Tube Lens Test Final Results

Have questions about the equipment used for macro- or micro- photography? Post those questions in this forum.

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Maxim46
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Post by Maxim46 »

Oh my...
Thank You Robert!!!

lonepal
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Post by lonepal »

Hi Robert;

You are awesome as always!
Regards.
Omer

ChrisR
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Post by ChrisR »

Great piece of work. It's a relief to all that some of the best results come from some of the cheaper options :D. Discovering that using "wrong" orientation and "wrong" distances was remarkable.
Chris R

kaleun96
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Post by kaleun96 »

Really appreciate all the testing you do Robert. Coming across your website a few months ago first got me on this addictive train ride that is >2x macro photography!

I'm having a little trouble understanding how you calculate effective focal length in these tests. I tried searching the forum and google but without much success.

Most of what was on Google related to the difference between front/back focal lengths as calculated from the front/back of the lens element, whereas effective focal length was calculated from the middle...or something along those lines. Though it doesn't seem to me that this definition relates to what we're talking about here, where we're reducing the tube length from the ~160mm (+flange depth) to the 144mm in the tests.

For example I can't work out how the focal length of the DCR-150 then changes from 208mm to 178mm (effective). Any pointers would be greatly appreciated!

ChrisR
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Post by ChrisR »

My take on that - Robert's may well be different:
The FL of a lens is the same, either way round.

What seems to get called back/rear focal length and front focal length are clearances really, nothing to do with focal length!

Some of these lenses are a little bit telephoto, so the measured distances change when you reverse them.

If you use a 208mm FL lens only 178 or whatever from a sensor, it can't focus, right? Well it does if you stick another lens in front of it. The rays between the "tube" lens and the objective aren't parallel, so performance should be all messed up, but somewhat bafflingly, Robert discovered that's the way some of them work best. :smt017 :-k :smt102 :smt119

The objective gives its marked mag on a 200mm tube lens, focused at infinity. It's a bit of an approximation, but if you use the tube lens at say 150mm from the sensor, you get magnification 0.75x what you would have got. Even if the maths is precise, the measurement isn't, unless you know exactly which point to use.
Chris R

RobertOToole
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Post by RobertOToole »

kaleun96 wrote:Really appreciate all the testing you do Robert. Coming across your website a few months ago first got me on this addictive train ride that is >2x macro photography!

I'm having a little trouble understanding how you calculate effective focal length in these tests. I tried searching the forum and google but without much success.
If I pick up a lens that is not labeled and I would like to know the focal length of the lens, I use a simple formula.

I take the lens, add extension so I can close focus, measure the magnification, than add some extension, lets say 28mm, and measure again.

FL = Added Extension / ( New Magnification - Old Magnification )
FL = +28 / ( 1.62 - 1.21 )
FL = 68mm
Most of what was on Google related to the difference between front/back focal lengths as calculated from the front/back of the lens element, whereas effective focal length was calculated from the middle...or something along those lines. Though it doesn't seem to me that this definition relates to what we're talking about here, where we're reducing the tube length from the ~160mm (+flange depth) to the 144mm in the tests.

For example I can't work out how the focal length of the DCR-150 then changes from 208mm to 178mm (effective). Any pointers would be greatly appreciated!
In the tube lens test I was only interested in the effective FL, that is how it effects the magnification. This was done by measuring the magnification twice. First with the system tube lens FL, and again with the shorter FL tube lens, this could be the same lens with short focus.

With this info you can use this formula.

New effective TL focal length = system tube lens focal length * ( New Magnification / Old Magnification )

Give it a try.

Robert

kaleun96
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Post by kaleun96 »

Thanks for the info. So there's no way to determine effective focal length before knowing the magnification? I think that's where I was getting confused.

Adalbert
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Post by Adalbert »

Hello Robert,
Have I interpreted correctly, that the Raynox DCR-150 performs better at 144mm then 208mm (for the crop-cameras as A6300)?
If yes, have you already detected the optimal position of the Raynox for the best quality (CA, etc)?
So, I have already started to collect the values for the DCR 150/250 and Mittys :-)
https://www.photomacrography.net/forum/ ... highlight=
BR, ADi

RobertOToole
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Post by RobertOToole »

kaleun96 wrote:Thanks for the info. So there's no way to determine effective focal length before knowing the magnification? I think that's where I was getting confused.
Not that I know of.

For what I am doing the results are always more important than the specifications.

Robert

RobertOToole
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Post by RobertOToole »

Adalbert wrote:Hello Robert,
Have I interpreted correctly, that the Raynox DCR-150 performs better at 144mm then 208mm (for the crop-cameras as A6300)?

