Making a teleconverter (TC) for UV

[nfoto]

As the topic title said, but forgot to mention a TC for the UV-Nikkor 105mm f/4.5. The same setup has been tested for the Coastal 60mm f/4 APO, but I would not use any TC for this short or shorter focal lengths except as a last-ditch resort.

 

The reason for my TC project was I would like a UV-capable lens for longer focal lengths than the UV-Nikkor itself. The Tele-Megor 180mm f/5.5  is quite OK, but less handy as I have to use it on a bellows to get the required  versatility. The 200mm Nikkors likewise are useable for UV, but not outstanding.

 

Any TC degrades the image to some extent, however with the superb quality of the UV-Nikkor as a starting point, hope is the final outcome would be acceptable. Thus time for rooting through my odd and end bins. 

 

I decided to base the TC on the very cheap (and old) Petri 2X converter, as a quick test showed it would pass a lot of UV unimpeded. Its internal optics are easily removed. I placed them in a suitable placeholder (C-mount adapter for m43) which then could be placed into a short extension ring such as the Nikon E2, or (with modifications) F-C. ring. These are easy to source on eBay etc.

 

The TC seen from above, with added internal black flocking and a lens mount shim as an additional security of keeping everything in position.

 

 

From the rear side, showing the threads that fit the barrel of the Petri optics.  This allows fine-tuning of the setup so the lens + TC gets infinity focus.

 

I also added a CPU to inform the camera of the new 210mm f/9 lens data. I had lifted the rear part of the F-C ring to use as a bespoke lens shade on my Olympus  38mm f/2.8 OM Macro lens thus added a stock F-mount bayonet so as to be able to mount the TC on the camera. If one starts with the E2 ring there is already a lens mount present here.

 

 

Shining the light of a 'Nemo' torch through the TC shows the glass inside is wholly UV transparent.

 

 

The TC on a UV-Nikkor, with a spare not completed TC at the right,

 

 

 

All right, so how does this solution stack up? Below are some comparison images first with the UV-Nikkor on its own,  then with the TC added, and finally, a frame with the Tele-Megor 180/5.5. All images were taken using an SB-140 flash by the way and with the respective lens set to f/11. The test subject is the ubiquitous Tussilago farfara and its likewise common companion, a Colorchecker Passport card. Camera is Nikon D3200 with its built-in Baader-U filter.

 

UV-Nikkor 105mm f/4.5

 

UV-Nikkor + 2X TC,

 

 

and then the Tele-Megor

 

 

These files were processed as usual in Photo Ninja and the "UV white" balance from the UV-Nikkor was used for the others. Obviously the Tele-Megor image could have been tweaked to match the others better, but that would defeat the purpose of the comparison. Plus, the lower sharpness would still be an issue.

 

Given the TC cost me less than $20 I won't complain. Even more so as the basic 'design' was completed in a very short time.

 

[colinbm]

Looks like you have made a successful conversion Birna

[JMC]

Nice conversion Birna.

[Andy Perrin]

The conversion looks successful although the telemegor image comparison seems unfair to me because the flower is not at the focal plane (the color checker looks plenty sharp!). 

[nfoto]

I could probably have made a sharper picture, but the probem was focusing as I had already extended the bellows to its maximum, and I had a precarious vantage point and needed to move further away.  However the main issue here is the different false-colour rendition compared to the others.

 

Below is another Tele-Megor capture of the endearing Tussilago. Not at the same spot although in its vicinity, and not on the same day. As can be observed, UV false colours are quite nice, but the ultimate sharpness is not present.  This image was processed in PN with its own w/b set. For such a relatively cheap lens, one should be well satisfied though if no better option is available.

 

[Andy Perrin]

There is a lot of CA in the new Tele-megor image not present in the other setup. 

[dabateman]

Looks good. Also has much deeper UV range compared to the megor.

 

If you want to have some fun, see if you can get a negative meniscus fused silica element or build something with concave fused silica.  But that may not look nearly as sharp as what you have here.

 

How does the Tc-16a compare? If its modified you can also get AF with a screw drive camera. 

[OlDoinyo]

Question: will this setup really outperform the Novoflex 400/600 lenses used with bellows? Obviously the base lens will, but I speak specifically of the combination you tested.

[nfoto]

Just briefly tested the TC-16a. It might render a tad sharper, but cuts more UV. There is no free lunch. And of course is 168mm f/7.1 that much better than 210mm f/9 when the total focal length is in focus (pardon the pun)?

 

As to the Novoflex 400, which I have, it quickly becomes unwieldy if the near range of subjects is the main area of use. Plenty of extension is to be added as the Novoflex is not a telephoto design. Whilst the 400mm f/5.6 lens head lets a lot of UV through, it has higher levels of chromatic aberrations that can be tricky to deal with, and contrast is less good than setups based on the UV-Nikkor.

 

I have other 400mm candidates to explore besides the Novoflex. Will try to get through some of them later before the spring season arrives.

