Focusing by Changing the Tube Length

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microb
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Focusing by Changing the Tube Length

#1 Post by microb » Fri Jun 14, 2019 11:02 pm

To get a sample in focus, typically either the stage moves up and down, or the whole head with epi and trinocular are moved up and down. The length between the objective and tube lens is not altered.

But could the objective be moved by itself in an infinity set up? Thor Labs sells a motorized z-axis for the objective. So since infinity focus between the tube lens and objective is supposed to be collimated light, the length shouldn't make a difference. Of course, nothing really has perfect parallel rays, so a possible visual shifts camera pixels with a change in height could happen and might require calibration so the shift could be handled in image processing. But maybe a 10mm z-axis difference does not cause a noticeable difference, so no calibration is needed.

The idea is to change distance B shown in the diagram below to get the sample in focus.
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zzffnn
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Re: Focusing by Changing the Tube Length

#2 Post by zzffnn » Fri Jun 14, 2019 11:44 pm

Focusing by changing tube length is not ideal at higher NA. Once you go over NA 0.6 or so, very few milometers count.

Also you probably cannot do focus stacking easily, when your magnification changes during focus.

microb
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Re: Focusing by Changing the Tube Length

#3 Post by microb » Sat Jun 15, 2019 12:30 am

zzffnn wrote:Focusing by changing tube length is not ideal at higher NA. Once you go over NA 0.6 or so, very few milometers count.
I don't understand the very few millimeters part.
zzffnn wrote:Also you probably cannot do focus stacking easily, when your magnification changes during focus.
A very tiny change might be noticeable since the rays may not be perfectly parallel, so if that tiny change is the magnification changes mentioned, then yes that would require some calibrated scaling of the image to image stack. I'm trying to figure out if that is a real issue or not. Thor Labs sells a cerna set up with a z-motored objective holder below a static mounted epi. I asked them about it. The only other usage maybe is automating parafocal changes. But buy Thor I would assume would mean spending the money for matching objectives.

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Re: Focusing by Changing the Tube Length

#4 Post by zzffnn » Sat Jun 15, 2019 5:50 am

Sorry for my typo, I meant "every few milimeters count".

Scarodactyl
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Re: Focusing by Changing the Tube Length

#5 Post by Scarodactyl » Sat Jun 15, 2019 6:12 am

Some stereo microscopes and macroscopes (like the Leica z16, some navitar lenses) use positioning of the front element/objective for fine focusing. I don't know how well it would work for stacking since the magnofication will be changing as well. Probably good enough for a stereo or macroscope setup, maybe kot for extra fine work.

abednego1995
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Re: Focusing by Changing the Tube Length

#6 Post by abednego1995 » Sat Jun 15, 2019 9:01 am

That's exactly how modern inverted microscopes work.
Just shift the revolver plus objectives in relation to the specimen.

As long as the objective exit pupil is within accepting range of the tube lens entrance pupil, the separation between them does nothing with magnification.

Cheers,
John

microb
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Re: Focusing by Changing the Tube Length

#7 Post by microb » Sat Jun 15, 2019 8:07 pm

abednego1995 wrote:That's exactly how modern inverted microscopes work.
Just shift the revolver plus objectives in relation to the specimen.

As long as the objective exit pupil is within accepting range of the tube lens entrance pupil, the separation between them does nothing with magnification.
Thanks. That really helps knowing that example.

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