Cheap tube lens

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Joaquim F.
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Post by Joaquim F. »

Another great find, very interesting John.

Regards

morfa
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Post by morfa »

Update: I might have been too quick to doubt the validity of the tube length stated by the seller – after some quick comparisons it seems as if the focal length of this tube lens very well could be closer to 172mm than 200mm. Haven't yet had time to do a proper measurement but just looking through the viewfinder and comparing the FOV from the tube lens and a regular 200mm (without objectives in place and focused to ≈5m) the tube lens is clearly wider!

To me this is a good thing since it could mean that the usable magnification range begin somewhere below what is specified on the objective. In any case I think it will be interesting to investigate how much flexibility in terms of magnification we can get away with just by varying the tube lens extension.

g4lab
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Post by g4lab »

John, Have you ever managed to track down who made the One Adapter to Rule Them ALL and where mere mortals might be able to sell their souls for one. :lol: :lol:

Gene

morfa
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Post by morfa »

Gene,
The only thing I know is that it said "Nikon M28-M52 Digital Adapter Ring" in the listing (#330458563486). It doesn't say "Nikon" anywhere on the adapter though and there was no box or any other info that came with it so I haven't been able to verify this.

g4lab
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Post by g4lab »


Blame
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Post by Blame »

Wow. 3.7mm diameter.

As soon as I get the bits I need from china, I am going to try with a cropped sensor and a tamron 90mm macro. That should give 5.3mm diameter, and 4.5x magnification, and a chance to challenge the nikon 4x 0.2NA.

Worth a try.

morfa
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Post by morfa »

Tonight I tried pushing the magnification down by decreasing the extension of the tube lens (so that it became focused "far beyond infinity").

Here is the 50X 0.55 pushed down to just below 30X (uncropped FF sensor):

Image

21MP version: http://farm6.static.flickr.com/5209/536 ... 5c7f_o.jpg

Overall appearance is degraded and the corner definition is not very good. But it's not as terrible as you might expect: center resolution is excellent and aberrations are not out of control. The general contrast was pretty low so I've adjusted curves and applied some unsharp mask.

FOV check with the 50x on a FF sensor (1/10mm tick marks)
Image

lothman
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Post by lothman »

morfa wrote:Overall appearance is degraded and the corner definition is not very good. But it's not as terrible as you might expect: center resolution is excellent and aberrations are not out of control. The general contrast was pretty low so I've adjusted curves and applied some unsharp mask.
Thanks John for your pioneer work :lol: So after all do you think this lens works better as a tube than the 200mm-Telelens-Solution?

Can we use a f=200mm achromat (+5 CU-lens) as a tube lens, and which diameter do we use for APS-C and FF.

Oskar O
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Post by Oskar O »

lothman wrote: Can we use a f=200mm achromat (+5 CU-lens) as a tube lens, and which diameter do we use for APS-C and FF.
That thought started to nag me too... if this lens works so well, then wouldn't an achromatic diopter around +5 of similar dimensions work just as well? Or is there more to it? very fascinating...

30X looks sort of ok, but would probably be pretty good when there is no real detail on the edges of the frame. Really interesting thread!

Craig Gerard
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Post by Craig Gerard »

Gene,

Can you be more specific. The Astro photographers have many useful attachments.

Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

ChrisR
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Post by ChrisR »

The Nikon 3T and 4T lenses are 52mm, and +1.5 and +2.9 diopters respectively. 5T and 6T lenses are 62mm, and +1.5 and +2.9 diopters.
They would provide a range of magnifications used in pairs - and keep the step-ring manufacturers happy. I believe they work better reversed on some telephotos, so there's a project for a wet afternoon. :smt101

realjax
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Post by realjax »

morfa wrote:Gene,
The only thing I know is that it said "Nikon M28-M52 Digital Adapter Ring" in the listing (#330458563486). It doesn't say "Nikon" anywhere on the adapter though and there was no box or any other info that came with it so I haven't been able to verify this.
It looks a lot like a thread based digiscope adapter, like this one for instance. Expensive though.
Jacco

Blame
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Post by Blame »

Morfa

You have dropped your tube length to about 120mm.

An infinite focus objective + tube lens = finite objective.

Previous threads suggest that finite lenses loose quality quickly when trimmed below recommended tube length.

120mm might just be too short for your 172mm tube lens. by rights, it should be. Any chance of you trying 170mm next?

This is frustrating. If the corners are good, it is proof that the objective can cope. If not, you are left wondering if it the tube lens's fault or the objective.

Still, I am very impressed with your tests.

Oh, is there some decentering? The left side looks better than the right. Objective and tube lens might be just slightly misaligned.

morfa
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Post by morfa »

Ok, so Nikon 4T (52mm) has a focal length of 344mm so with a 50X objective designed for a 200mm tube lens it should give 344/200*50=86X magnification. That would mean a horizontal FOV of 36/86=0.42 mm.

FOV with Nikon 4T+Nikon 50x/0.55 (FF)
Image
It's probably a tiny bit more than 0.42mm so I ideally I should have added some more extension but I figured it's close enough.

Stacked result with this combo:
Image
6MP version: http://farm6.static.flickr.com/5121/536 ... b5b2_o.jpg

I think I see some focus banding so I should probably have chosen a smaller step size than 0.002mm. Also, the top left looks much sharper than the bottom right so results could probably be improved with some careful realignment. The dark corners are not caused by the objective or tube lens – one of the extension tubes used has a mask which is obstructing some light rays (I know this because the effect disappear at shorter extension or if the ET is removed).

Other than this, diffraction is probably the big detail killer here. Correct me if I'm wrong but I'm pretty sure that NA 0.55 means a lot of empty magnification when used above 80X with this sensor/resolution.

morfa
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Post by morfa »

Another one with Nikon 4T as a tube lens. This time "pushed back down" to approximately 50x by shortening the tube lens extension.

Image
12MP version: http://farm6.static.flickr.com/5170/536 ... f1d6_o.jpg

Like before the lower right is worse than the upper left, most likely due to some misalignment somewhere.

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