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UltravioletPhotography

A Focotar II 1:5.6/100mm from 1981. Not very good for UV-photography


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I have measured a Focotar II 1:5.6/100mm from the last production year.

 

post-150-0-50014700-1526557877.jpg

The serial #3065653 indicates that it was produced 1981, the last year it was made.

https://www.l-camera...-_1:5.6_/100_mm

 

The spectrograms below show my transmission measurements of this lens.

As the right group of measurements in the diagram show, the Focotar did not have a wide UV-transmission.

For comparison I also included graphs of my Nippon Kogaku EL-Nikkor 80/5.6 (old metal version).

 

LEFT, green: Nippon Kogaku EL-Nikkor 80/5.6 (metal). RIGHT, purple: Leitz Wetzlar Focotar II 100/5.6.

post-150-0-36549900-1526618901.png

 

All graphs are normalised to an average of 100% within the gray area, just before the light-source become too weak.

The noise in the graph further into VIS is due to that the light source for those wavelengths is switched off to reduce crosstalk into the UV-measurement.

 

 

EDIT: Replaced spectrogram. Improved graph, new colours, slightly better normalisation range.

EDIT II: Improved the title.

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Ulf, this is nice work and thank you !!!

 

Permit me to add a small caption to the chart so that the two transmission curves are readily identified? If you do not like it, I will instantly remove it!

 

When looking at the these two transmission curves, I think that perhaps the Focotar II is not entirely unuseable but that the UV-pass filtration should be carefully chosen to maximize transmission between 370 - 400 nm? Perhaps something like the StraightEdgeU? What is your opinion on that? Or perhaps we should simply not use the Focotar II at all for UV.

 

 

ADDED LATER: I add the following two things to the Lens Sticky entry for Focotar II.

  • half-max 370-400 nm
  • Not very good for UV. See: LINK

The LINK is for Ulf's topic.

I'm not sure how to phrase what I want to say about the transmission. I'm trying to indicate where reasonable UV transmission begins. Maybe I should write only this: "half-max 370 nm". Opinion please? Thanks.

 

ALSO: I realized that I do not understand why there is normalization between 410 -428 nm?

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I wrote this post very quickly with available material.

 

Andrea, I like the caption. Thank you for the help.

 

That will indeed make it easier to identify the different lenses directly from the chart.

 

Do you think it improves readability much to change the curves for each lens to the same, dIfferent colour?

I could do that and replace the graph.

 

I agree that the Focotar II is not entirely unusable for UV, but it is just barely usable, and then only for the near-VIS range with the StraightEdgeU or possibly some UV-BG-stacks.

I didn't even care to try it on the camera before returning it. It costed too much to just sit on a shelf collecting dust.

 

When I got it I hoped it could replace my EL-Nikkor 80/5.6 as an even better lens. That hope was not fulfilled. :(

I'm not really ready to spend enough money to buy an UV-Nikkor 105. Maybe sometime in the future.

 

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About beginning of UV-transmission range I would say Half-max > 370nm or even Half-max ≈ 750nm

 

Maybe we should define a more formal posting format for separate posts for individual lenses and filter types.

Do you think that is a good idea?

 

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The normalisation is needed due to my method of measuring lenses with my relatively primitive spectrometer setup.

The metod do not give an absolute transmission value due to the lack of integrating sphere etc.

 

I will soon write a post about the method I use, describing the procedure and why I think it is valid with given limitations.

I hope the normalisation will make better sense then.

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I think that if it is not too much trouble, different colors for each transmission curve is useful.

 

I started a Lens Gallery section. I could relabel that Lens Gallery and Transmission Charts.

Let me think about it and consult with Bjørn. :)

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I have been using 100mm Focotar-2 lens for years, alongside 50mm Focotar-2, 35mm Kyoei and a number of other lenses that transmit down to 320 nm. 100mm Focotar-2 is one of the best non-dedicated UV lenses I've ever used. Your tests do not agree with my observations.
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Andy Perrin

I have been using 100mm Focotar-2 lens for years, alongside 50mm Focotar-2, 35mm Kyoei and a number of other lenses that transmit down to 320 nm. 100mm Focotar-2 is one of the best non-dedicated UV lenses I've ever used. Your tests do not agree with my observations.

Ulf's is from the last year it was made. Possibly it is different from yours by the addition of a coating or something?

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I do not think my measurements can be that much off.

All other lenses I have measured so far agree reasonably with other published findings.

The reason for the difference you se must be caused by something else.

 

Alex, could you please reveal the serial number of your lens?

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Thank you Alex.

Then your lens is from the 250 batch made two years before the one I measured.

The design formula or coating specifications might have changed quietly by Leitz in that time.

 

I found such a change for the old Focotar - 1 1:4.5 / 50 mm - DOOCQ, where the earliest one I have is distinctively different in a way unlikely caused by ageing or dirt.

 

Is it possible that you have the transmission characteristics for your lens and can share?

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I have always heard the Focotar2 100/5.6 was good for UV. From Dr. Klaus I think. But it's been a long time since I heard that so my recall might have faded.

 

We had some comments recently somewhere that a particular lens might have changes over time due to different manufacturer or coating changes, etc. Could be the case for Focotars also. I think Schneider Kreutznach made Focotars IIRC.

 

But that transmission curve above seems to indicate enough UV for Ulf's Focotar. Perhaps longer exposures and concentration in the higher UV. So a wise choice of filtration would be called for.

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The graph above indicates a bad lens for UV, definitely NOT a lens one would want to think of using, let alone investing the kind of money in that a Focotar 2 100mm would usually cost.

I would retest, because that lens has a reputation of being very good for UV.

An El-Nikkor 80mm (chrome mount/base) is a pretty good UV lens, but the other plot is not. I mean 360nm at 20%? Na, not worth the space to collect dust.

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I would retest, because that lens has a reputation of being very good for UV.

 

That good reputation and the result from my FOCOTAR-2 50mm was the reason buying the100mm lens in the first place.

I was very disappointed with the result, as I had been quite convinced this would become my main UV-lens in the future.

I had already prepared a helicoid and everything else to start using the lens.

 

When I got te result I retested in all possible ways I could imagine, to find any grave flaw in my method.

I also retested several other lenses I have, where I have found statements, curves or indications on the net about their UV-response.

Every lens except the FOCOTAR-II 100mm I had gave reasonable responses.

Even my NEWPORT ORIEL 41775 FUSED SILICA PLANO CONVEX LENS 150mm FL. X 50.8mm gave reasonable results, with a transmission down to 230nm.

My light-source and spectrometer work well almost down to a 200nm.

 

I returned that Focotar-II 100mm.

 

My conclusion is that the reputation of being a good lens for UV is not valid for the full population of FOCOTAR II lenses out there.

 

The Componon-S 100mm I have is made by Schneider-Kreuznach. It is even slightly worse for UV.

The FOCOTAR 50mm (LFE) was produced by the same company.

https://www.l-camera..._1:4.5_/_50_mm.

That lens is not very good for UV either

 

I have not given up yet, but will try to find and buy or borrow one of the earlier lenses to test, preferably one from the same year as the lens Alex has.

 

EDIT: Correction and addition about the fused silica lens, (that I intend to use for collimation in my test setup).

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My conclusion is that the reputation of being a good lens for UV is not valid for the full population of FOCOTAR II lenses out there.

 

Yes, I agree. There must have been some changes in the line over the years. I'll add something to the Lens Sticky about this.

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