[Arjay Bee]

That looks awesome

[ThorSanchez]

The time is 0230. I thought it was 0130 but the clocks went forward apparently.

That's the lens finished - well - the Mk-20 (42.8mm) is finished!

I'll lie in tomorrow (because it's Sunday and I can do that!) and then put together some demo shots!

It's so good to finally get an actual functional lens after all that work! It took ages! It took 48 revisions for the Mk-20... I was hoping to stop at 42 because I like that number (it's Douglas Adams' fault that I like it!) - but I ended up moving the aperture slot back as far as it would go to reduce vignetting.

A quick play around gives us the following specs:
Focal length 42.8mm
Aperture is F/1.7 wide open to about F/16 before severe vignetting occurs (this can be reduced further with a special clip-in adapter that goes in the front and holds a different plate a few mm in front of the lens).

Focusing range is 10cm to infinity, which I consider to be very good.

It is a single-element lens and has severe CA, which is to be expected.

Demo shots coming soon!

That's awesome. The staff absolutely needs to to an in-depth, front-page review of this, like this. Completely dead-pan, serious review like Pentax just released a * lens. It would be great, ridiculous, hilarious; perfect for these shut-in corona virus times. You need to come up with a name for it, like the Robertson Prime Optics DQT 42.5mm f/1.7 ZP NW.


I have a new hot end on order for my 3D printer, you've inspired me a little to print some more but it's acting up. All kinds of little faults and problems, it's only a $200 Monoprice mini, but I need to get it back in top form. Experimenting with stuff like this is wonderful. Closest I've come (and that's not very close) is printing a K-mount telescope adapter, some Pentax battery holders, and hotshoe covers.

[cprobertson1]

That's awesome. The staff absolutely needs to to an in-depth, front-page review of this, like this. Completely dead-pan, serious review like Pentax just released a * lens. It would be great, ridiculous, hilarious; perfect for these shut-in corona virus times. You need to come up with a name for it, like the Robertson Prime Optics DQT 42.5mm f/1.7 ZP NW.


I have a new hot end on order for my 3D printer, you've inspired me a little to print some more but it's acting up. All kinds of little faults and problems, it's only a $200 Monoprice mini, but I need to get it back in top form. Experimenting with stuff like this is wonderful. Closest I've come (and that's not very close) is printing a K-mount telescope adapter, some Pentax battery holders, and hotshoe covers.

Man, I'd love that - I should print a second one and send it in!

I'm actually pretty sure that's a rebadge of the same printer as mine (or rather, we're both rebadges of some random Chinese make) - I have a wanhao I3 mini. Given the price it's am amazing printer - obviously not up to the same standard as a £500+ model, but it's remarkably good for what it is. Granted, you do spend more time doing maintenance on it than you would with a more expensive one...

I never managed to get the demo shots today - I'm still at work just now so I decided to spend today getting housework done since I wont get much of it done during the week. I will, however, hopefully take the camera out after work tomorrow for our daily permitted walk as-far-away-from-other-people-as-I-can-manage!

I've also discovered that my F/16 aperture is not directly visible, but causes a weird inverse-vignetting.... the edges of the image are white. If you move the sensor (composition adjust) you can see its white, then a rainbow, then the edge of the aperture - so it appears that the aperture size is SO close to the field-of-view of the lens that it's diffracting into it. (Kife-edge diffraction).

Basically, the penumbra of the aperture is in the field of view, even if the aperture itself isn't.

I seem to recall that camera designers use twice the FoV when designing their lenses - but I didn't know the specifics. I suspect this is part of the reason.

It would appear that the edges of the frame are aperture-limited - but it does still work at F/16. I suspect a lens hood would help too by reducing off-angle light from hitting the aperture in the first place (although it is already pretty limited).

I'm currently print F2, 4, 5.6, 8, 11 plates, along with a few fun ones (snowflake-bokeh is amazing. It looks cluttered and angular - like you're looking through a kaleidoscope!)

