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03-15-2020, 01:51 PM - 1 Like   #31
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Project update!

Got the rear bits printed (the moving bits) which is good enough to test the idea, but the front is missing...

I had to stop printing because the filament started to fail - looks like moisture/humidity problems, its snapping before it even makes it to the printer!

New filament ordered.... I took the suggestion of @ThorSanchez and bought black PLA - actually, it's "matt black", but to be honest, I dont think it will matter too much!

03-16-2020, 04:46 PM - 2 Likes   #32
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Just a mini-update!

Using the fragments of filament that remained on the reel, I printed off about half of the lens. Managed to run tests with the focusing mechanism, allowing me to adjust some of the dimensions to hopefully improve the final model.

Here's a pic taken while I was testing the infinite focus - the shot is wide open (aperture F/1.7 - the new glass lens element is 42.3mm focal length and has an clear aperture of 25mm giving an t-stop of F/1.7 ish).

Center sharpness is decent, especially given that it's wide open.

Hazing is ridiculous as practically everything is a light leak. It improves drastically when I use my fingers to make a crude aperture! Just need the new filament and we should be good for a real test run!
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03-18-2020, 03:57 AM   #33
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QuoteOriginally posted by ThorSanchez Quote
I own the Lomography Daguerreotype Achromat 64mm. I'm not selling it, but if you have some specific questions I could potentially answer them.
Could you do me a quick favour of you can spare the time?

I'm curious about two things that can be figured out by looking into the front of it.

1 - the appropriate distance from the aperture plate to the lens element. I reckon it should be less than the focal length?

2 - are there baffles or ridges on the inside barrel, or is it smooth? Part of my using problem is due to reflected light inside the barrel of my lens assembly - baffles may help - was just wondering if they had the same problem that I did.
03-18-2020, 04:07 AM   #34
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QuoteOriginally posted by cprobertson1 Quote
Could you do me a quick favour of you can spare the time?

I'm curious about two things that can be figured out by looking into the front of it.

1 - the appropriate distance from the aperture plate to the lens element. I reckon it should be less than the focal length?

2 - are there baffles or ridges on the inside barrel, or is it smooth? Part of my using problem is due to reflected light inside the barrel of my lens assembly - baffles may help - was just wondering if they had the same problem that I did.
1 - About 145mm

2 - Smooth

Hope this helps. I could take some pictures if you need it.

03-18-2020, 11:18 AM   #35
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Wow - that's much longer than I expected!

That should be fine for now - many thanks!

I think I'll add ridges to it - pretty sure a lot of the hazing I'm experiencing is caused by stray light reflecting off the inside of the barrel before it hits the lens.

I'm having problems with the distance between the aperture plate and the lens - it seems to be too far, but I can't shrink it much without causing other problems.

I've decided the Mk20 will just have to have to live with the vignetting at *smaller* apertures - though I have a cheat-ey plan to make a smaller aperture plate that can be held on an insert that goes on the front of the lens, and holds it much closer to the element. It's kludgey, but it'll work if it comes to it!
03-18-2020, 12:40 PM - 1 Like   #36
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I feel privileged to gaze upon such greatness. Epic work here!
03-19-2020, 09:49 AM   #37
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QuoteOriginally posted by Wasp Quote
I feel privileged to gaze upon such greatness. Epic work here!
Fingers crossed it works out!

...fingers crossed I get my new filament soon!

Serious question for folks...

The current design uses a 42.8mm lens (25.1mm dia, max F/1.7) - it is a single lens element and will suffer from (probably quite severe) chromatic aberration.

I can solve this with a dichromat lens.

I have a 39.5mm dichromat lens (18mm dia, max F/2.2) that will fix that.

Which should I go with? Extra light, shorter DoF... Or less light but longer DoF and greatly reduced CA?


Last edited by cprobertson1; 03-19-2020 at 10:36 AM.
03-19-2020, 10:54 AM   #38
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I would go with less chromatic aberration.
03-19-2020, 01:27 PM   #39
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QuoteOriginally posted by cprobertson1 Quote
Which should I go with? Extra light, shorter DoF... Or less light but longer DoF and greatly reduced CA?
If you have sufficient hardware to assemble both, print up both. Otherwise use the dichromat.
03-19-2020, 01:47 PM   #40
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QuoteOriginally posted by Arjay Bee Quote
If you have sufficient hardware to assemble both, print up both. Otherwise use the dichromat.
I'm a little short on long thin (M2x30mm) screws - the bigger problem is the extra 3 degrees FoV (diagonal) on the 39.5mm dichromat - I'm worried about mechanical vignetting from the lens body.

