Lens design #2

[Stefano]

I found the way to change the input wavelength, I slightly modified the lens to optimize it for 340 nm (I just changed some distances, the elements are the same).

 

I'm trying to make the first 3 elements fit inside a Thorlabs 1" tube, which is a bit tricky when trying to preserve image quality, but I am almost there.

[ulf]

Please do not see me just as pessimistic and boring guy when discussing this project.

 

I have been along for a long time involved in many different development projects.

During that time I have learned that such projects are not only limited by laws of physics.

There are some other laws that almost always also come into play, the Murphys laws:

https://www.cs.cmu.edu/~fgandon/miscellaneous/murphy/

 

Projects are driven forward by entusiasts and project leaders.

I am glad to see the bold drive about this from Stefano.

It seams like he is doing this as a learning exercise and that the goal mainly is the journey, not the final physical lens.

To follow the progress is quite interesting.

 

I hope this will turn out to give a useful design in reality too, but fear that some will set their expectations too high and then get disappointed by the results.

 

[lukaszgryglicki]

Yep, but I'm under impression that this is for learning & fun/joy of creating a tool.

 

[Stefano]

The main goal is trying to build a reasonably cheap UV lens (at least cheaper than Universe Kogaku lenses or a UV-Nikkor) that has still reasonable image quality. I would like to build one such lens myself for UVB photography and UV TriColour, although I won't do that in the near future, I want to perfectionate the design first.

 

My camera (Canon EOS M) has a sensor coverglass that is at least partially transparent to UVB light, but being a color sensor I need long exposure times at high ISO to get an image at all, and I can't afford to stop down the lens beyond about f/8, otherwise there would be too much noise in my images. That's why a lens with acceptable image quality already at f/2.8 or f/4 would benefit me.

 

I am sharing the designs both to have them checked and improved upon and also to make it possibile for anyone interested to build such a lens. The learning process is also worth it.

 

I think this is an example that shows how necessity drives ideas. The main reason I am doing this is that commercially-available UV lenses are too expensive for me at the moment.

[lukaszgryglicki]

I'm warning you:

- Nikon D600 with CFA intact is almost totally useless for UV-B with any lens: UV-Nikkor Or UKO 50/3.5.

- Nikon D600 with CFA removed is also almost useless (I wonder why?) - it should be about the same as mono Fuji 50R right? In Live-view? But Fuji allows focusing with just a bit of lag while Nikon's Lv is black, even on mono one - you need to guess everything - not a fun.

 

I will soon convert my another Fuji 50R to full spectrum but without touching CFA, so I will have a comparision (I'm buying Fuji 100s as a normal daily camera - unconverted, so I can convert my 50R).

 

[Stefano]

Do your Nikon D600's have a fused silica window?

[lukaszgryglicki]

It has 280+ nm passing glass (Schott 280 or something - not quartz/fused silica) but then still - supposed to pass from 280nm - live view is black, but then I'm able to make a photo just fine, but must guess focus and is overexposed according to what Lv says, possibly by 3 stops or more, so I also have to guess exposure. It's not very comfortable to do UV-B photo with it - but possible. There are also many light leaks when pushed too far with ISO or time (I've black-taped everything possible long time back) - looks like Nikon bodies start to leak everywhere when pushed too much - this is not happening at all with Fuji GFX.

 

[Stefano]

A few updates:

 

I slightly redesigned the lens to make it "buildable" (I only changed some distances, the elements are the same). I tried to make a diagram, it took me a few hours because I had to resize the images to have the same scale etc. Probably it is precise to 0.2-0.3 mm, which is about the same precision I think I might achieve when building a lens.

 

 

The aperture should be mounted in the middle (connecting the tubes), using an M42 to SM1 adapter on the left side (Thorlabs SM1A49). My aperture (which can be easily found on eBay) has male and female M42 threads.

 

I measured some distances with a caliper:

 

This is the front of the aperture, with a Thorlabs SM1A49 adapter screwed on top. This side (with the adapter shown in the image below) should be screwed on the left tube in the lens diagram.

 

This is the other side, with a Thorlabs SM1A48 adapter. Since the threads of the adapter are 3.1 mm deep (according to Thorlabs), the distance between the flat part of the adapter and the aperture blades is 9 mm.

 

Below some simulations, using this new design:

 

Spot diagrams, at f/8 and various wavelengths (some field curvature is evident in the corners). The maximum ray angle on a Canon APS-C sensor is about 15° in the corners.

 

Field curvature improves at shorter wavelengths.

 

200 nm:

 

310 nm:

 

400 nm:

 

Distorsion (I still have to understand how to use this tool, there are "F-tan theta" and "F-theta" settings). Below distorsion at F-tan theta:

 

This lens has noticeable barrel distorsion.

 

Focal length (I') as a function of wavelength:

200 nm: 44.8424 mm

250 nm: 49.2878 mm

300 nm: 51.6018 mm

350 nm: 52.9759 mm

400 nm: 53.8643 mm

 

"Image Distance" (I think the distance between the rear element and the sensor) as a function of wavelength:

200 nm: 50.4706 mm

250 nm: 55.4114 mm

300 nm: 57.9624 mm

350 nm: 59.4712 mm

400 nm: 60.4445 mm

 

It is clear that refocusing is needed when changing wavelength (and chromatic aberration would be very noticeable in wideband photos).

[Stefano]

The lens displays mechanical vignetting at f-numbers smaller than about f/4.55.

 

This does not take into account vignetting caused by lens rings or Thorlabs tubes.