[OlDoinyo]

Another useful test would be to examine the images taken with and without the teleconverter, adjusted to the same scale, and compare the sharpness carefully. Is the teleconverter actually gaining you any angular resolution in the final image, or are you merely magnifying image imperfections? I would be interested to know the answer.

[dabateman]

Thanks Birna.  I guess if you had a full spectrum converted D850 with focusing bracketing,  the Tc-16a would allow you to do your stack easier, without a macro rail.  But you have many of those already. So no savings there.

I too would be interested in OlDoinyo test. I think the teleconverter image will be sharper based on your already provided images. But maybe 1:1 is better to compare then down sampled full view. 

[nfoto]

9 hours ago, OlDoinyo said:

Another useful test would be to examine the images taken with and without the teleconverter, adjusted to the same scale, and compare the sharpness carefully. Is the teleconverter actually gaining you any angular resolution in the final image, or are you merely magnifying image imperfections? I would be interested to know the answer.

 

Actually such a test is impossible to conduct if *all* parameters should be equalised. One has to move the camera to make the image magnification equal, which leads to an altered perspective.  If we don't do that, there are introduced problems as to how the scaling or resampling of the image is conducted. And so on.

 

In theory, if one can avoid empty magnification (resolution is infinite), we could enlarge the image without problems so just the base lens would be required. In practice adding a TC is often to be preferred.

 

[Doug A]

@nfoto Impressive transplant and results. You know no fear when it comes to modifying equipment. I like that.

 

The Nikon lens is so sharp, it could probably handle even more magnification. Perhaps add a modified 1.4 teleconverter to the 2X? It might even be possible to find a very old Nikon mount 3X made by the company responsible for the Petri.

 

Thanks for sharing,

Doug A

[Guest]

Birna. Overtaken by ennui...

 

Dragged over from the TC-16 thread. 

 

On 4/14/2021 at 1:00 PM, nfoto said:

Apparently the TC-200 delivers better results than the optically much simpler Petri-based 2X, at least combined with the UV-Nikkor.

 

[nfoto]

Well, there are lots of combinations to test. That example referred to landscape shooting, if I remember correctly.  My setup here is for flower photography. I'll certainly continue experimentation, once the dreadful white fluffy stuff covering everything disappears.

[OlDoinyo]

7 hours ago, nfoto said:

Actually such a test is impossible to conduct if *all* parameters should be equalised. One has to move the camera to make the image magnification equal, which leads to an altered perspective.  If we don't do that, there are introduced problems as to how the scaling or resampling of the image is conducted. And so on.

 

In theory, if one can avoid empty magnification (resolution is infinite), we could enlarge the image without problems so just the base lens would be required. In practice adding a TC is often to be preferred.

I guess I have not made myself clear. I did not mean that you should reframe between shots. Rather, I proposed that pictures with the base lens alone and with the base+TC combination be taken from the same place (you may already have done this.) Then, you could take the former image and:

 

-crop it so the frame boundary matches that of the second image; and

 

-upsample it so that the pixel count matches (I.e. the angular pixel pitch is the same.)

 

Then, assuming near optimum focus for both images, you could determine which one is sharper.

 

The reason I suggest this is because, unless something freakish is going on, a teleconverter will never increase the actual angular resolution beyond what the base lens is already capable of; it can only magnify the image that it is given. If your digital sensor has higher sensor-plane linear resolution than your base lens (which is quite commonly the case,) using a teleconverter will not resolve any finer detail, but merely magnify image faults.

 

In the film era it was very different, because most films other than specialty copying emulsions have fairly low linear resolution compared to the lenses in use.

[OlDoinyo]

52 minutes ago, nfoto said:

dreadful white fluffy stuff

 Aw, come on--it's pretty!

[nfoto]

Resampling is a tricky aspect a it never can be done without influencing the pixel content and image information  Upsampling can never add more detail. Downsampling can seemingly mask image defects. If, on the other hand, we can add magnification of the scene directly, more information might be gleaned.

 

Snow is pretty, I grant you that. But it interferes with my flower photography.

 

 

[Guest]

I would be so grateful if it was snow that kept me locked down in curfew and away from the flowers (nature) these last two years.  

[Andrea B.]

I'm enjoying the fact that you put a spare Petri adapter to use to make the UV-Nikkor TC.

 

Do you have a reference for that black flocking?

[nfoto]

The black flocking was pulled from my scrap bins. I think it originally came from a model train kit (?); very thin velvet with an adhesive backing. I have used lots of the material to flock lens mounts etc. over the years.

[Guest]

This is where I bought mine stateside. It came as a huge roll inside a cardboard tube. 

 

http://scopestuff.com/ss_flok.htm

 

[ulf]

38 minutes ago, Blazer0ne said:

This is where I bought mine stateside. It came as a huge roll inside a cardboard tube. 

 

http://scopestuff.com/ss_flok.htm

 

That is a very good flocking material. I bought some of it ca 2009.