How I created an Autofocus fused silica lens for UV photography

[dabateman]

I have been wanting an autofocus lens for UV macro type imaging. As I like the in camera focus stacking mode on Olympus cameras.  So I have been looking seriously at the Nikon Z-mount cameras just because there is the Megadap autofocus adapter, which would allow my home made lenses to autofocus.

But the Nikon's don't have incamera focus staking and they have PDAF points on the sensor which causes banding. Then I noticed for the current autofocus adapters to work you need PDAF point on the sensor, which is something I don't like. I would like to get a nice contrast detection sensor moving forward. The Olympus EM5mk2 is already excellent this way and has in-camera focus stacking.

 

So I need to build a lens, and one that not only has autofocus but could be used on multiple camera systems.  So I looked at the current Canon EF mount lenses.  There are quite a few that use linear focus extension for the focus system. 

These include the EF 28mm F2.8, EF 35mm F2, EF 50mm F1.4 USM, EF 50mm F1.8 II, EF 50 mm F1.8 STM, EF 50 mm F1.2 L USM, EF 85mm F1.2 II USM, EF 50mm F2.5 compact macro and the Yonguo 50mm f1.8 lens.

 

The best option might be the Canon 50mm f2.5 compact macro, as it extends quite a lot. This is its component manual:

EF 50 2.5 MACRO (C0MPACT).PDF

 

But I was able to get a cheap EF 50mm F1.8 II lens and there are many YouTube videos online to show you how to take one apart. This is its component manual:

EF 50 1.8 II.PDF

 

The back comes off easy exposing the rear group. This group of three elements easily turns out, counter clockwise. I scanned the lens with and without the back 3 elements and the UV cut off was the same at about 360nm. This is the Raw scan:

 

This is an image of the Canon 50mm F1.8 II back with back 3 element group removed:

 

 

I then drilled through the back of the lens, through the front three elements and then carefully with a hammer and a bolt, taped out the front glass bits.  The first and third elements are very thick and of the same type of glass. The middle element is very different, it broke apart very easily when the drill hit it and sparked and flickered due to the drilling heat. 

So be careful in the removal.  The back end (maximum aperture opening is 20mm)

This is the font without any glass:

 

 

There is lots of room to add elements and since the whole front moves during focusing, it works well. 

This is the front with a 25mm filter element in place:

 

The cheapest option might be to add the SvBony 0.5x element to the front. My elements are 58mm focal length and work. But I would need a to make a spring and holder to place it for infinity focus.

 

So for mine I used black epoxy and glued in a fused silica plano convex 75mm focal length element in C-mount adapter. This is my final lens:

It focus very fast, almost too fast as its probably expecting more glass weight than just a single element.  So for best results you need to tap the shutter button twice. Once to get there and a second to lock on tight. The in-camera focus stacking feature works very well.

 

I don't have a good image with it yet, as my sun has been low and I didn't use a tripod for my first quick images sets. But will post some in-camera stacks later.

 

I also added a cheap empty metal 52mm ring to the front of the lens as the front filter thread is plastic. 

 

 

 

[Dmitry]

What are you used as EF-m4/3 adapter?

I tried Canon 50mm F1.8 II on my Viltrox EF-M2 and its high current focusing motors made adapter dead after like 10 minutes of use.

[dabateman]

I have been using it on a Metabones pass through adapter.  The Metabones adapters tell the camera that a Panasonic Leica micro four thirds 14-150mm f3.5/5.6 lens is being used.  So focus stacking works with them. All other adapters pretend to be the Olympus MMF3 adapter,  which doesn't support focus bracketing. As the camera thinks a four thirds lenses is on the camera. 

 

So far so good.  The Yongnuo 50mm might also be a good option,  as the focus motor is slower and its all screws not plastic snap in parts. Also a cheap lens.

 

This is how the Yongnuo comes apart:

https://www.lenstip.com/425.3-Lens_review-Yongnuo_YN_50_mm_f_1.8_Build_quality.html

 

You just need to glue into the large front section as the whole thing moves forward and back like the Canon. Also the Aperture control can be protected.  I needed to fix mine after hammering,  as I got a shard stuck near the aperture.  But my aperture works and is fully controlled by the camera.  Only F8 is really F12, both being 6mm diameter opening.  I just have a 75mm not 50mm lens. 

 

The IBIS reports as a 50mm lens. So I have to be careful handheld. 

 

[Stefano]

Nice experiment. I think there is room there to build a nice multi-element lens, with maybe 3-4 elements. Do you still have the aperture blades? If so, are they close to the lens?

