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Olympus auto focus bracketing with afocal 160TL objectives

 
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zzffnn



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PostPosted: Tue Feb 06, 2018 5:54 pm    Post subject: Olympus auto focus bracketing with afocal 160TL objectives Reply with quote

I already have a collection of no-cover LWD 160TL objectives that go all the way to 40x NA 0.55, completed with their respective compensating eyepieces.

So I wonder, why not use my Olympus E-M10 II's automatic focus bracketing and drive my Sigma 30mm F/2.8 lens, which is tightly connected to compensating eyepiece + 160 mm (flocked M42) tube + objective.

Is there any potential optical issues with the above rig, in theory? Or is the above rig that much worse than an infinity rig, for the same cost?

I am assuming the reason why people are not doing afocal focus bracketing is because:

1) infinity objectives are mechanically easier to connect, since no eyepiece is required and automatic camera lens can work as tube lens; and
2) modern infinity objectives generally provide better image than older 160mm tube objectives.

I can overcome 1) by centering and holding compensating eyepiece inside flocked M42mm tubes, with plastic set screws. My Sigma 30mm lens costs very little ($100 or so) in used market and it functions well in automatic focus bracketing (and it connects easily to M42 tubes via its front threads and adapter rings). That way I don't need to use expensive auto m4/3 tube lens of around 200mm.

I am willing to take 2) as a compromise.

I just don't want to spend that much for infinity objectives just for stacking, since I already have good working 160TL LWD metallurgical objectives and compensating eyepieces.

Please feel free to talk me out off my current thinking. Any comment is appreciated. Thank you.
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enricosavazzi



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PostPosted: Tue Feb 06, 2018 11:41 pm    Post subject: Reply with quote

There is nothing "magic" about infinity systems, and the only clear advantage they provide over finite ones is that additional optics can be added at the rear of the objective without changing the optical properties like magnification and focus. Infinity systems usually work better than finite ones because they are newer and better built, not because of any intrinsic advantage.

Both types (infinite and finite) may or may not require compensating eyepiece, but compensating eyepieces are less frequently required in modern systems. This, in turns, means that a higher number of infinity objectives require no compensating eyepieces.

Since my Zeiss microscope is currently disassembled and stored, I cannot test whether focus bracketing with the camera lens in an afocal setting produces a set of stackable images. I don't see any reason why focus bracketing at the camera lens should behave differently in finite vs. infinite systems, but some eyepieces might be more sensitive than others to coupling with a camera lens not focused at infinity. I guess one has to try to see how it works in practice.

I verified that the Sigma 30 mm f/2.8 (smooth version) does focus bracket on the E-M1 Mark II.
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Pau
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PostPosted: Wed Feb 07, 2018 3:47 am    Post subject: Reply with quote

Quote:
So I wonder, why not use my Olympus E-M10 II's automatic focus bracketing and drive my Sigma 30mm F/2.8 lens, which is tightly connected to compensating eyepiece + 160 mm (flocked M42) tube + objective.

Is there any potential optical issues with the above rig, in theory?


I assume that your idea is to maintain the camera lens to eyepiece distance fixed so focus bracketing will alter the eyepiece-objective distance if the camera lens extends during focusing.
This will work but not without issues as the objectives and eyepieces are designed for a fixed distance. The effect will be some change in magnification and spherical aberration. The first effect can be compensated by the stacking software like with ordinary lenses, the SA will be low, even not noticeable, with low magnification/NA objectives but can kill resolution with high magnification/NA ones.

I think that counterintuitively you will need smaller steps with low magnification objectives than with higher magnification ones (not sure)

As Enrico suggests it will be equivalent to forcing infinite objectives to rear focus other than infinite

If your camera lens has internal focusing the effect will be equivalent to change the lens to eyepiece distance, it could change geometry (flatness of field and distortion) and maybe induce other aberrations, vignette and uneven illumination.

But, this is as you say in theory, having the parts at hand, why not to test it yourself and tell us?
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zzffnn



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PostPosted: Wed Feb 07, 2018 5:55 am    Post subject: Reply with quote

Pau,

I think Okympus in-camera auto focus bracketing changes distance between lens elements.

So in effect, if we consider the lens a single simple lens, then we are changing distance from lens to eyepiece (and sensor to lens distance), in amount of a few millimeters.

My Sigma 30mm lens focuses internally, if I remember correctly, so sample to objective distance should not change. For an external focusing lens, sample to objective distance may change as well.