Apart from that, there is nothing stopping me from building both! New reel of filament is at the post office, I can pick it up tomorrow after work!


What I can do is build an adapter to see if the achromatic doublet will work with the existing housing (adapter can move it back 3mm easily enough) - it might just work!

I'll focus on the standard lens to get a working prototype, since everything is ready to go with that one.

Not long now!
03-19-2020, 08:55 PM   #41
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Well I'm impressed! I was looking at your focus attachment thinking "but focus is a rotational movement, that will never work.." until I saw the finished article and mentally turned the knob if you see what I mean. Now it becomes clear, well done!
03-25-2020, 09:45 AM   #42
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Had a bunch of failed prints - teething problems with the new filament, missed layers, and warping.

Trying to work out relationship between aperture stop (distance form lens) and vignetting - not looking good.

My estimates are poor (I don't know enough about the field of geometric optics) but seem to suggest that vignetting will start around F/2.8.

Yes, STARTS at F/2.8. This is the size that the aperture itself impinges on the field of view and becomes the "field stop" instead of the sensor in the camera. increasing the F/stop only makes it worse after that point.

I know I can reduce the aperture further without causing vignetting by physically relocating it closer to the lens.

This has something to do with chief rays, and possibly the exit pupil - but this is where my knowledge breaks down: I don't know how to calculate WHERE the light starts falling off.

My estimates so far are based off incomplete understanding of geometric optics - meaning I might be wildly off. Based on experience so far, I'm in the ballpark - but not accurate (nor precise) by any stretch of the imagination.

While I can determine the minimum aperture experimentally by attacking an aperture plate with a tapered drill, I would rather I understood the actual geometric optics for working it out... Especially when I change other parameters in the system to adapt it for other lens elements.

Oh, speaking of which, my achromatic doublet's focal length is too short ; the mirror impacts it. Gone back to the single element. Oh well! I tried!
03-25-2020, 01:52 PM - 1 Like   #43
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Finally making some headway! I might keep the white focusing ring - it will look nice against the black (though, it will get dirty... heck, you can see dirt on it already!)

Printing a black one just now as a spare (and so I have a colour-matching set with no grime on it), and I should have a prototype up and running by tomorrow evening, assuming I get the inside barrels printed alright.

This design should work with 42mm to 85mm by changing out the rear part of the inner barrel, and there is, in principle, no hard limit on how long the focal length can be - just need to extend the inner barrel to some arbitrary distance (though with the current screw length, short-distance focusing will suffer).

With that in mind I will hopefully grade to an achromatic doublet in the future, and maybe even experiment with a petzval-type arrangement with two achromatic doublets in opposition, and the aperture in the middle.

FOR NOW though, let's get this party printed! It's been too long coming! Mk-20-45A (A is the 42.8mm version, B is the 29,5mm version - and this is the 45th revision of the Mk-20 design - B-version is currently non-functional due to the aforementioned mirror-collision problem... hope my camera's alright :hides: )
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03-26-2020, 04:01 AM   #44
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QuoteOriginally posted by cprobertson1 Quote
There we go - that's it finally built - just need to build a support for the bottom (there is a mount for a second tripod on the driver-ring.

Seems to work exceptionally well - at least for macro work - I took a few random macro shots of things in the livingroom and I was thoroughly impressed with it actually!

The mechanism runs as smooth as butter and is pretty damn sturdy - there is a tiny amount of slop in the wooden rod - which I believe is the rod itself flexing - but it really isn't noticeable (by which I mean once you set the focus, it stays there even if you move the camera around a bit).

All-in-all, I'm very impressed with it!

There's room for improvement (there always will be!) but I'm certainly happy with it so far! Can't wait to give it a go with some astro work!

Ps - note the finish isn't great - I've still to sand it and paint it - so there are a few burrs here and there; but the moving surfaces have all been cleaned up - the movement is ::Barry White voice:: Smooooooooth. Oh yeeah.

--EDIT--
Added images showing use of lens-support
Have you considered interpgrating a tripod mount into the rear part. That is a lot of load cantilevered off the camera body tripod mount
03-28-2020, 06:45 PM - 3 Likes   #45
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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!
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