I've just plugged the SD card in, so I can grab a sample shot or two - though they won't be anything of terrible interest (just shots I took around the house to make sure it was working right)

Will post up shortly.

--edit-- I'll need to get this posted up on thingiverse as well - I'll aim to get that done this week

---------- Post added 2020-03-29 at 15:00 ----------


Shot through the back window (which is filthy... that mess is on the window, not on my lens!)


Quick and dirty sharpness test at F/8


Here's that white vignetting I was talking about - I believe it to be knife-edge diffraction from the aperture edge, but it occurs right down to F/5.6 (in fact, it appears to be wider at 5.6, making me think something interesting might be happening (though I cannot say what exactly)

Still, it is definitely something I can live with - if I need a smaller aperture I can make an adapter for them (I'm thinking of a cone that holds a little disk near the lens element - just slot it in the front).

Speaking of slotting things in the front, a lens hood might help too - I've built the lens to accept a 49mm thread, so I plan to put a 52-49mm thread later in there to give the lens a 52mm filter thread that is metal and won't wear down as quick as plastic will.

---

Will hopefully get actually-interesting pictures taken during the week and I can really show it off properly!

[cprobertson1]

[NOTE: just for fun becuase I have a spare ten minutes and could do with a cheering up. ]

So how I was thinking of names... how does this sound?


The Robertson Mk-20 "Squiggles" FDM 42.8mm F/1.7 AA RR SSMC MC WA-M-PK*



Mk - "Mark" - like a version number but cooler-sounding. Kinda.

"Squiggles" - the operational codename. It is best not to ask how it got this name.

FDM - Fused Deposition Modeling (3D printed) - how it was constructed

42.8mm - the absolute minimum lens focal length you can fit in the unit without it hitting the mirror.

F/1.7 - the aperture of maximum aberration, coinciding with the maximum aperture

AA - Anti-Achromatic - this is not just achromatic, it's actively chromatic

RR - "Rainbow-Ready" - the lens produces more rainbows than a My-Little-Pony-Convention, but the resulting images are noticeably less-weird. Probably.

SSMC - "Super-Smudgy-Multi-Coating" - multi-coated with super-smudgy fingerprints. Quality control isn't great, but I guarantee all lenses have at least six layers of fingerprints.

MC - "Marco" - this is a Marco lens - similar to a Macro lens, but something got reversed somewhere.

WA - "What Abberation?" - If an image is 100% aberration, then there is no aberration - it's just an image.

M - Manual - this actually refers to the lack of a user manual, rather than the focusing mechanism.

PK - not just the mount, also stands for "Potent Kewlness" referring ot the level of kewlness inherent in the system.

* - a footnote that doesn't actually refer to anything - it's just here to confuse people.

---

Last edited by cprobertson1; 03-30-2020 at 12:04 PM.

[ThorSanchez]

[NOTE: just for fun becuase I have a spare ten minutes and could do with a cheering up. ]

So how I was thinking of names... how does this sound?


The Robertson Mk-20 "Squiggles" FDM 42.8mm F/1.7 AA RR SSMC MC WA-M-PK*



Mk - "Mark" - like a version number but cooler-sounding. Kinda.

"Squiggles" - the operational codename. It is best not to ask how it got this name.

FDM - Fused Deposition Modeling (3D printed) - how it was constructed

42.8mm - the absolute minimum lens focal length you can fit in the unit without it hitting the mirror.

F/1.7 - the aperture of maximum aberration, coinciding with the maximum aperture

AA - Anti-Achromatic - this is not just achromatic, it's actively chromatic

RR - "Rainbow-Ready" - the lens produces more rainbows than a My-Little-Pony-Convention, but the resulting images are noticeably less-weird, probably.

SSMC - "Super-Smudgy-Multi-Coating" - multi-coated with super-smudgy fingerprints. Quality control isn't great, but I guarantee all lenses have at least six layers of fingerprints.

MC - "Marco" - this is a Marco lens - similar to a Macro lens, but something got reversed somewhere.

