A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can build

[Nodda Duma]

Yesterday was kind of slow at work, so I played in Zemax while I worked on some other stuff. Basically I was wondering if I could design a Cooke Triplet in a reasonable large format focal length using lenses you can buy out of a catalog. I figured some intrepid DIY'er might be interested in the results and maybe put one together. The lenses are all available from Thorlabs.

Of course, keep in mind that "you get what you pay for" :) There's a limit to what you can do with off-the-shelf optics. But that aside...


This covers a 4x5 format. Reference the layout for lens orientation (showing f/8 aperture setting).

129094

Focal length: 177 mm.
Sharp on-axis, soft off-axis at f/8
Should be pretty sharp at f/16.

Lens 1: Part # LA1384-A Diameter: 2"
http://www.thorlabs.com/thorproduct.cfm?partnumber=LA1384-A

Airgap between Lens 1 and Aperture stop: 9.0 mm
Airgap between aperture and Lens 2: 2.0 mm

Lens 2: Part # LD4735 Diameter: 1"
http://www.thorlabs.com/thorproduct.cfm?partnumber=LD4735

Airgap between lens 2 and lens 3: 4.0

Lens 3: Part # LA1050-A Diameter: 2"
http://www.thorlabs.com/thorproduct.cfm?partnumber=LA1050-A

Back focus is ~156 mm.



Blacken all lens edges with Speedball Black India ink (I forget the part #, you can get it at a crafts store like Michael's).

Notes for lens 1: Mounting should allow for a clear aperture diameter of at least 35mm.

Notes for aperture: The aperture should provide a maximum 22.2 mm for f/8. For the aperture, you can use Waterhouse stops or an off-the-shelf aperture such as Thorlabs D25S: http://www.thorlabs.com/thorproduct.cfm?partnumber=D25S Make sure you maintain the 2mm airgap.


Notes for lens 2: The fused silica material acts as the "flint" I couldn't find a larger diameter lens like this, which limits the f/# to 8. When mounting you need to provide for a clear aperture diameter of 23mm or you'll get vignetting. This gives you about 1mm edge mounting thickness which is sufficient to mount the lens. Alternatively, you can edge bond in a tube with RTV *not* hard epoxy.


Notes for lens 3: Mounting should allow for a clear aperture diameter of at least 33 mm.


The design consists of two outer plano-convex outer lenses (the positive crowns) and an inner equi-concave lens (the negative "flint"). Finding a suitable flint was tough because catalog lenses are made out of either N-BK7 or N-SF11 and the combination of those two is not conducive to color correction. The fused silica of the chosen lens seems to work well, but performance does suffer. A custom lens would help performance quite a bit, but I'm limiting this to what you can buy off-the-shelf.


Blacken the inside of your barrel.


Good luck, have fun, and share the results!


Regards,
Jason

P.S. Feel free to ask questions

[Old-N-Feeble]

Kewl!! :)

[IanG]

Interesting, thanks.

I have a number of LF triplet lenses, including a couple of modern Multi Coated Geronars, however the 3 TTH Cooke triplets I own are quite different - the element are surprisingly quite thick and they are large lenses (length & weight) for their FL and aperture.

Ian

[Nodda Duma]

edited the thread title to clarify this is a build-your-own-lens project.

[Will Frostmill]

Well, this is very exciting!
Any tips on barrel material?

[Nodda Duma]

Well, this is very exciting!
Any tips on barrel material?

Anything will work that will hold them in place without blocking the optical path and which allows you to mount to a lens board. I once mounted a laser beam focuser in a pair of rolled-up notebook paper tubes. Had a sliding focus and everything. Only cost me about 3 swear words.

There are lens mounts available at thorlabs, Edmund optics, etc, but they are a little pricey. Since they're 1" and 2" diameter lenses, maybe you could make the barrel out of standard PVC pipe. Or look on McMaster-Carr's website. If it were me I'd go by Home Depot and dig around or root around in the basement.

If you have access to a lathe then you can of course machine up a barrel with lens mounting steps and retainer rings. Allow about 0.005" clearance for the lens diameters (so the lenses don't get smooshed when the metal contracts at lower temperatures). Paint the inside with flat black!

