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Calculating vignette, cropping on tube lens?
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cube-tube



Joined: 10 Oct 2017
Posts: 105
Location: Durham, NC

PostPosted: Sun May 27, 2018 8:14 pm    Post subject: Calculating vignette, cropping on tube lens? Reply with quote

I am using a 180mm f3.5 tube lens, and there is a significant sharp vignette with all of my objectives. I may buy another fast telephoto lens soon, but only if I could use it as a tube lens for my microscopy.

I understand that the vignette is caused by the small aperture on my tube lens, but how do I know if there will be a vignette or not with a new lens? For example, the Canon 300mm f4 or 400mm f5.6?

Also, will the image be cropped (higher magnification) with a longer lens? And, will the image quality be affected in other ways?

Thanks
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Pau
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Joined: 20 Jan 2010
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Location: Valencia, Spain

PostPosted: Sun May 27, 2018 11:49 pm    Post subject: Reply with quote

What sensor format?
For APSC many lenses will do, for FF the selection will narrow a lot.
A diopter like a Raynox or Sigma LSA on bellows or tubes is less prone to vignette.
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JohnyM



Joined: 24 Dec 2013
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PostPosted: Mon May 28, 2018 3:16 am    Post subject: Reply with quote

It's not about F-stop, but entrance and exit pupil. In example, im using Repromaster 135mm f8-9 that covers full frame, bit with only ~1mm of infinity space.

Formula is:
(Tube lens etrance pupil - Objective lens exit pupil)*Tube lens focal lenght/Sensor diagonal = Infinity space

If infinity space is greater than 0 - you should get no vignetting, if you manage to get microscope objective within that distance to tube lens.



If you cannot measure entrance / exit pupil they can be approximated by formula:

Exit pupil = 2* Focal lenght * Numerical Aperture

Tricky thing is that this formula is rarely accurate due to lens designs, and can only hint if soething might or might not work. Best if you can somehow measure it first.





Yes, image will be "cropped" with longer tube lenses, due to higher magnification. Image quality should be very similar, but image will be significantly softer due to diffraction. On the other hand, if you're using some high quality objectives, you might get more usable resolution.
With my system (A7RII with 4,5 um pixel) i tend to get best sharpness with EA below F8 and capture all the detail with EA=16.
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cube-tube



Joined: 10 Oct 2017
Posts: 105
Location: Durham, NC

PostPosted: Mon May 28, 2018 7:13 am    Post subject: Reply with quote

@Pau: I'm using a Canon 6D (full frame)
Something else I'm confused about:
How do people connect camera body, rayonox, bellows, and objective? Is there some diagram that you know of that has a list of all of the adaptors etc.? That would be very helpful to me.

Edit: I think I found what I'm looking for:
https://www.stackrail.info/images/r3.jpg

Thank you.


Last edited by cube-tube on Mon May 28, 2018 7:27 am; edited 1 time in total
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cube-tube



Joined: 10 Oct 2017
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Location: Durham, NC

PostPosted: Mon May 28, 2018 7:24 am    Post subject: Reply with quote

@JohnyM is the diffraction happening because the effective aperture of the tube lens is so small, or because the effective aperture of the objective is so small?

I'm thinking it is probably worth it to just get a rayonox or similar.
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mawyatt



Joined: 22 Aug 2013
Posts: 2078
Location: Clearwater

PostPosted: Mon May 28, 2018 8:57 am    Post subject: Reply with quote

If you do a search on here you will find lots of details on lens construction (extension tubes, bellows etc.) for objectives, and lenses for use as a "tube" lenses for infinite objectives.

Peter at MJKZZ has a tube lens kit will all the adapters and such:
http://www.mjkzz.com/product-page/variable-extension-for-tube-lens
http://www.mjkzz.com/product-page/variable-tube-system-with-clamps

WeMacro also has tube lens kits.
https://www.wemacro.com/?product=raynox-reversed-m42-adapter-rings-pack
https://www.wemacro.com/?product=m42-extension-tube42mm-x-1-0

Regarding what makes a good "tube" lens is best evaluated by test. Known good "tube" lenses are the Sigma LSA (200mm) and Raynox 150 (208.3mm) mentioned by Pau. The old Nikon 200mm F4 "Q" also works well as a "tube" lens.