WA - "What Abberation?" - If an image is 100% aberration, then there is no aberration - it's just an image.

M - Manual - this actually refers to the lack of a user manual, rather than the focusing mechanism.

PK - not just the mount, also stands for "Potent Kewlness" referring ot the level of kewlness inherent in the system.

* - a footnote that doesn't actually refer to anything - it's just here to confuse people.

Yes! That's perfect.

[cprobertson1]

Here we go - had the Roberson Mk-20-48A out today after work (not long after naming it actually!




It has surprisingly good center sharpness! I haven't touched up the image (which is why it has a coloured haze (haven't a clue why its purple) and isn't correctly colour-balanced on top of that (my bad).



This is the snowflake aperture, it gives a very cluttered looking background that I just love.

Again, note I never touched up the images - practically straight-out-of-camera.




Here's that weird vignetting I was talking about... Having had a think about it, I think it is first-order fresnal diffraction from the edge of the aperture. I am not sure how to prevent it, but a lens hood might help, maybe.


There are more images in the album - I'll hopefully post more up as I play around with it. It's a terrible lens - but it remarkably fun to use. I definitely loves closeup shots - landscape shots aren't great. I suspect it'd be an interesting portrait lens as well!


Now... onto phase II! Upgrading to an achromatic doublet. The Mk21 will be a longer-focal length achromatic doublet, and will hopefully have the aperture close enough to the lens to avoid that weird vignetting!

The 21 will consist of a modification to the inner-barrel of the lens, rather than an outright upgrade. The entire "outer" barrel should be able to stay as-is.

I purchased some achromatic lenses:

FL 66.5mm (22mm dia) (max F/3)
FL 75mm (20mm dia) (max F/3.75)
FL 110mm (42mm dia) (max F/2.6)

The 110mm is interesting in that the maximum theoretical aperture is pretty wide - but I think I'll start at the lower focal lengths and work my way up.

[Arjay Bee]

These are great!

Kind of replicates the vision one has after going for a check up at the opticians after having those drops - wide open and very bokehed around the edges but sharp as in the centre. Like you I enjoy the near focus ones of the gorse blossom. Blues ad yellows look good -how is it on oranges and reds?

I look forward to the other focal lengths.

[cprobertson1]

These are great!

Kind of replicates the vision one has after going for a check up at the opticians after having those drops - wide open and very bokehed around the edges but sharp as in the centre. Like you I enjoy the near focus ones of the gorse blossom. Blues ad yellows look good -how is it on oranges and reds?

I look forward to the other focal lengths.

You know something... those drops cause dilation of the pupil, right? I can't say I've ever had them (I'm not even sure its a thing we do in the UK - I've never heard of anybody telling me they've had them? Can anybody else confirm?) - but I reckon that if they do cause dilation, then the effect you've seen with the drops in is in fact the exact same phenomena observed with the lens - your eye's aperture (about F/2.1 when fully open) lets in lots of stray light from high angles of incidence - just like my lens is doing.

How cool is that!

That's a good shout on the oranges/reds - I can't think of anything offhand that is actually orange or red... but as soon as I find something I'll snap a shot of it!

I'm looking forward to trying out the higher focal length lenses - though they'll be a while in getting here (about a month).

Because the longer focal lengths are physically further forward inside the inner barrel, they'll be closer to the aperture, which should greatly reduce the vignetting (and allow me to close down the aperture more), and since they're achromatic doublets, CA should be greatly reduced too. Based on playing around with lenses held in front of the camera body (DUST NIGHTMARE by the way), the longer focal lengths should have reduced spherical aberration as well.

At some point I'm going to play around with a two-element (technically three-elements in two groups if an achromatic doublet is used in front) - it will be interesting to see what happens in terms of CA when you add a second lens into the mix (and it will be interesting to see how the textbooks say how to deal with it).