[Will Frostmill]

Thanks, that's a really, really, comprehensive answer. Particularly nice that you added the bit about the 0.005" clearance.

[richardman]

Sounds like may be a project for 3D printed mounting plates and PVC tubes...

[Nodda Duma]

If you can make a 3D printed mounting plate then you can make a 3D printed lens barrel.

[AndrewButts]

Hi, thanks for putting the effort into this design! Your description made me curious what it's like to design a lens when constrained to only off-the-shelf parts. Does your software allow for automatically varying the choices of elements from a preselected catalog, or do you have to pick the elements and then just vary the distances? I've spent the last few evenings working in OSLO-EDU trying to get some intuition for how lens designers work. Reading through some of your other posts it seems like you're a great resource for us here, thanks for joining!

[Nodda Duma]

Hi Andrew,

It was my pleasure! I've got some more to try when I can find the time.

If I remember correctly, the process went something like this:

Set up a Cooke triplet with the appropriate image plane size and focal length. Set the positive glass types to N-BK7 ( what most catalog positive lenses are made of). At this point you'll have a really nice Cooke with RMS spot diameter ~20um for 4x5 image plane.

Constrain the outer lenses to plano-convex. This is a better approximation to the lens shapes of an ideal Cooke than an equi-convex. Also set the outer glass types to N-BK7. Reoptimize at this point. I optimized for P-V Wavefront (this is my preference), but I track spot size. I also set the stop either before or after the middle lens, depending on what gives better merit function value.

Set the element focal lengths to be equivalent to common values in the catalog. It's fairly straightforward to find a suitable combination.

Now I've basically reduced the variables to the center negative flint lens. In a Cooke, the choice of glass types corrects color. So I have to do this now. The outer glass is set to N-BK7. The most common flint for catalog negative lenses is SF-11, which doesn't work well with N-BK7 to correct color. If you were to optimize for a flint to pair with BK7 youd find the design settles on N-F2, but that isn't an option for catalog optics.

There's no catalog flints really available to pair with catalog crowns to correct color, which never made sense to me (except maybe catalog lenses are for tinkerers, professors, or people who just don't know any better).

So the real sneaky part that makes this work is that Fused Silica is another choice for "flint" that has low and similar dispersion like BK-7 (Abbé of both are in the 60s), but index is different enough that it's not the same glass. If you use a fused silica lens for the flint, then at least color is reduced enough for acceptable spot sizes. Set it up as an equi-concave lens for design symmetry.

Now it's just a matter of finding catalog lenses that are close to what you have. Zemax has a tool which allows me to rapidly filter through stock lenses from all the vendors for focal length and diameter, element type. I don't think Oslo has that unless they've incorporated it recently. If not, then just flip through the Edmund and Thorlabs catalogs. Once selected, optimize lens spacings to give you the proper focal length and final aberration correction. Don't constrain focal length too tightly, just keep it roughly normal or it'll go short (smaller focal lengths make smaller spot sizes).

The diameter of the fused silica lens constrains the f/#. If that weren't the case, then I would instead stop the design down until the RMS spot diameter is about 100um across the field. This allows you really nice contract prints and maybe enlarge 2x or so from a wide open photo. As it is, this design is about there anyways and provides good performance. Not as good as a true Cooke of course, but surprisingly good enough with maybe some pleasing softness.

[Nodda Duma]

Btw a good reference for how to design a Cooke is "Lens Design" by Joseph Geary. Has a great Cooke design example in there, among others.

[AndrewButts]

Thanks for your answers! I have another question:

In your design you mention distances to the second lens. Are these distances to the center of the lens or to the edge? Is the convention to always do it one way or the other?

[richardman]

Has anyone made one yet?!!

[Henrim]

This is interesting. I would like to try to do something like this at some point. May be not in the near future though. Anyway I drew the attached diagram to check if I'm following your description correctly. My interpretation is to measure the air caps from the edge of the lens to the edge of the lens. Air cap between lens and aperture is measured from the edge of the lens to the centre of the opening. Is that correct? If that is, there is a clearance issue if the suggested aperture assembly is used (as shown with blue dotted line).

http://melaanvuo.com/share/triplet.png

[Nodda Duma]

Thanks for your answers! I have another question:

In your design you mention distances to the second lens. Are these distances to the center of the lens or to the edge? Is the convention to always do it one way or the other?

