Hello from Oregon. Questions concerning use of extension tubes.

[Greg Howald]

I have a B660 Amscope microscope. Although it is professional in size, objectives are finite achromatic. I have add a 20mm extension tube under the head of the microscope with an open slot so I can add/remove the polarizing analyzer. I have had good result with it but magnification has changed in value due to the added tube length. It's not 160mm anymore. Tube length is now 180mm. So now for you wizards who have been doing this for a long time...
By what percentage has the magnification increased?
I can't do good science unless I know the answer. Otherwise I'm just playing because I have exceeded industry standards.
I did some math by adding percentages of additional length based upon standard tube length but that is still just a guess on my part. Your thoughts would be appreciated.
Thanks. Greg

[viktor j nilsson]

I would be less concerned with magnification, and more concerned about image quality.

Here's an often cited figure:


I believe it's based on how spherical abstain increase with differences in tube length.

With a 20mm difference in TL, you should start seeing pretty severe spherical aberration already with a 20x 0.4 objective. That's why all intermediate tubes intended for finite microscopes have telan lenses that compensate for the increase in TL.

[viktor j nilsson]

Also, the change in magnification depend on the focal length of the objective, so it will differ between your objectives.

[Hobbyst46]

I have a B660 Amscope microscope. Although it is professional in size, objectives are finite achromatic. I have add a 20mm extension tube under the head of the microscope with an open slot so I can add/remove the polarizing analyzer. I have had good result with it but magnification has changed in value due to the added tube length. It's not 160mm anymore. Tube length is now 180mm. So now for you wizards who have been doing this for a long time...
By what percentage has the magnification increased?
I can't do good science unless I know the answer. Otherwise I'm just playing because I have exceeded industry standards.
I did some math by adding percentages of additional length based upon standard tube length but that is still just a guess on my part. Your thoughts would be appreciated.
Thanks. Greg

The magnification has only increased by 20/250, 8%, but the problem is that the optics are designed for peak performance at 160mm tube length, so the image quality is probably impaired. With a "standard" slide, like a stage micrometer, a comparison of the images with and without the extension tube can qualitatively show the maleffect. I would also check if dust on the filter is made visible in the extended tube configuration.

[Greg Howald]

Many thanks to you folks. I haven't noticed a change in image quality, probably due to old eyes. I greatly appreciate your input. Sounds like I need to rethink my stinkin thinkin.
Greg

[Scarodactyl]

As a side note, for scientific work the nominal magnification of the objective isn't that useful for establishing scale. You'd need to know the field of view, whixh you would probably want to directly measure by imaging something of known scale.

[hans]

Here's an often cited figure...

Interesting figure, if anyone is curious, Google reverse image search finds Rik's Links which points to the original source:
https://doi.org/10.1111/j.1365-2818.1959.tb04454.x

Does not appear to be freely available, unfortunately.

[Greg Howald]

Thanks to all of you for your advice. I have removed the extension tube. Now my calibrations concerning measurements with my camera will continue to be accurate and I'll have more clarity. Thanks a bunch.
Greg

[75RR]

.
One work around would be to place a polarized filter in the head
.

[Greg Howald]

To 75rr

I have placed a polarizing disc in the head of the scope. I do not know if I have an inferior disc or not but it certainly seems to dull the image. I see a stark difference between plain polarized light and bright field. This has been to the point of continually removing the head and putting it back on to switch from polarized light to bright field and vice versa and is the reason I wanted to try the extension tube in the first place. I have certainly not noticed the dull image effect with plain polarized light in the petrographic scope. I think I will have to try a different brand of polarizing sheet.
Thanks, Greg

[75RR]

.
There is an illumination penalty with leaving it in, how troublesome that is depends on how much reserve illumination you have and how often you use polarized light.

I am able to remove mine with the flick of a lever and yet unless I am attempting high power Darkfield I tend to leave it in place.


An infinity system, assuming the tube lens is in the head, will allow the use of an extension below it.

[hans]

I have add a 20mm extension tube under the head of the microscope with an open slot so I can add/remove the polarizing analyzer. I have had good result with it...

I have placed a polarizing disc in the head of the scope. I do not know if I have an inferior disc or not but it certainly seems to dull the image.

Are you using the same polarizer in both cases? Is the polarizer that is causing the image degradation just a piece of plastic, or is it laminated with glass on both sides?

[Greg Howald]

It's plastic.

[hans]

It's plastic.

Is it the same one you used successfully in your 20 mm extension tube?

[Greg Howald]

Yes. It is the same, although I must tell you I have different plastic from a different manufacturer now and am about to do some testing with that and a glass polarizer. It may be morning before I get to it. I've had a busy other things day.

[Greg Howald]

Got to it. Cut a new analyzer and mounted it in the head. Used a new glass polarizer. Looks nice and not blurred in bright field. Thanks for your patience and concern. Because of your help and encouragement this problem is solved and I can rotate the polarizer to change the degree of polarization.
Thanks again.
Greg