[dabateman]

Yes my element is about 4mm infront of the aperture. The aperture is fully working.

What's odd is the maximum opening is 20mm. So the front group is acting like a 100mm lens and the rear 3 elements is also working like a 100mm element,  to boost that apparent f2.5 to f1.8. 

However,  with the lens set to f8. The opening is 6mm in diameter.  That's the correct opening for a 50mm lens but not a 100mm. So not sure what's going on. 

Anyway set at F8 via the camera results in a 6mm opening,  which is f12 for my 75mm focal length element. Which is a nice sharp point. 

 

The rear 3 group is pushed into a plastic holder and glued into place.  You could use that holder as well for lens design.  The plastic easily slots into the main lens and moves with the aperture and front group. 

 

[colinbm]

Good that it works for you Dave.

[JMC]

Love this David. Great little project, thanks for sharing.

[dabateman]

30 minutes ago, JMC said:

Love this David. Great little project, thanks for sharing.

Yes after I had it apart,  it was clear that if I ever get a Thorlabs 103mm Achromatic doublet cheap or given to me (I open to anyone sending one as a Christmas present) it would mount perfectly inside. 

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=6802

 

You would just need to add electronic extensions tubes to gain closer macro focus. 

I currently use a 10mm tube to get closer focusing with my 75mm as it will focus beyond infinity and without tubes the closest focusing distance is 150 cm.

 

[colinbm]

Very neat Dave, I am understanding it a bit better now that you have shown the lens that you want to place onside the Canon lens body.
Besides having the lens straight & square in the opening, does it matter for its exact location, the contrast focusing takes care of it ?
So you still have electric aperture & focus control ?

 

[dabateman]

As close as possible to the aperture is really it.

The automatic aperture works,  but what the camera says wouldn't be true. As in setting the camera to f8 results in 6mm diameter opening.  So for a 75mm element thats F12. For 100mm element that would be F16.

Yes the contrast autofocus takes care of autofocus.  So the camera tells the lens where to move until the on sensor information is correct. 

 

Thus why I looked for the best linear extension focus lenses.  The Canon 50mm macro moves the barrel more forward and back.  So might work better with a 100mm element stack.

 

I have to measure the plastic rear holder. But it might also support a 100mm element stack.

[colinbm]

Good, thanks Dave
You had better start putting flowers in the kitchen & note on the fridge door...

[Doug A]

What a project. Never considered drilling thru a lens and knocking the glass out. You are dedicated.

 

Thanks for sharing,

Doug A

[dabateman]

5 hours ago, Doug A said:

What a project. Never considered drilling thru a lens and knocking the glass out. You are dedicated.

 

Thanks for sharing,

Doug A

Well if you look at the front image without glass,  you can see scratch marks around where the first element was.

I first tried to cut out the elements.  But they are all glues and pushed in tight within the plastic.  For both the front and rear sets of three. These clearly are not meant to be removed. 

I also didn't want to take apart the front section,  as risk of breaking a plastic clip that holds the autofocus together.  Since autofocus was the whole point. So then figuring out solvents to dissolve the glue and not the plastic, was out of the question. 

Thus left with brute force of drilling and taping out.

Since I first cut away parts from the first front element,  I have it intact with slight drill indentation. Might be fun to see what an image through  it looks like.

I snagged my copy with dust on the underside of the first group. Which is super easy to clean.  But allowed me to get my copy for under $50.

So for cheaper than a manual aperture control ring and helicoid, I got autofocus and auto aperture. 

 

[Doug A]

Thanks for the extra info. Project was a lot of work but the payoff is big. You probably have the world's only AF UV lens. Well done.

 

The Thorlabs page is interesting. Their UV doublets appear to have AR coatings. So there must be some coatings that don't kill UV. Good to know.

 

Thanks,

Doug A

[Andy Perrin]

Quite cool, nice work David. 

[JMC]

Thanks for starting this topic Dave. I've got a Nikon 50mm f1.8 AF lens from a charity shop and intend to fit either my 79mm UV aspheric from Thorlabs, or the 80mm OptoSigma triplet UV lens into it (not decided which way to go yet). Nikon 50mm lens is currently apart and all original elements removed. Some more tinkering to do, and want to get some AF extension tubes as the whole thing needs moving about 20mm further from the camera with these 79mm and 80mm lenses. The idea with using a Nikon AF lens was to retain a manual aperture ring as well as being able to use autofocus and aperture on the camera. Also, with a Nikon lens body I can use it on my Nikon, Canon and Sigma Sony bodies.