Yes, prescription thicknesses are always along the optical axis (airgaps and lens thicknesses are usually vertex to vertex, for example).

[Nodda Duma]

This is interesting. I would like to try to do something like this at some point. May be not in the near future though. Anyway I drew the attached diagram to check if I'm following your description correctly. My interpretation is to measure the air caps from the edge of the lens to the edge of the lens. Air cap between lens and aperture is measured from the edge of the lens to the centre of the opening. Is that correct? If that is, there is a clearance issue if the suggested aperture assembly is used (as shown with blue dotted line).

http://melaanvuo.com/share/triplet.png

Nope, center to center (technically along the optical axis). This is convention.


Airgaps and thicknesses are never measured edge-to-edge because lenses are almost always of different diameter. Doing that would lead to over-complication of the callouts.

The D25S is just an example of an aperture you could use. I make no claim to having rigorously analyze the setup. :) it's hard to tell from the drawing but I think the aperture is offset to one side. You'd have to look at the solid model to know for sure. I thought I had checked it but it's been a while. Now you know why engineers insist on design reviews to check their work. You can try the D36S too. Or just use Waterhouse stops.

[Nodda Duma]

Has anyone made one yet?!!

I haven't heard of any yet...I'm still waiting for someone to try it and report the results!!

[Ken Lee]

One thing I noticed in the catalog was that they offer new lens diaphragms with what appear to be many blades. Hmmm...

[Henrim]

Thank you, I get it now.

By the way, what is the acceptable tolerance for the airgaps?

I was thinking that the aperture on the D25S is probably offset but that is not indicated in the drawing and none of the photos show the other side either. Would be on the safe side to try with the bigger model.

And an other question about the aperture. I googled for triplets and in many cases the aperture is drawn behind the second lens element. What difference does it make if it is on front or behind the element?

[Nodda Duma]

Ken: yes, it's very important for the type of lab setups those are typically used in.


Henrim: I didn't do a tolerance analysis but if I had to guess I'd say keep the airgaps (lens seats) to within at least +/- 0.010" and even better to be within +/- 0.005". That's very easy to achieve when machining the barrel on a lathe.


Stop before or after the second lens: stop position impacts some of the Seidel aberrations such as coma, astigmatism, etc, and was selected based on what gave better performance.

Generally speaking, the ideal position for the general positive-negative-positive design form is centered, maintaining design symmetry. Physical necessity, however, requires offsetting the stop position to one side or the other of the negative lens.

[Henrim]

Thanks again. Yes, it would be hard not to keep it within +/- 0.005" :)

I'm tempted to make the lens and I have all the material needed for the barrel. Only thing I don't have is time...

[Nodda Duma]

Awesome please do try to find the time and let me know how it turns out. Feel free to ask questions along the way. First light will be very rewarding.

I'll try to do a tolerance analysis this weekend and verify my educated guess.

[J. Patric Dahlen]

Now I've basically reduced the variables to the center negative flint lens. In a Cooke, the choice of glass types corrects color. So I have to do this now. The outer glass is set to N-BK7. The most common flint for catalog negative lenses is SF-11, which doesn't work well with N-BK7 to correct color. If you were to optimize for a flint to pair with BK7 youd find the design settles on N-F2, but that isn't an option for catalog optics.

How about using a cemented achromat and a meniscus, like the Aldis Uno, instead?

[Nodda Duma]

That would make it not a Cooke.

I mean it could be done, but you don't need a lens designer to tell you how to slap together a meniscus and achromatic. There's no real challenge to it from a "design using only catalog lenses" standpoint. That and the available meniscus are the wrong glass type for color correction.

[J. Patric Dahlen]

That would make it not a Cooke.

I mean it could be done, but you don't need a lens designer to tell you how to slap together a meniscus and achromatic. There's no real challenge to it from a "design using only catalog lenses" standpoint. That and the available meniscus are the wrong glass type for color correction.

