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UltravioletPhotography

Kuri 35mm vs. Noflexar 35mm


Andy Perrin

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Sometimes, I have seen deeper UV transmitting lenses look pinker straight out of the camera.

Given the yellower content of your Kuri pics it may indicate more UV depth, which is what I have seen from some spectrometer tests,

but the RAW version between the two lenses may look more pink with the Kuri. Just something to look for.

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The ClubSnap forum link in the first link is a piece of history. I was about to start middle school in September 2012.
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Andy: Re optical benches: they aren’t magic, you know.

 

No, of course they aren't. And I personally do not think MTF charts tell the entire story, but neither should we discount them. Everything in balance.

 

One does need an optical bench to more precisely check for aberrations, decentering, sharpness and FFD of a lens.

 

I have often wanted to BEG Roger Cicala to run a couple of my UV lenses on Olaf to see just how "good" they actually are compared to the usual hype. :grin:


 

Here is a nice write-up about how to do a brick wall test.

Selecting the Proper Brick Wall for Photographic Tests

 

By Roger Cicala

 


 

This next link is also a cool article. Andy's Kuri seems to have a slightly off FFD. But I was truly surprised at the variation in tested camera body FFDs for some of the brands as shown in this link. A bit scarey.

The Great Flange-to-Sensor Distance Article. Part II: Photo Cameras

 

By Roger Cicala

 

This is also interesting reading:

https://www.dpreview...t-my-own-lenses

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The ClubSnap forum link in the first link is a piece of history. I was about to start middle school in September 2012.

 

Stefano, I think the two links I posted are very informative links, and need not be criticized.

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I didn't criticize them (at least, not in a bad way). It is nice to see this old stuff "emerge" every now and then.
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Andrea, going back to my particular Kuri:

 

2) As determined with the NEX-7, the center of the lens is incapable of infinity focus on the M42-NEX adapter. It focuses fine on a helicoid. It’s unquestionably an issue with this particular lens.

 

I think that is fixable, by adjusting the focus ring, the same way as I did adjust the fixed focus setting of my Sunex for different filters.

I think there are some locking screws on the focus ring that if losened can allow shifting the rings end ange a bit.

For shorter FL-lenses the back flange distance is more sensitive.

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Andy Perrin
Hm. Well I could consider it but the helicoid also works and I’m a bit worried I could decenter the lens or something if I mess with it.
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I agree with Ulf, that is how they are. You can adjust the infinity, but you should get an infinity focus M42 adapter with built in lens for infinity. Just a cheap one, because those probably wont have coatings and will pass UV as good as the Kuri does.
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Andy Perrin
You lose UV due to reflection at two more surfaces then, Cadmium. I think the helicoid works fine and doesn’t add any more glass or risk messing with the lens alignment.
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Well, I use a helicoid all the time, I also focus with it, keeping the lens at infinity. UV losses due to reflections are probably comparable to the losses of an un-glued filter stack vs. glued, so not that much, even if measurable.

 

(Yes, the point is that you would introduce other losses, and a helicoid is just a simple solution, but still the reflection losses are not a lot).

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I agree with Ulf, that is how they are. You can adjust the infinity, but you should get an infinity focus M42 adapter with built in lens for infinity. Just a cheap one, because those probably wont have coatings and will pass UV as good as the Kuri does.

That statement is valid only for cameras with a looong flange focal distance (FFD), longer than the 45.46mm that is specified for the M42 mount.

Steve needs a lensed adapter to use these lenses om his Nikon DSLRs (FFD 46.6mm) for focussing at infinity.

 

Andy, I think, is mainly using a Sony A7S, with a FFD of 18mm, the same FFD as Canon Eos M has.

There is plenty of room for avoiding any extra lenses, in the light path.

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I just checked the infinity focus capability of my camera + Soligor + helicoid system. I set the lens at the widest aperture (f/3.5), and I took some photos of very distant structures (more or less 1 km away). In UV I just reach infinity at the maximum (as stated before), in IR I reach a bit beyond. It really depends on the lens, how much focus shift it has, where the sensor is (my Canon has been converted by Alan Burch and he re-alignes the sensor to reach infinity), and so on. Maybe Andy was just unlucky.