I'm not at that stage yet - still playing with single-lens designs. In fact, I probably won't bother with a two-group setup unless I need a longer or shorter focal length and have to "extend" or "shrink" the back-focal-length (BFL) to fit in the design envelope - otherwise a 300mm lens would be ~255mm long (300mm in front of the sensor) - which is possible but unwieldy. You reach a point where your lens is just a big ol' telescope!

In the meantime though, time to play with this lens and learn what I can from it before building the next one!

I feel kinda bad about dismantling this one to build then ext one - I'm out of the nuts and bolts I used to construct this one, and I only have one 52mm-filter>PK adapter... Hrm, decisions decisions.

[Arjay Bee]

Your efforts reminded me of this article:

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

Something to read while you await the next resources. There are more resources on the Interwebs and a full doc was created about the manner in which Frazier and Co was ripped off.

[cprobertson1]

That lens looks like so much fun. I had very briefly skimmed the article about a month back when I was looking at achromatic arrangements (it was one of the "see also" articles listed at the bottom of a random page) - but I never looked closely enough to realise it had an articulating head - I just saw "massive depth of field" and promptly forgot about it.

That's a really cool lens - it apparently works by using a small aperture and by tilting the focus plane (which I believe would be considered a distortion of object-space? Again, don't quote me on that - I'm not a master of the terminology (or even the theory) - more of a bumbling novice! The one thing I know is that I know nothing!).

It makes me want to make a periscope lens with an articulating head as well - it'd take some designing though. The actual lightpath isn't hard to design - but the lens alignment would have to be spot-on to avoid distortion - which means everything will have to be very finely adjustable. I'm not saying I can't do it, but I dont think it'd be worth the effort. In terms of time, I'd "pay" the same as I would for a professionally made one.

On the other hand, I did draw up a sketch for a semi-submersible "probe lens" version of the Mk-20. Using two planoconvex lenses in opposite directions I can create a collimated "tunnel" of light - allowing me to place the end of the probe maybe a foot away from the camera body (the longer it is, the harder it is to collimate). It'd be made from PVC pipes/plumbing parts, because it's cheap and easy to work with - the submersed end could be made watertight with o-rings and gaskets, and the probe would just fit on the inner-barrel of the Mk-20, allowing focusing across a small range.

It'd be a terrible lens, again, but it would be able to take closeup shots underwater. I have no idea what I'd use such a lens for, but with a submersible lens, I'm sure I'd find something eventually.

Purely theoretical idea though, but I don't see any technical reason why it can't be done. Obviously, the image quality would be poor again (roughly the same as the above shots - but zoomed in 1.4x because of the refractive index of water), but for the sake of having fun, I reckon it'd be very possible.

---

Speaking of possibilities - I have a problem. I dont have enough screws and nuts to build a second lens with a different focal length... and I dont have enough adapters for the metal K-mount.

But I just had an idea - bore out the inner barrel and put a bayonette on it - we can then slot in a new lens-assembly while keeping the focusing mechanism intact.

Basically, we'd be removing the inner cone with the lens attached.

Ah, wait... the aperture plate goes through the outside of the inner barrel... that won't work... but wait! Yes it will, only the 42.8mm needs the aperture at that location: lets suppose I keep the Mk-20 [42.8mm-F/1.7] as an intact unit and build a new assembly for the 66.5, 75 and 110mm versions, with interchangeable inner lens assemblies. It's like how a caveman might build a zoom lens.

That sounds like a useful way of building it actually - a portion of the inner barrel would need to stay, but with a bayonette on the front we could make a new "liner" with the lens in it. It might be a problem for longer lenses - but having said that, 110mm is only 30mm in front of the existing assembly so it's not terribly awkward.

I think the Mk-21 [66.5-110mm F/3.0>3.75>2.6] is going to be a very special lens indeed... if only for the weird way I had to write the f-stop range!

I also feel I should keep the "original" Mk-20-48A (now known as the Mk-20 [42.8mm-F/1.7]) intact - it'd feel wrong to kill the firstborn (excusing the fixed-focus versions that came before it).