True true, and that would be a topic for another thread. I'm not an expert on lens design, but I'm interested in old classic lenses and love reading about them, even if I don't understand the calculations and such. Finding a lens designer like you on a forum like this really inspires me. A photograph is not just a picture to me, but something that was seen through a lens of a certain design.

[Nodda Duma]

J. Patric: I'll take a look at that design and add it to my "to do" list. It is a lot like one of the steps taught in the design process from the landscape lens to the double gauss.

Greg: Unfortunately no. For several reasons including the long focal lengths and the index/dispersion properties of the polycarb material, those lenses aren't conducive to designing better performing camera objectives, even when combined with available catalog lenses.

[J. Patric Dahlen]

J. Patric: I'll take a look at that design and add it to my "to do" list. It is a lot like one of the steps taught in the design process from the landscape lens to the double gauss.

Please report back with your findings. The Aldis Uno is interesting. For example, it should be cheaper to make than the Tessar, but does it have any advantages over the Cooke triplet except maybe contrast?

[Nodda Duma]

No, the Cooke is inherently a better design because there are enough variables to correct for all the Seidel Aberrations. The doublet / meniscus combination is short 2 needed to correct them all, as well as not being a symmetric design. In this day and age, the number of air-glass interfaces are inconsequential thanks to coatings...there will be no contrast advantage.

[PCC]

Nope, center to center (technically along the optical axis). This is convention.


Airgaps and thicknesses are never measured edge-to-edge because lenses are almost always of different diameter. Doing that would lead to over-complication of the callouts.

The D25S is just an example of an aperture you could use. I make no claim to having rigorously analyze the setup. :) it's hard to tell from the drawing but I think the aperture is offset to one side. You'd have to look at the solid model to know for sure. I thought I had checked it but it's been a while. Now you know why engineers insist on design reviews to check their work. You can try the D36S too. Or just use Waterhouse stops.
Would you be able to draw a diagram of this? I'm a simpleton and work best with visuals. Thanks! Yes, I'm tempted to give this a try depending on the cost of materials involved.

[J. Patric Dahlen]

No, the Cooke is inherently a better design because there are enough variables to correct for all the Seidel Aberrations. The doublet / meniscus combination is short 2 needed to correct them all, as well as not being a symmetric design.

Excellent points, and it makes me wonder why they came up with the idea in the first place. Maybe it was cheaper to make despite the cemented doublet?


In this day and age, the number of air-glass interfaces are inconsequential thanks to coatings...there will be no contrast advantage.

True, but the Aldis Uno was designed in 1900 or thereabout.

[Nodda Duma]

Excellent points, and it makes me wonder why they came up with the idea in the first place. Maybe it was cheaper to make despite the cemented doublet?


Very likely..that and it was "good enough"

[Nodda Duma]

I have enlisted the help of an enthusiastic mechanical engineer. Would it be useful if he were to generate a model / layout of a lens barrel for these optics?

[brandon allen]

My 4x5 folder and accompanying wooden tripod projects are now complete. I suppose making a lens would be the next logical step...

What sort of performance could one expect from this lens design? Would it be comparable to commercially made lenses? Certainly making images with a self-made lens would be worth a lot, but would building this lens be really "worth it" as far as how well it would function? (Assuming it was built properly)

I think what I'm trying to do is talk myself out of starting this project...

[Nodda Duma]

I wouldn't rely on it as your only lens but as you stop down it should transition from soft to sharp in a pleasing manner.

I took some RMS spot size data down for you to review. By convention I believe the desired circle of confusion for 4x5 film is ~0.11 mm or less.

See attached, showing spot diameters for various points on the film plane. Hope this helps and my apologies for it being a non-film image.

135556

[brandon allen]

I have enlisted the help of an enthusiastic mechanical engineer. Would it be useful if he were to generate a model / layout of a lens barrel for these optics?

That would certainly help me!

[pdh]

When you have some idle time (!), Nodda, would you be able to scale this for 8x10?

[Nodda Duma]

Sorry pdh, I had tried that originally. It is easy enough to scale the design on paper, but the available catalog lenses don't support the larger format.

Even this size was hard. That's why the lens is not faster than it is.

If I made the center flint custom then it's much easier. But then they wouldn't be all catalog lenses.

