Question on tube length and focus

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Jmurphy18
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Location: Newport News va
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Question on tube length and focus

Post by Jmurphy18 »

Good Morning
I have a question on tube lens length. I have posted before and a couple of you folks have answered and thank you. I did read the whole thread Rik sent me the link on at least twice.
I am a bit puzzled and I do have a couple of questions.

Here is what I have for my Raynox 150:
Tube length published is 208.3mm
1) Camera Nikon Z6 II. Flange length 16mm
2) Nikon FTZ adaptor 30.5mm.
Combining #1 & #2 yields the normal Nikon flange distance of 46.5mm
208.3 - 46.5 = 161.8mm.

ThorLabs tubes:
My caliper is only 150mm (6”) so I have to split the tubes to measure the length.
#1. 59.8mm includes RAF SM2 - F mount
#2. 100.5mm includes SM2A24 & 52 - 43 step down ring & land on Raynox
100.5 + 59.8 = 160.3mm
This puts me 1.5mm short of actual. I have some pictures of focus I took at what I “thought" was focused. Note I am suffering some cataract issues that a to soon to come out yet ~ Feb 2022 and I am having some issues. 1st set is take down my street ~ 1/4 mile from camera. I tried for the far house and missed I remember Rik has said to Ultra Gena longer and I went to Yorktown and the bridge is > 1mile and the VIMS building is ~ 1.25mi. Conditions were 4PM, bright and sunny with sun 90 deg to camera so not the best. I was eager to try the focus. I was getting a lot of glare on the rear screen and hard to see in the viewfinder.

Here is a shot down the street
_Z622244.jpg
shot from the center area and you can see I blew the focus. I got the sign not the house behind. I Thought I got the house. 200%
_Z622244-1.jpg
This is from Yorktown
_Z622245-1.jpg
center area ~ 200%
_Z622245-1-2.jpg
across the river at VIMS
_Z622246.jpg
center area
_Z622246-1.jpg
How far a distance should I be focusing the lens? I need to go out and do this again as I blew the focus. I am going to bring my wife as backup to help
should I be at the 161.3 distance?

Thanks so much!
John
JohnM

JKT
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Re: Question on tube length and focus

Post by JKT »

Theoretically you should use moon, but in practice it doesn't matter as you can easily calculate the correction if you know the focal length and the distance you had it focused. The correction is db = f - 1 / (1 / f - 1/a), where f is the focal length and a is the distance to the target you used. Just use same units for f and a and the result is in the same units as well.

Chris S.
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Re: Question on tube length and focus

Post by Chris S. »

John,

For your purposes, you can treat all the images you showed as demonstrating that your tube lens is at an appropriate distance from your sensor. Even if your assembly is theoretically 1.5mm too short, don't worry about it. What you have right now is good to go.

Having the tube lens at the precise point of infinity focus is not important for most photomacrography, particularly at the 10x magnification you've named as your goal in another thread. In fact, there are good reasons that photomacrographers sometimes deviate intentionally from infinity focus.

The numerical aperture (NA) of the objective determines how much departure from infinity it can tolerate. The lower the NA, the more departure you can get away with. For this purpose, most 10x objectives have a conveniently low NA (between 0.25 and 0.30). At these NA's, one can move the tube lens quite far from infinity focus without sacrificing image quality. If you eventually move to higher magnifications, the NA of the objective will increase, and tolerance of departure from infinity will decrease. But even then, 1.5mm should not matter for any objective one would place on a macro rig.

When I first built my tube lens, I was very careful to set it to infinity focus. Later, I chose to shorten the tubes by a couple of millimeters, as this allowed me to swap some parts and increase structural rigidity. (I got rid of an adjustable tube that permitted slight wobble and and replaced it with a solid tube that did not.) I use it with a full set of Mitutoyo objectives (from 2x/0.06 to 100x/0.70), and none of them seem bothered by those missing millimeters.

Here is a graph that is probably illustrative of the amount one can depart from perfect infinity tube lens focus, based on objective numerical aperture. The graph depicts data from varied tube lengths with finite objectives, not varied tube lens focus with infinite objectives. But these two paradigms share a lot of physics; if the study were repeated today with infinite objectives and tube lens' focus departure from infinity, the graph would probably much the same.

And in addition to that graph and my own experience, what I've written above has been demonstrated by many photomacrographers in our community. Shooting pictures of the moon may be a theoretically hypercareful way to focus one's tube lens, but this level of care in tube lens focus has been demonstrably unimportant in real-world photomacrography.

So forget about focus and move on--you've already nailed focus for photomacrography, in that your distance images convincingly demonstrate that you've already built an effective converging lens assembly that is quite ready to play well with a 10x infinite objective, and even objectives of much higher NA.

By the way, "converging lens" is a term that I and some others prefer to the term "tube lens," unless the lens under discussion was officially manufactured for placement in the lens tube of an infinite microscope. Infinite objectives produce parallel light beams that must be converged on sensor by a second lens. Our community has discovered a wide variety of lenses that will perform this function of convergence--hence my calling this second lens a converging lens. Many of these good lenses were never intended for the purpose of converging infinite optics. To me and some others, it communicates most clearly to reserve the term "tube lens" for converging lenses designed specifically for use in a microscope lens tube, and use "converging lens," for the much larger body of all lenses than perform this function well. Some lenses specifically designed for converging within lens tubes have particular properties--which I'd call true "tube lenses"--have particular properties that sometimes warrant discussion. Hence the value in maintaining this distinction.

--Chris S.

Jmurphy18
Posts: 22
Joined: Sun Jun 13, 2021 12:07 pm
Location: Newport News va
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Re: Question on tube length and focus

Post by Jmurphy18 »

JTK & Chris S: Thank you both for taking the time to answer my question!! Migrains stopped me from doing much this weekend.

Thanks for the info Chris and I will lock down the size here and run with the settings on the "Converging lens". Yes I do plan on purchasing a Nikon 10x objective new month when I get paid. Retired and only get paid once a month now. Just doing a roof, skylights and other items blew any items I would have wanted to purchase for a while. :o With your info I added the 1/2" or 13.4mm tube back into the system. With this I have ~ 1mm of play now on the adjustable and I have that locked both inner and outer. Gives me an odd size, flange is 13.5 + 1 + 2.7 for locking ring or 17.2 to replace with a solid tube. So with only 1mm of movement and 99% of the thread internal I figure I should be good for now. When I go to larger magnification I will need to get a solid tube though as you recommend. FYI this ThorLabs adj tube is pretty solid. I do not detect a lot of movement. However my dial indicator is now dead 35 years old and just quit so I cannot get a reading. Shaft moves but no dial action :roll: I do have 3 ThorLabs clamps on a 300mm rail holing this assembly plus a linear rail supporting the rear that Ray Parkhurst helped me with.
I am slowly working to make my station as stable as I can on a very limited budget.

Thanks for the links and your setup looks nice ! I read the charts and I am trying to understand. I might be an Engineer but optics is not my area (ME). But I did see what you were doing to I even got my 4x (AmScope) to work on this long tube w/o Raynox. NA .1. Just took a chart shot of a focus chart for a laugh.

Thanks again for the info and I will proceed on with this tube setup and converging lens and I will keep my numbers for my finite lens also so I can use my 4x. My RAF mount just came today so I should be good when I can order the objective.
JohnM

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