Eyepiece to objective distance should not change.

I do have most parts, but I need to ask my machinist friend to make an eyepiece-to-lens adapter with M42 tubes. So I wonder if such an approach is flawed or worth it. I do not want to bother him with unworthy ideas.
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ChrisR
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PostPosted: Wed Feb 07, 2018 8:20 am    Post subject: Reply with quote

Could you use something like a T42 telescope adapter . ebay 253024850611 ?
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zzffnn



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PostPosted: Wed Feb 07, 2018 8:47 am    Post subject: Reply with quote

ChrisR wrote:
Could you use something like a T42 telescope adapter . ebay 253024850611 ?


I like that has male or female M42 on either ends. Not sure how the set screws hold the eyepiece though (do set screws directly contact eyepiece, or screws press on an internal ring, which in turn presses on eyepiece). For microscope eyepieces, I may need longer set screws?

I do know that I can get microscope eyepiece to 1.25" adapters, which are slightly more expensive, but still quite affordable.
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Pau
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PostPosted: Wed Feb 07, 2018 9:58 am    Post subject: Reply with quote

zzffnn wrote:
So in effect, if we consider the lens a single simple lens, then we are changing distance from lens to eyepiece (and sensor to lens distance), in amount of a few millimeters.

My Sigma 30mm lens focuses internally, if I remember correctly, so sample to objective distance should not change. For an external focusing lens, sample to objective distance may change as well.

Eyepiece to objective distance should not change.

Afocal is a kind of infinite corrected system, at its optimal point the eyepiece projects its image at infinite, an adequate position for relaxed eye vision. Changing the lens to eyepiece distance while maintaining the subject to objective and objective to eyepiece distances will not change it.
In this case I guess that your approach wont work well.

If you change the distance between your eye and the eyepiece you don't change the focus point. Yesterday I was testing a phone afocally for a friend and this was also my result, if placed at wrong distance of the eyepiece pupil the image is just not good.
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jojm



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PostPosted: Wed Feb 07, 2018 11:54 am    Post subject: Reply with quote

Last year, I've done a test of Olympus focus bracketing with an afocal system.
Here, you could read (in French) the memo of this test.
http://www.lenaturaliste.net/forum/viewtopic.php?f=233&t=17286&start=70#p107588

The test was done with the M. ZUIKO DIGITAL 30mm 1:3.5 and Unilink adapter within eyepiece sold by Brunel Microscopes.
http://www.brunelmicroscopessecure.co.uk/acatalog/digital.html

The microscope objective was a no-name PLAN 4x/0.10, 160.0.17 sold by Amscope.
http://www.amscope.com/4x-plan-achromatic-microscope-objective-lens.html

The combination of M. ZUIKO 30mm and th Unilink.


Whole view of the tested system.


The result
Vendée beach sand with garnets
FOV 4.1mm
Focus bracketing step: 5
93 images in the stack



Corners are really bad.

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zzffnn



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PostPosted: Wed Feb 07, 2018 5:05 pm    Post subject: Reply with quote

Thank you very much, Jean-Marc!

Was the focus start point again 10 feet for the lens? Did you run the focus rail by hand or by stepping motor?

I would be happy enough with a stacking results like yours. Those corners may be due to mismatched eyepiece compensation.

I did an experiment today. 8 slices, about 100 frames, 10x NA 0.25 160TL objective, focus differential "5" and focus start point 10 feet. My stacked result is of low quality due to lack of rig rigidity/stability/weight, but it did show that it would work.
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zzffnn



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PostPosted: Thu Feb 08, 2018 2:01 pm    Post subject: Reply with quote

I have decided not to use in-camera focus bracketing for magnification over 3x.

Just did a stacking experiment with a 10x microscope objective on my microscope, and found that I can get good results quickly, without the limitations of in-camera focus stacking (constantly stop at infinity and restart from 19feet focus again). And my microscope rig is more stable and rigid.

What I do is simply use my Olympus E-M10 II's pure electronic shutter, set camera to continuous shooting at frame rate of 5 frame per second. Start camera using WiFi + my phone's Olympus Image Share app (to avoid vibration from hand pressing), rack up microscope focus during shooting while keep holding shutter release on my phone, then slow down when I hear the camera slowing down (when buffer fills up).

My inspiration is from Beatsy: http://www.photomacrography.net/forum/viewtopic.php?t=34810&highlight=electronic+shutter+focus
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