[pdh]

I did wonder if that was the sort of problem. thanks for replying.

[StoneNYC]

Sorry pdh, I had tried that originally. It is easy enough to scale the design on paper, but the available catalog lenses don't support the larger format.

Even this size was hard. That's why the lens is not faster than it is.

If I made the center flint custom then it's much easier. But then they wouldn't be all catalog lenses.

So I guess no lenses for 14x20 then... Shame...

[Nodda Duma]

Not catalog Cookes at least.

[dodphotography]

Anyone have success? I'm going to dick around with this for a while. Not expecting anything amazing but some cheap lens elements online makes for a low cost / low risk project.

[richardman]

Notice that the original Cooke, they are fairly small up until 7" or so. Smaller than a plasmat. Then the size jumps until around 10 1/2", which is still usable size for 4x5. But then anything longer than 10 1/2" are HUGE.

I have no machining skills but love Cooke lenses, sigh... someone needs to make this....

[Nodda Duma]

If I had something larger than a micro lathe I'd happily turn a barrel for you. I really need to get a respectable lathe but I've been spending my hobby money exploring Nikon camera bodies. I have an FM3a arriving this week, which is very exciting--paid for by an unexpected "good job" bonus at work. Maybe I will start saving up for one of the antique lathes I see on Craigslist. They are fairly common in New England.

[dodphotography]

If I had something larger than a micro lathe I'd happily turn a barrel for you. I really need to get a respectable lathe but I've been spending my hobby money exploring Nikon camera bodies. I have an FM3a arriving this week, which is very exciting--paid for by an unexpected "good job" bonus at work. Maybe I will start saving up for one of the antique lathes I see on Craigslist. They are fairly common in New England.

I live in Littleton, Ma... We are practically neighbors.

[Nodda Duma]

Alternative to machining a barrel, try rooting around Home Depot or Lowe's for suitably-sized plumbing hardware. Small sections of Schedule 40 1" and 2" pipe may work well.

[coisasdavida]

In the book Primitive Photography, Allan Greene describes a very clever way to hold elements inside a PVC pipe, it uses rings made of pipe as well, I'm afraid my vocabulary is limited to explain.

[dodphotography]

That's the fun in DIY... If I wanted complete optical perfection I'd go buy the Schneider, Nikkor, Fuji.... Sometimes you fail, it's ok... Still fun man.

[axs810]

I wish there was a DIY 8x10 version...I would try and build one :P


Seems like it would be a fun little project I'm surprised no one has tried to make one yet!

[alanbutler57]

I hope someone builds one. Is there a source for "homebrew" formulas for other popular designs? For example since Rapid Rectilinears are pretty common, how would one go about using one as the chassis to build a Verito? That is to say, if you have a RR of focal length x, glass diameter y, how would one calculate the focal length and diameter of a positive meniscus to achieve a proximity of a Verito?

[Tim Meisburger]

Funny you should mention that. I was just looking at another thread and admiring the look of a Verito. Thinking about buying one if I could find it relatively cheap, but if it could make one, that would be cool...

[Nodda Duma]

I hope someone builds one. Is there a source for "homebrew" formulas for other popular designs? For example since Rapid Rectilinears are pretty common, how would one go about using one as the chassis to build a Verito? That is to say, if you have a RR of focal length x, glass diameter y, how would one calculate the focal length and diameter of a positive meniscus to achieve a proximity of a Verito?

It's more of a question of .. If you have the required focal length and f/#, how do you define the glass types, radii of curvature, center thickness, and all the other parameters necessary to fabricate, mount, and align the lenses to get good image quality.

I do have a Rapid Rectilinear design. Coming up with a design is pretty easy from my point of view as a lens designer..in fact, many classical forms are already in the lens design "public domain" packages that come with optical design software such as Code V or Zemax. The problem is the fabrication cost. Prototype quantity (like 1 or 2) lenses cost upwards of $1000 each. It's not until you're making 500-1000 copies that the cost gets down less than $100 each. So you can see why something like a new large format Cooke triplet costs so much...they're just not making a lot of them.

