NA and tube length

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mawyatt
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Post by mawyatt »

JH wrote:I use a microscope. The distance between the microscope lens and the tube lens I fixed. The distance between the tube lens and the sensor is set so the tube lens is focused at infinity (in this case a three top at my neighbours). I focus the microscope on the butterfly scale and take a picture – the second picture in the first row in the series above.

If I increase the distance between the tube lens and the sensor the lens focus closer. If I decrease the distance the lens focus further away. When I did this the picture was totally out of focus and I needed to refocus the microscope before I could take a new picture.

My point was that a few mm wrong in tube length (deviation from infinity) at NA 0.6 could be OK at least in the center of the picture.

[Edit: I am sorry, I should have mentioned in the beginning that the 150mm apo gerogon lens that I used cannot focus by itself, it needs to be used on bellows or tubes]

Best regards
Jörgen Hellberg
Jorgen,

Thank you for doing this, the completely confirms what I had thought.

Best,

Mike

mjkzz
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Post by mjkzz »

If I increase the distance between the tube lens and the sensor the lens focus closer. If I decrease the distance the lens focus further away. When I did this the picture was totally out of focus and I needed to refocus the microscope before I could take a new picture.
OK, I am too "focused" on simple doublet type of tube lens and your 150mm apo gerogon is a bit too complicated and your test might not be the same (as simple doublet chromatic type tube lens), but has same effect and that is what it counts. Thanks for results.

Being stubborn, I still think people are paying multiple times of money to get the best performance out of what they paid for, deviating from design constraints will lead some kind of issue, particularly, forming an image using rays that are not optimum.

I do not have any good objectives, so I can not do any tests to show :-)

mawyatt
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Post by mawyatt »

microman wrote:Btw, how long distance do i need to focus on with the tube lens to set inf. ?
From my yard the farthest object i can see is around 600m away. If i need to focus farther i need to take my gear down the street. Then i can focus on somthing a couple of km away. Will it make a difference you think ?

I used a star or planet, but as shown getting set at "infinity" isn't necessary for the IQ, a few mm off won't make a difference.

Best,

Mike

mjkzz
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Post by mjkzz »

I used a star or planet, but as shown getting set at "infinity" isn't necessary for the IQ, a few mm off won't make a difference.
Then why do they even specify a focal length requirement for tube lens? Just to make sure at that focal length, it will have the magnification specified?

OK, I take that.

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Post by ChrisR »

:smt023
Chris R

mawyatt
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Post by mawyatt »

mjkzz wrote:
I used a star or planet, but as shown getting set at "infinity" isn't necessary for the IQ, a few mm off won't make a difference.
Then why do they even specify a focal length requirement for tube lens? Just to make sure at that focal length, it will have the magnification specified?

OK, I take that.
Peter,

Probably, and to meet the specified WD. If the tube length is different, the WD will move and magnification will change. Not a big deal for the kind of stacking I do, and suspect many others do as well, since precise magnification isn't required and stacking (moving the camera/lens combo) moves the in-focus position across the object anyway.

For machine use, where magnification may be critical, then the tube length becomes critical as well.

Anyway, this is what I thought was the case in the other thread and thanks to Jorgen for confirming so.

Best,

Mike

microman
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Post by microman »

ChrisR wrote:
microman wrote:Btw, how long distance do i need to focus on with the tube lens to set inf. ?
From my yard the farthest object i can see is around 600m away. If i need to focus farther i need to take my gear down the street. Then i can focus on somthing a couple of km away. Will it make a difference you think ?
You can work that out, from the
thin lens equation ,
1/(image distance) + 1/(object distance) = 1/fl

so for example
If you take a 200mm thin lens, and put it 205mm from the sensor, it'll focus at 8200mm, = a car and a half away!*
So a far off power line would be fine, but as I found, my neighbour's TV aerial wasn't really far enough.

* That's the same as taking a 200mm camera lens, set the focus ring to infinity, then put it on a 5mm extension tube, on the camera.
Thanks for the explanation :)

mjkzz
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Post by mjkzz »

Thanks Mike,

I actually did a quick test, with DCR-150 on bellow, 1 mm each way (plus or minus, so total 2mm), I still get pretty good focus on something about 2km away. So a few mm is probably not a big deal at all. The DCR-250 is a bit restrictive, maybe 0.5mm each way from "dead" on point.

Yes agreed, magnification, particularly for measurement with computer vision, but then again, each individual system can be calibrated.

Regards
Peter

Lou Jost
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Post by Lou Jost »

Thanks for this test Jorgen! Very helpful.

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