 

He can fix this with a helicoid, but if he couldn't maybe a solution could have been to add a piece of glass behind the lens to shorten the optical path. That introduces losses but can work.

 

In general, to avoid this issue one can ask to have the camera converted to reach beyond infinity. Hope I didn't go too much off-topic.

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Andy Perrin

Stefano, the lens wasn’t focusing on either camera. It was not the fault of the conversion.

 

Also, isn’t the terminology backwards there? The optical path (geometric path times refractive index) should get longer if you increase refractive index by adding glass behind the lens:

https://en.m.wikipedia.org/wiki/Optical_path_length

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Well... I know that if you convert a camera without replacing the filters with something else you may not be able to focus to infinity because of that issue. It is as if the lens is further away from the sensor. It is like when objects seem closer when they are underwater (and your eyes are not), for example when you see your feet in the sea at the beach.

 

From here:

 

"When using rear mounted filters, it is important to realise that they shift the focusing towards infinity as the filter-glass gives a shorter optical path length mimicking a forward shift of the lens".

 

Do they actually mimick a backward shift (towards the sensor)?

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Andy Perrin

I think your quote is using the terminology in reverse also? Possibly what people in this board are calling optical path length does not match the standard definition in optics...

 

In the case of camera conversion without replacing the IR cut filter, I think the lens is effectively closer to the sensor after conversion because you have replaced a segment of the optical path with n=1 instead of n=1.5.

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It probably is a terminology issue. What I mean is a sort of apparent distance, something that is actually shortened by high n materials, like water or glass. There is a term probably, I will search it now.
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Andy Perrin
Yes, the term is optical path length. Geometrical length is the alternative. But it’s lengthened not shortened by high n.
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I made a little drawing in Paint to better show this:

post-284-0-60490900-1620488556.png

 

The cone of light is coming from the lens and (ideally) has the "tip" on the sensor. With the piece of glass, the system behaves as if the lens was closer to the sensor, so the focus point is shifted towards infinity.

 

The glass pane is not perfectly centered, I hope this doesn't itch anyone... it is a quick drawing.

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Andy Perrin
The picture is fine, the optical path length is longer in your solid line than in the dashed.
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  • 3 weeks later...
Andy Perrin

Round 2 matchup!

 

I redid the test, the white balance is still arbitrary but the same for both images, but this time both lenses were set to infinity focus and placed on the helicoid, and I then adjusted the helicoid to focus the image on the most distant object. There was a very tight lens hood in both cases, but for some reason it vignettes a lot in the Noflexar.

 

Full images

Settings for both were F/11 2.5" ISO100. Camera was balanced on window sill.

 

Kuri

post-94-0-33434000-1622218888.jpg

 

Noflexar

post-94-0-63893200-1622218908.jpg

 

Noflexar with a re-whitebalance on the same pixel, vs. Kuri

post-94-0-19379000-1622219492.jpg

 

1:1 crop center

 

Kuri

post-94-0-42211100-1622218952.jpg

 

Noflexar

post-94-0-59102600-1622218966.jpg

 

1:1 crop corner

 

Kuri

post-94-0-87552400-1622218981.jpg

 

Noflexar

post-94-0-10298700-1622219635.jpg

 

 

Conclusions are basically unchanged: this Kuri is not the sharper lens, alas. Also, Noflexars are rightly prized despite the narrower bandpass. I've owned two Noflexars and both are quite sharp. The contrast does seem better on the Kuri, and certainly the colors are better (this was true even after re-whitebalancing).

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Andy the 1:1 crop center for the Noflexar has a dark brick patch that the kuri doesn't.

I find that interesting.

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Andy Perrin

Andy the 1:1 crop center for the Noflexar has a dark brick patch that the kuri doesn't.

I find that interesting.

It's actually quite boring -- it's sensor dust. When I took the second photo, it was a few pixels off from the first photo, so some dust moved into the frame. I should probably clean that sensor up.

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It's actually quite boring -- it's sensor dust. When I took the second photo, it was a few pixels off from the first photo, so some dust moved into the frame. I should probably clean that sensor up.

Ok that makes sense. I couldn't figure out what architecture detail was discovered and if a sign of some off angle IR leak or not.

As you have previously shown that IR was better at revealing things in brick work.

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