Why not use a combination of catalog lenses? Catalog lenses are very, very difficult to combine to form a useable objective with any reasonable f/# or image quality. If you want to know why...well, I'd love to know why as well. I think it's a combination of cost and that catalogs sell simple lenses which are great for laser optics but almost useless for imaging lenses. The flint and crown typically found in a catalog are a horrible combination for color correction, but they are inexpensive and work well for lasers.


The prescriptions are out there, and tell you everything you need to know to make and assemble the lenses...if you know how to interpret the data and turn it into real hardware (and thank God not many do..I'd be out of work!). But the cost to put together a single example is going to make you choke. Unless you're independently wealthy of course. In which case, any lens will be cheap. :D

[richardman]

...Prototype quantity (like 1 or 2) lenses cost upwards of $1000 each. It's not until you're making 500-1000 copies that the cost gets down less than $100 each. So you can see why something like a new large format Cooke triplet costs so much...they're just not making a lot of them.

...

I am assuming you are referring to the Cooke PS945 (which I have one), in which case, is that a triplet? It's suppose to have similar optic quality of the Pinkham and Smith Visual Quality IV but it's not clear whether it has similar optical formula.

Speaking of Cooke Triplet - I just got the 150mm Geronar, nice to find a modern coated triplet at that focal length.

Lastly, Miyazaki san in Japan has been making small batches of Leica M compatible triplets for the last few years. Starting with the 35mm/2.5, he added 28mm, 24mm, all 3 are triplets. He just added a 21mm (not sure if it is a triplet) and will be releasing a fast Gauss-based 35/1.4mm. I have the 35 and 28 and they are fun little lens to play with.

[Nodda Duma]

Yeah that's the one I had in mind when I wrote that. Obviously I don't know the prescription but the point is making only a few = $$$$

[Steven Tribe]

Before I start my own DIY lens thread, I would mention that there is another prescription available for a RR on pages 48 and 49 of the many times reprinted Orford book. There is also a complete step by step guide to making the brass and iris/WHS.

My thread will be reverse engineering. Taking the front lens cell from an Aplanat (Voigtlander Euryskop series II) and making a duplicate rear cell, based on radii, thicknesses and refractive indices of the surviving front cell.

[Dan Fromm]

I am assuming you are referring to the Cooke PS945 (which I have one), in which case, is that a triplet?

Richard, if you have the lens count reflections from each cell and then you'll know whether it is a triplet. If you decide that it is a triplet, count again. From the horse's mouth (http://www.cookeoptics.com/l/largeformat.html):


4 elements in 2 air-spaced doublets

[Nodda Duma]

Quick clarification on the airgaps. Note that the reference dimensions given in the original post are center-to-center airgaps. That can be a little difficult to measure unless you can generate a solid model and extrapolate what the mounting-to-mounting distances need to be. What may be of more practical use is the the edge-to-edge measurements. For the distance between the flat of lens #1 to the shutter, that's the same 9mm. For the shutter to lens 2 and from lens 2 to lens 3, obviously the measurement is different due to the sag of the surface. Here's an illustration giving the center airgap thicknesses and the edge airgap thicknesses:

141037

So for example you'll need an edge spacer between lens 2 and 3 of thickness 2.83 mm to properly set the 4mm center airgap.

Hope this helps.

-Jason

[Nodda Duma]

Also... if anybody who uses Solidworks or other 3D CAD software wants a STEP file of the lenses, PM me a note saying so with your email address and I'll send you a STEP file of the lenses and the rays.

[rataflo]

Hi,
I found your post while trying to found info on cooke triplet to make one in diy with off the shelf parts. Nice post :. I'm not an expert in optics so i have some questions for you but i can easily design a barrel to 3d print.
I build an afghan camera box with some electronic (light meter, shutter, rgb light for making print contact) base on arduino and the last part is to found a way to have a decent diy and open source lens. Some crappy shot of my box here => https://www.flickr.com/photos/127501997@N06/
So my questions:
If i use fused silica for both plano convex it's ok?
Can i put the stop outside the group lens? More easy to do with cardboard with holes than an iris diaphragm.
Thanks
Flo

[rataflo]

Hi,
I finally do some research and found that my questions where dumb :)
I play a lot with winlens 3D and finally design a lens with a minimal back focus (136mm), f/8 and able to cover a size of 180mm (i user 10*15cm ilford paper). I 3D printed a barrel to fit the lens.
My first tests are promizing but the exposure time calculation give me very dark shot. I use a diy exposure meter inside the camera so probably problems with incident light.
142136
142137

Next step is to create stop with 3D printed iris.