Who knows, it might even become a collector's item someday - a super-creative art-lens (which is another way of saying "bad image quality" but we need to make it marketable) for super-creative peoples. £500. No, £800 - it's one-of-a-kind! I mean, I wouldn't pay anywhere near that... maybe £50... but it's a super-art-lens for the super-creative-type photographers - which is why I will give it to the first person who gives me £800, plus handling and shipping fees, tax, error reduction, homeostatic processes, electricity generation, customisation options, and spellchecking - call it an even £2000?

[Arjay Bee]

Or follow Frazier and take it to Hollywood. Sell it to studios so as to be used in art and independent movies.

[cprobertson1]

Or follow Frazier and take it to Hollywood. Sell it to studios so as to be used in art and independent movies.

I wonder how that lens would behave in a cinematographic setting, actually... walking around with it would probably make the audience feel drunk.

[ThorSanchez]

Here we go - had the Roberson Mk-20-48A out today after work (not long after naming it actually!




It has surprisingly good center sharpness! I haven't touched up the image (which is why it has a coloured haze (haven't a clue why its purple) and isn't correctly colour-balanced on top of that (my bad).



This is the snowflake aperture, it gives a very cluttered looking background that I just love.

Again, note I never touched up the images - practically straight-out-of-camera.




Here's that weird vignetting I was talking about... Having had a think about it, I think it is first-order fresnal diffraction from the edge of the aperture. I am not sure how to prevent it, but a lens hood might help, maybe.


There are more images in the album - I'll hopefully post more up as I play around with it. It's a terrible lens - but it remarkably fun to use. I definitely loves closeup shots - landscape shots aren't great. I suspect it'd be an interesting portrait lens as well!


Now... onto phase II! Upgrading to an achromatic doublet. The Mk21 will be a longer-focal length achromatic doublet, and will hopefully have the aperture close enough to the lens to avoid that weird vignetting!

The 21 will consist of a modification to the inner-barrel of the lens, rather than an outright upgrade. The entire "outer" barrel should be able to stay as-is.

I purchased some achromatic lenses:

FL 66.5mm (22mm dia) (max F/3)
FL 75mm (20mm dia) (max F/3.75)
FL 110mm (42mm dia) (max F/2.6)

The 110mm is interesting in that the maximum theoretical aperture is pretty wide - but I think I'll start at the lower focal lengths and work my way up.

I love it. This is what I wanted my Helios or Lomo Daguerreotype to look like but never quite get there on APS-C. I was going to try reversing the front element of the Helios 44M to get a look like that, but it turns out that it's other Helios lenses that you can do that with; I took apart the 44M, couldn't get the front element reversed and it took me months to put it back together correctly.

I will definitely download your files once they're on Thingiverse and play around with this. I'll just need to find a source for some glass...

[cprobertson1]

I love it. This is what I wanted my Helios or Lomo Daguerreotype to look like but never quite get there on APS-C. I was going to try reversing the front element of the Helios 44M to get a look like that, but it turns out that it's other Helios lenses that you can do that with; I took apart the 44M, couldn't get the front element reversed and it took me months to put it back together correctly.

I will definitely download your files once they're on Thingiverse and play around with this. I'll just need to find a source for some glass...

Alibaba is surprisingly decent for lenses, including achromatic doublets.

Of course, the sky's the limit - you can pay £500 for a single glass cell if you want - I just... wouldn't!

I had better get those files prepped for thingiverse soon actually - I'm starting to procrastinate over renaming the currently unlabelled parts of it. It's workable as-is, but I'd rather label things properly so future users can figure out what's what (where's where (and why's why)))

[cprobertson1]

Robertson Mk-20-48A 42.8mm F/1.7 Art Lens by cprobertson1 - Thingiverse

That's it now on Thingiverse, source files included.

I'll post up assembly instructions at some point (it involves a lot of sanding, some filing, and then you have to assemble it in a particular order. It's fun!)