Long distance.

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Greg Howald
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Joined: Tue Oct 20, 2020 6:44 am

Long distance.

#1 Post by Greg Howald » Wed Jul 14, 2021 5:35 pm

I know what I think I did but I don't really know What I did. How about help with an explanation.

I placed an O-ring having a thickness of 5/32 of an inch on my 10x objective and mounted it into the scope. This increased to focal distance between the objective and the specimen from 4.5mm to 6.3mm. Is this in fact a means of increasing working distance or is my image (which looks fine to me) badly distorted?

hans
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Re: Long distance.

#2 Post by hans » Wed Jul 14, 2021 9:31 pm

I would double-check the working distance measurement. Assuming a 160 mm finite microscope (adding a spacer under an infinity-corrected objective shouldn't significantly change the working distance) then I think the 1.8 mm increase is much larger than expected and also going in the wrong direction for a 4 mm increase in tube length. Increasing the tube length should move the specimen focal plane slightly closer to the objective.

Greg Howald
Posts: 1185
Joined: Tue Oct 20, 2020 6:44 am

Re: Long distance.

#3 Post by Greg Howald » Thu Jul 15, 2021 2:04 am

It is an Amscope finite plan 10x objective and the stage had to be lowered significantly to regain focus. The distance increased. I cheat a lot but I did not expect the results I got. Hmmm.😃
Greg

hans
Posts: 987
Joined: Thu May 28, 2020 11:10 pm
Location: Southern California

Re: Long distance.

#4 Post by hans » Thu Jul 15, 2021 3:32 am

To make sure I follow: The 10X is normally 4.5 mm from the specimen before you lower it 4.0 mm (5/32 inch) by adding the O-ring. Then to regain focus after adding the O-ring you have to lower the stage by 5.8 mm (1.8 mm more than you lowered the objective) meaning the distance between the objective and specimen has increased from 4.5 to 6.3 mm?

You can also try lifting one eyepiece while looking through it, which increases tube length while leaving the specimen and objective in the same position, so may be easier to keep track of what is going on.

Greg Howald
Posts: 1185
Joined: Tue Oct 20, 2020 6:44 am

Re: Long distance.

#5 Post by Greg Howald » Thu Jul 15, 2021 3:55 am

Thanks for that tip. Remember that what I am doing with this experiment probably consists of more play than anything else. 😆 Greg

hans
Posts: 987
Joined: Thu May 28, 2020 11:10 pm
Location: Southern California

Re: Long distance.

#6 Post by hans » Thu Jul 15, 2021 6:31 am

Agree it is interesting to play around with the optics and attempt to explain the results. I did some simple experiments like this while trying to understand the common formulas related to camera setup.

For a change in tube length the resulting change in working distance can be estimated by pretending the objective is a simple lens and using the thin lens equation. (Linked page has a handy calculator in the first section.) For example say 18 mm object distance and 180 mm image distance which is close to a 10X 45 mm parfocal DIN objective with 195 mm total distance object to image. To find the change in working distance when lifting an eyepiece by 4 mm using the calculator on the HyperPhysics page:
  • Enter 1.8 and 18 cm for the object and image distances then click the "1/f" part of the big red equation image to calculate a focal length. (Result 16.364 cm.)
  • Edit image distance to 18.4 cm then click "1/o" to calculate a new object distance while keeping the same focal length. (Result 1.7961 cm, or a 39 um decrease in working distance.)
A bit tricky to do the measurement accurately by eye since the eye's own ability to focus affects the result. (Camera would be more repeatable but is not on my microscope at the moment.) But trying a few times I get results in the 30-50 um range. (I am focusing on the label on a prepared slide, no cover glass, and going by the graduations on the fine focus knob. Cover glass complicates the calculation, although probably not too significant with the 10X objective.)

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