Lens test( Canon 24mm/40mm)( My EL Nikkor 80's head to head)

[Nate]

I found I had a lens that didn't glow under UV, and no coating, so I made a camera lens with it on a helicoid. I put the zwb1 2mm/S8612 2mm on the front.

One of the rear elements in one of the EL's got a massive cleaning with everything I had in the house when I first got the lenses, but I can't remember if I'm using that one.

 

Would this be considered as a legit test Even though it's not a pinhole?  I did turn up the saturation a bit to see the color better.

[Fandyus]

Well, even though you're not using a pinhole, this makes it clear how much worse the transmission gets with the extra glass added, so I suppose it's a good way to tell some basic things. But pinhole is best for optimal results.

[Nate]

The Industar 50-2 faired even better than expected too, but with a little less transmission.

[Fandyus]

Yes, the Industar 50-2 is pretty good. Not as good as the Soligor 35mm f/3.5, but it has the benefit of being very sharp stopped down.

[Andrea B.]

Nate, here's something to consider. In this kind of test, if you don't shoot straight down the lens along the lens axis, then there will be a kind of vignetting effect which darkens the result on one side. I can see this happening, for example, in your EL-Nikkor 80 photos. The lens on the left looks a bit darker on the left half of the blue-violet area. 

 

The result of having to photograph each lens individually by shooting along the lens axis was what gave me so much trouble when I was working out the protocol for the pinhole test because each photo had to be identically converted and white-balanced in order to be able to meaningfully compare the results.

(Note: I was not the one who first thought up the Pinhole Test.)

 

Also note that increasing saturation alters the luminance of that color. So it may not be a good idea to increase saturation unless you have an app which will permit separation of the luminance and chrominance effects? OTOH, given that the Pinhole Test is a relative test, small changes in overall saturation probably wouldn't change the results all that much. A few simple experiments will sort this out.

[Nate]

Thanks Andrea, I might try again sometime when I can get a sharper pinhole image. Question, does the darker purple/blue color indicate not as deep uv pass on a white balanced pinhole image?  Thanks

[ulf]

A much faster alternative to a pinhole, for these tests, still with a really deep UV reach, is to cobble together a simple lens based on an uncoated fused silica lens element with a suitable focal length.

A one lens element design like this surpasses the UV-Nikkor 105mm all the way down to 200nm. 

It naturally have all kinds of uncorrected aberrations but can be be quite usable anyhow.

 

The best shape is planoconvex lens, but a convex-convex one works too. For a practical build a FL in the range of 70-100mm will do. The lens do not have to have a big diameter at all to improve the speed compared to a pinhole.

For reasonable image quality it needs to be stopped to at least f/8 or more.

If focussing on the test objects is done by moving the camera the design can be super simple, just some extension tube and a disk with a hole at the center at the end, to mount the lens on.

 

Sometimes the FS PCX lenses are found on places like eBay to a nice price.

A quality lens from Thorlabs is not painfully expensive either:

https://www.thorlabs.de/thorproduct.cfm?partnumber=LA4327

https://www.thorlabs.de/thorproduct.cfm?partnumber=LA4600

 

Such a project can then be expanded to include an iris mechanism and helicoid if that is desired.

[Nate]

Thanks for the info @ulf I might order one to play with. Throughout the years, I've torn apart many optics and saved a few of the lenses from it. Is there any way to test them to see if they're fused silica? 

[ulf]

39 minutes ago, Nate said:

Thanks for the info @ulf I might order one to play with. Throughout the years, I've torn apart many optics and saved a few of the lenses from it. Is there any way to test them to see if they're fused silica? 

There is no reason I can think of to use fused silica in any lens designed for normal VIS light.

 

Fused Silica and Fluorite are used in the UV-Nikkor.

Fluorite is used in some really high end lenses, but it is more expensive and delicate material and thus avoided for more normal lenses.

[dabateman]

Fluorite and fused silica would be rare, but BK7 glass isn't.  You may want to test your elements.  One of the elements from a cheap Canon 50mm f1.8 ii lens had transmission down to 300nm.

300nm and above is good enough. Also since most cameras can't see below 330nm anyway,  you dont need fused silica.

[Nate]

32 minutes ago, dabateman said:

One of the elements from a cheap Canon 50mm f1.8 ii lens had transmission down to 300nm.

I have 2 Canon 40mm pancakes, I wonder if one of those elements goes down that far. I only use one, so I don't mind tearing into one of those.

[dabateman]

2 minutes ago, Nate said:

I have 2 Canon 40mm pancakes, I wonder if one of those elements goes down that far. I only use one, so I don't mind tearing into one of those.

I wouldn't risk it.  The Canon 50mm f1.8 was all glued together.  Nothing would come out easy. Instead of spending hours cutting out the front element,  I just drilled through the back and knocked them all out. I think it was the front one I tested though that was partially intact that had the deep range. 

[Stefano]

Usually it's the flint elements that absorb UV more. Crown glasses (such as BK7) easily reach UVB.

[Nate]

I think with the lens I already have, it goes as low if not lower than the EL Nikkor 80, and I wouldn't need to test anything below that at this point in my UV journey. Thanks guys for the input.

[ulf]

All of what you say above about BK7 is more or less true for an uncoated lens.

Especially if you use a Baader U as a test filter. That too limits the test range.

However the same method could be applied for deeper UV using a more UV-capable camera and a different UV-pass filter.

Then a BK7 might not be enough.

 

The original idea with pinholes is to have no transmission limitations.

My point above with the fused silica lens was just that there are no doubts, what so ever, for a good transmission with fused silica elements and pinholes, but that lens setup is faster.

 

All other alternatives are less efficient, even if they still might be OK for this testing.

 

It would be sad if anyone misunderstood and wrecked a 50/1.8 just to harvest oen of the lens elements.

Then it would be cheaper and better to buy one of the lenses form Thorlabs. 

 

 

[Andrea B.]

Question, does the darker purple/blue color indicate not as deep uv pass on a white balanced pinhole image?

 

Answer:

https://www.ultravioletphotography.com/content/index.php?/topic/2213-pinhole-lens-test-with-summary-of-test-protocols/

 

That's the link to the Pinhole Test instructions ("protocol").

I wrote there that:

• Lenses which have low UV transmission will show a dark violet-blue.
• Lenses which have high UV transmission will show almost white.

I also wrote there that that the Pinhole Test is not nuanced. I didn't write there but should have, that the results depend on what UV-pass filter you are using over the Pinhole.

 

And as a fair warning:  There were some points about using a Pinhole Test which we never did figure out.

[Nate]

Thanks Andrea