[dabateman]

I got a Canon 50mm f2.5 macro to work on now. Its a little bit more complicated to remove the elements.  I will need to most like drill through the three sections. 

Between the Canon 50mm f1.8 ii and the Canon 50mm f2.5 macro,  the ii was easier to preserve some of the glass.

I want to spec the 3 sections first before major damage.  Maybe one group is ok for UVB, or not. Who knows.

 

How are you working the Nikon lens on a Sigma body? Do you mean the Fp? I don't think you can mount it to the SD14 unless you have a mount kit.

Also wouldn't it only autofocus on the Nikon? Although if you go with the Z-mount,  would also work there. Canon EF just seems so universal now, for autofocus. 

[JMC]

Sorry, I meant to say Sony not Sigma. I'll correct that.

 

Good luck with the macro lens. I'd like to use a Nikkor micro, if I can find one at a reasonable price.

[dabateman]

Ok just scanned the 50mm f2.5 macro sections.

The first 3 elements glued in the plastic holder cut off at 340nm.

The middle 4 elements, which are very hard to remove cut off at 366nm.

The back two elements which pop out with the plastic mount frame cut off at 330nm.

 

The single plano convex front element from the Canon 50mm f1.8 ii lens that has a bit of a drill hole in it, cuts off at 317nm. So if I was more careful cutting that out, then putting it back in. You could get a decent UVB autofocus lenses. 

[lukaszgryglicki]

Man, just start to produce and sell them :P (actually I'm a bit serious).

[dabateman]

Well an update on the Canon 50mm f2.5 macro lens. I drilled through the internal four elements easily.  It was very clean and quick through the aperture.  Everything works, but I got some glass dust into the focus ring. Now it makes an odd noise when you try to focus manually. 

Things I don't like about the 50mm macro:

The autofocus is very slow. 

It hunts from end to end.

You can't manually focus really, it must be on and you can't override it like the 50mm f1.8 ii.  It ignores your ring motion if not set to MF.

Thing I like:

There is a huge space to add lenses. 

A 100mm element works inside. 

 

I bought mine cheap from Japan,  as it was suppose to have fungus on the lens. But it didn't.  Only the first element had spots and they are most likely oxidation.  It could have been polished off and I probably could have sold it for twice what I paid. But I want it as an auto aperture autofocus tube. So that so far is working. 

 

I might design a CaF front element with fused silica back for correction into UVC. Or I might just keep it as a fun AF tube. That project might move to the 50mm f1.8 ii lens. 

 

The Canon 50mm f2.5 macro tube works very well with the Milar 50mm f4.5 lens on front for automatic in camera focus stacking on my Olympus cameras. 

[JMC]

Minor update from me on my autofocus experiment. I got a Nikkor 50mm f1.8 AF as my starting point. Lenses are all out, and the focus and aperture still works. Out of interest I thought I'd measure the transmission of each element. The design of the lens is a 6 element 5 group, with one cemented group. It's not multicoated according to Ken Rockwell - https://www.kenrockwell.com/nikon/5018daf.htm

 

This is the lens design, again from Ken's page linked above. I've added in 1-6 in red for the elements.

 

And here's how they looked after removal.

 

Transmission was quite interesting.

 

Note, I've included a 'Theoretical' line as well, which would be all the elements combined. Unfortunately I was so excited when it arrived that I took it apart before measuring the transmission of the lens, so don't have an 'unmodified' lens graph to compare against.

 

The cemented doublet seems to be the one restricting the transmission the most (as expected). What surprised me though was element 6 (the rear element). This is a plano convex lens with a focal length of about 65mm, and it has a really deep UV transmission unlike the 3 single elements at the front of the lens. I wonder what glass they used for that?

 

Anyway, sharing in case it is of interest.

[colinbm]

Looks good Jonathan, can you match the glass with quartz ?

[JMC]

Hi Col. At the moment I'm just going to use the lens body to hold and focus basic fused silica singlet lenses. I'm not planing anything more complicated, like trying to make a multi element lens.

[dabateman]

2 hours ago, colinbm said:

Looks good Jonathan, can you match the glass with quartz ?

Colin that will not work.  Different glass types and coatings are used to correct for distortion as well as the different shapes.  You typically have lower refractive index glass in the front followed by high refraction index element behind it.

For us than fused silica is the middle, between CaF2, which has lowest refractive index,  than Al2O3 (sapphire lens) has the highest refractive index. 

Its hard to find elements in sapphire.  But CaF2 and fused silica are easily available. 

Also air between the elements also acts like a lens and its thickness matters too.