The Raynox 250, old Vivitar 135 F3.5 (Komine) and Zeiss 135 F3.5 also make good "tube" lenses for reduced objective magnification (assumed infinite objective requirement of 200mm), but will vignette on a full frame sensor and only slightly on a APC sensor.

All of these lenses can be found on eBay for reasonable prices.

Best,
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harisA



Joined: 03 Jul 2011
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PostPosted: Mon May 28, 2018 12:42 pm    Post subject: Reply with quote

With the exception of Mitutoyo objectives the only way to achieve FF frame coverage is to use a tube lens with a focus distance significant higher than the objective's designation. Nikon objectives require 200mm tube lens and Olympus 180mm .You can start with a 300mm lens but have in mind that this will also alter the magnification of the objective.For example a Nikon objective (designed to work with a 200mm tube lens) will give a magnification 1.5x higher with a 300mm lens.
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Pau
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PostPosted: Mon May 28, 2018 1:50 pm    Post subject: Reply with quote

Some Nikon objectives like the Plan CFI 10X 0.25 (golden shell) have an image quality circle even larger than the most Mitutoyo, of course not the same quality.
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rjlittlefield
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Joined: 01 Aug 2006
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PostPosted: Mon May 28, 2018 2:24 pm    Post subject: Reply with quote

harisA wrote:
With the exception of Mitutoyo objectives the only way to achieve FF frame coverage is to use a tube lens with a focus distance significant higher than the objective's designation.

I am curious to know what experience is the basis for that statement.

I re-checked just now, and I get good full frame coverage on Nikon D800E using Raynox DCR-150 with Nikon CFI 10X NA 0.25 (part number MRL00102); with Nikon CFI BE 10X NA 0.25 (part number MRN70100); and with Nikon CFI BE 4X NA 0.10 (part number MRN70040), hood removed as shown at http://www.photomacrography.net/forum/viewtopic.php?t=18182 .

--Rik
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JohnyM



Joined: 24 Dec 2013
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PostPosted: Mon May 28, 2018 2:24 pm    Post subject: Reply with quote

Also CFI PA 20/0,75 cover full frame corner to corner sharp. Coverslip only tho.
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cube-tube



Joined: 10 Oct 2017
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PostPosted: Mon May 28, 2018 5:48 pm    Post subject: Reply with quote

@harisA I have both Nikon and Mitutoyo objectives and I get similar vignetting with all of them.

Also @Pau I have that Nikon 10x (the one in my profile photo?) and get a vignette.


Last edited by cube-tube on Mon May 28, 2018 5:50 pm; edited 2 times in total
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cube-tube



Joined: 10 Oct 2017
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PostPosted: Mon May 28, 2018 5:48 pm    Post subject: Reply with quote

@mawyatt thank you so much that is all very useful.
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cube-tube



Joined: 10 Oct 2017
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PostPosted: Mon May 28, 2018 8:45 pm    Post subject: Reply with quote

Also, is there a consensus on whether a forward or reversed raynox is better?
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mawyatt



Joined: 22 Aug 2013
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PostPosted: Mon May 28, 2018 9:03 pm    Post subject: Reply with quote

cube-tube wrote:
@mawyatt thank you so much that is all very useful.


You are welcome.

Regarding Raynox used forward or reversed. Either way I don't think matters much. I tend to use them (150 & 250) reversed, others use them in normal direction. Probably best to test in your system.

Best,
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JH



Joined: 09 Mar 2013
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Location: Vallentuna, Stockholm, Sweden

PostPosted: Mon May 28, 2018 11:29 pm    Post subject: Reply with quote

From my own experience, a short empty tube might clear the corners. I know that this changes the set up from infinity to something else - so you have to test to see if the image quality is still ok (I usually does not see any change with a short tube)

Best regards
Jörgen Hellberg
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