "conventional condition" and as long as the objective is at this position, you can do anything you want above it.

For example, you could insert a negative lens in the tube, and put the eyepiece much higher... but since the objective has not been informed of this, and has not been moved, the correction is still optimal... and the light, in passing through the negative lens, thinks it is travelling a shorter distance than the actual new MTL.

The big 160mm Zeisses are an example... they have all sorts of optics in the image path, and the MTL can be up to 300mm, but the OTL remains at 160mm whatever you do.

So, if you were to focus your Leitz with original-equipment Leitz eyepieces, and assuming that the OTL in this case is exactly 160, then change the eyepieces for another make, if you don't have to re-focus the microscope then the new eyepieces are fully compatible, whatever their appearance in terms of empty tube etc.

If you do have to re-focus, then you are deviating from optimal correction, the image will be affected (but probably not by much).

Your procedure would be to focus (using a 40x) on your specimen, leave the focus as it is and insert the new eyepieces. Image in focus? Fine. If not, pull the eyepiece out a bit. If you can get focus this way, great, you need to make up some spacers to keep the new eyepieces spaced... o-rings are good for this. Can't get focus? This would mean that the new eyepieces sit too high, and in theory you would have to machine some metal off the seats to get them sitting lower.

My personal opinion is that a mismatch between 160 and 170 is not particularly serious. I know many people (I have done as well) who chop and change between 160 and 170 objectives on a Zeiss or a Leitz, doesn't seem to do much harm at all. And the objectives which would be most affected (high-dry) have a collar so you can correct the image anyway.

Having written all this, and I have to stop because I am late for the market, you will find it all much better explained here.

http://microscopy.fsu.edu/primer/anatom ... ength.html

Enough said.

Peter

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