Got microscope objective, is tube length important?

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Bearthunder
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Got microscope objective, is tube length important?

Post by Bearthunder »

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

Chris S.
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Post by Chris S. »

Bearthunder, welcome to the forum! :D

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.

Bearthunder
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Post by Bearthunder »

Thank you. :D

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

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

Chris S.
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Post by Chris S. »

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

Bearthunder
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Joined: Thu Mar 29, 2018 3:28 am

Post by Bearthunder »

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. :)

Deanimator
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Post by Deanimator »

Bearthunder wrote:Thank you. :D

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

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

cube-tube
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Post by cube-tube »

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.

rjlittlefield
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Post by rjlittlefield »

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

cube-tube
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Post by cube-tube »

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

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