Hi ADi,

Well I like to think so but when I downsize the infinity focus image or upsize the short focus image to match the other and compare the difference the advantage seems to be less but they do seem to look better to my eyes.
If yes, have you already detected the optimal position of the Raynox for the best quality (CA, etc)?
For the 10X?

No I have not tried the Raynox yet on the 10x yet. The performance of ITL200 in reverse seems like it will be very hard to beat.

Best,

Robert
Last edited by RobertOToole on Wed Oct 24, 2018 11:59 am, edited 1 time in total.

Adalbert
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Post by Adalbert »

Hi Robert,

Your test: https://www.closeuphotography.com/tube-lens-test under: “Tube Lenses shorter than 200mm
Thorlabs ITL200
https://www.closeuphotography.com/thorlabs-itl200/
Set-up:
• Short focus, 144 mm extension from sensor to lens
• Reverse mount
• 75mm from tube lens to objective
Image Quality:
• Overall: 9.6 out of 10
• Center: 8.5
• Corner: 8.5
• Chromatic Aberration Control: 10
.
.
.

RAYNOX 208MM (43mm x 0.50 mount)

Also know as: DCR-150, Macroscopic Lens Model M-150, Model CM-2000 1.5X, +4.8 diopter

Closeuphotography.com/raynox-tube-lens
Set-up:
• Short focus, 144 mm extension from sensor to lens
• Normal mount
• 50mm extenstion from tube lens to objective
Image Quality:
• Overall: 9.6 out of 10
• Center: 9.5
• Corner: 10
• Chromatic Aberration Control: 9.5
So, at 144mm is the quality in the corner for Raynox = 10 and for ITL200 = 8.5.
Independent of the typo at ITL, Raynox has achieved the full 10 points in any case.
The image quality in the corner is very important for me, therefore I’m asking for that :-)

BR, ADi
Last edited by Adalbert on Wed Oct 24, 2018 8:38 am, edited 1 time in total.

Lou Jost
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Post by Lou Jost »

Don't forget that you can sometimes get higher corner quality also by increasing the focal length of the the tube lens so that the image circle of good quality gets spread out to the corners. There is a limit to this, as too much expansion gets into empty magnification, as others have pointed out.

Adalbert
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Post by Adalbert »

Hello Lou,
Thank you for the hint but I only try to analyse the results of the test of the given configuration.
As far as I can see the DCR-150 performs better at 144mm than 200mm (infinity focus).
Set-up:
Infinity focus
• Normal mount
• 50mm from tube lens to objective
Image Quality:
• Overall: 8 out of 10
• Center: 8 
• Corner: 8 
• Chromatic Aberration Control: 9
.
.
Set-up:
Short focus, 144 mm extension from sensor to lens
• Normal mount
• 50mm extenstion from tube lens to objective
Image Quality:
• Overall: 9.6 out of 10
• Center: 9.5
• Corner: 10
• Chromatic Aberration Control: 9.5
With the A6300 and Mitty 5x, of course.

BTW, normal mount or reverse?

BR, ADi

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Post by rjlittlefield »

Let me add a cautionary note about generalizing Robert's results.

When the rear lens is focused at infinity, the situation is relatively simple. The objective is being used at its design point. Changing the spacing between objective and tube lens will not affect the magnification, and it will not affect aberrations that originate in the objective. Slight misalignment between the objective and tube lens will have no significant effect on the image. Really the only effect of changing the spacing or alignment of the objective and tube lens is to change aberrations that originate in the tube lens, due to changing the distance from axis where pencils of light enter the tube lens.

When the rear lens is not focused at infinity, the world gets much more complicated. In this case, light rays between the two lenses are not parallel, but instead are converging or diverging. In that configuration, changing the spacing between objective and tube lens will also change magnification. It will also change aberrations that originate in the objective, because the objective is no longer being used at its design point. Misalignment between objective and tube lens becomes a much more potent source of new aberrations that originate in the decentering, not really from either lens alone. All of these effects can vary significantly from one objective to another, and some of them become much more important with larger NA objectives. A non-infinity configuration that works great at 5X NA 0.14 can fail miserably at 50X NA 0.55.

--Rik

Adalbert
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Post by Adalbert »

Hello Rik,
A non-infinity configuration that works great at 5X NA 0.14 can fail miserably at 50X NA 0.55
Yes, and therefore I would like to collect the optimal values determined empirically a tube-lens, microscope-lens and size of the sensor.

BR, ADi

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