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Got microscope objective, is tube length important?

 
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Bearthunder



Joined: 29 Mar 2018
Posts: 4

PostPosted: Wed May 29, 2019 3:28 pm    Post subject: Got microscope objective, is tube length important? Reply with quote

I bought a couple of cheap microscope objectives on ebay, a 4x and a 10x, both are finite marked with 160. So in my case 160 is the tube length I should use. But is this the distance as measured from the mounting flange of the objective to the camera sensor? Getting correct distance isn't a problem as I have both bellows and extension tubes. But how important is this 160 mm distance? Is it just that at this distance you get the right magnigication, or does it also have something to do with the image quality? Or are there other things I don't know about?

Clueless in Stavanger, Norway
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Chris S.
Site Admin


Joined: 05 Apr 2009
Posts: 3279
Location: Ohio, USA

PostPosted: Wed May 29, 2019 7:33 pm    Post subject: Reply with quote

Bearthunder, welcome to the forum! Very Happy

Objectives with a small numerical aperture (NA), such as 4x and 10x objectives, are tolerant of widely-variant tube lengths. With higher NA objectives, tube length eventually does become problematic. There is an old, oft-posted graphic that depicts where problems occur—if you’re interested, someone here will probably know where to find it.

I’m not sure exactly what you mean by the mounting flange of the objective. You want to measure from what is often called the “shoulder” of the objective—the place where the threads and objective body join. And yes, you measure from this point to the camera sensor.

Also, be aware that the 160mm marking on your finite objectives really means that you want to place the objective shoulder 150mm from the sensor. An extra 10mm has been included in the specification to allow for the eyepiece of the microscope the objectives were intended to be used with.

--Chris S.
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Bearthunder



Joined: 29 Mar 2018
Posts: 4

PostPosted: Thu May 30, 2019 4:20 pm    Post subject: Reply with quote

Thank you. Very Happy

Ah, excellent! Yupp, shoulder is what I meant. OK, so 150mm it is then! Thank you. I'm less clueless now. Razz

Hoping to be able to buy a WeMacro focus stacker this fall, manual focus stacking with a microscope objective wasn't easy. Rolling Eyes
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Chris S.
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Joined: 05 Apr 2009
Posts: 3279
Location: Ohio, USA

PostPosted: Thu May 30, 2019 7:50 pm    Post subject: Reply with quote

Bearthunder wrote:
Hoping to be able to buy a WeMacro focus stacker this fall, manual focus stacking with a microscope objective wasn't easy. Rolling Eyes

How well I know. I once shot a 450-image stack manually, only to decide that I should have used a polarizer. Redid the stack, only to decide that I should have cross-polarized. After the third iteration, I decided that the motorization project that had been sitting on my worktable was going to get done before I shot anything else.

--Chris S.
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Bearthunder



Joined: 29 Mar 2018
Posts: 4

PostPosted: Sat Jun 01, 2019 10:19 am    Post subject: Reply with quote

Chris S. wrote:

How well I know. I once shot a 450-image stack manually, only to decide that I should have used a polarizer. Redid the stack, only to decide that I should have cross-polarized. After the third iteration, I decided that the motorization project that had been sitting on my worktable was going to get done before I shot anything else.


A stack of 450 images manually? Holy cow! The most I've done is 70! But I'm really looking forward to do more precise stacks with higher magnifications. There are so much I want to see in more detail. Smile
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Deanimator



Joined: 23 Oct 2012
Posts: 781
Location: Rocky River, Ohio, U.S.A.

PostPosted: Sat Jun 01, 2019 4:10 pm    Post subject: Reply with quote

Bearthunder wrote:
Thank you. Very Happy

Ah, excellent! Yupp, shoulder is what I meant. OK, so 150mm it is then! Thank you. I'm less clueless now. Razz

Hoping to be able to buy a WeMacro focus stacker this fall, manual focus stacking with a microscope objective wasn't easy. Rolling Eyes


And the cheaper the manual rail is, the more painful it is.

I tried it a bit with a cheap Chinese rail from eBay and it was literally impossible to achieve consistent step sizes. And that was at 1:1 or less.

You will REALLY like the Wemacro rail. The company is a pleasure to do business with.
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cube-tube



Joined: 10 Oct 2017
Posts: 105
Location: Durham, NC

PostPosted: Mon Jun 03, 2019 6:03 am    Post subject: Reply with quote

Chris S. wrote:
With higher NA objectives, tube length eventually does become problematic.


Is this also true for infinity space, for infinity-corrected objectives? I have seen conflicting information about the importance of optimizing the distance between the objective and the tube lens.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19980
Location: Richland, Washington State, USA

PostPosted: Mon Jun 03, 2019 11:18 am    Post subject: Reply with quote

cube-tube wrote:
Chris S. wrote:
With higher NA objectives, tube length eventually does become problematic.

Is this also true for infinity space, for infinity-corrected objectives? I have seen conflicting information about the importance of optimizing the distance between the objective and the tube lens.

It's a different game with infinites.

With finites, using the wrong tube length introduces aberration by changing the distance from subject to objective. This aberration increases rapidly with larger NA and degrades the image even in the center of the field.

With infinites, used as designed with the rear lens focused at infinity, the objective's focus point does not change if you change the focal length of the tube lens. In this case there's no added aberration at the objective. The only effect of changing separation between the objective and tube lens is to tweak aberrations at the tube lens, by changing the distance off-center for rays at any particular angle. The image at center of field will not change, but off-center images can.

However, if you focus the tube lens away from infinity, say by adjusting bellows length behind it, then you do drag the objective away from its designed focus point, and in that case you add aberrations at the objective that are very much like the finite case.

--Rik
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cube-tube



Joined: 10 Oct 2017
Posts: 105
Location: Durham, NC

PostPosted: Mon Jun 03, 2019 1:10 pm    Post subject: Reply with quote

Thanks for clarifying that for me, Rik. That's all good to know.
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