[rataflo]

First test :

142180

Yes the center is blurry. It's not a lens fault but the system to lay flat my paper is a bit broken so the paper is in a convex shape :\
I'm more worried about the blurry effect on the blanket in the lower left corner. Need more tests for sure.

[Nodda Duma]

Hi rataflo,

Sorry I didn't respond earlier...have been so busy that I haven't been able to check the forum.

I am glad you found the answers. There is no such thing as a dumb question (unless you're an engineer).

Your results are very nice. Congratulations!

You have blur in the lower left corner because one or more of your elements is slightly decentered. You will need to align them better...run out the lenses with an accurate test indicator to 0.005" or less or shim-to-center the lenses (you should be able to figure a way to do this...look up Yoder's book on mounting optics if you get stuck).

[pdh]

..look up Yoder's book on mounting optics if you get stuck).

will he also tell you how to use The Force ? ;)

[Nodda Duma]

will he also tell you how to use The Force ? ;)

He says don't Force lenses into their mounts. That leads down the path to broken-optics-darkness. ;)

[rataflo]

Hi rataflo,

Sorry I didn't respond earlier...have been so busy that I haven't been able to check the forum.

I am glad you found the answers. There is no such thing as a dumb question (unless you're an engineer).

Your results are very nice. Congratulations!

You have blur in the lower left corner because one or more of your elements is slightly decentered. You will need to align them better...run out the lenses with an accurate test indicator to 0.005" or less or shim-to-center the lenses (you should be able to figure a way to do this...look up Yoder's book on mounting optics if you get stuck).

Thanks!
And a big thank you for your post. It's the one i was hoping for a long time.

I will try to build a test rig with a laser.
But before i have to change the way i print the pieces and found a better way than scotch tape to fix the lens to the box :P

[jeffreythree]

Thanks for posting this. I like to take my hobbies as far as possible into the DIY deep end, and my first LF camera(4x5) is about half built. You said an 8x10 lens was not possible off the shelf, but what about something smaller in the 100mm range for use with roll film backs? I don't know much about optics design, but have become interested in it recently.

[Nodda Duma]

Hi jeffreythree

You're welcome. 4x5 was the largest I could do. Conversely smaller formats are possible but I haven't had the time to explore that aspect.

-Jason

[Nodda Duma]

Recently acquired a mini-mill and mini-lathe, and started turning a barrel for this project.

https://uploads.tapatalk-cdn.com/20170404/536ea65263e03f624298fcbccdb2c617.jpg

O.D has been partially turned (note flange for mounting against lens board). OD is 2.2". Last night I bored out the I.D. for the aperture stop and for the airgap between the first and second lens. Took the picture after setting up the boring tool and before I started turning the I.D.

Material is Delrin.. I happened to have some suitably sized round stock. Lathe is a 7x14" Chinese lathe that you can buy off ebay for a handful of Benjamins.

[Ted R]

Very interesting thread.

Congratulations on acquiring the lathe :-) I miss mine (left behind in England when I emigrated).

[Nodda Duma]

Thanks. I would get a Hardinge if I could get it into my basement. This will work fine though.

Here is the drawing I am working to.

https://uploads.tapatalk-cdn.com/20170405/53a7623db2ccc20b3d95e64faf520993.jpg

[Nodda Duma]

Front side of aperture and lens #1 seat done

https://uploads.tapatalk-cdn.com/20170405/aae8a1bb807993e81862c7eb3b2d88f2.jpg

[Nodda Duma]

Mounting flange and back side O.D. mostly done.

Time to part it off to work on the back.

https://uploads.tapatalk-cdn.com/20170405/c060cb035ae15d0b79fd2d1f9d1169db.jpg

[Nodda Duma]

So I parted it off and made a plug to fit inside the hollowed out front, so when I chuck it up the barrel won't collapse (at worst) or warp (at best). So I've done that, and have runout the barrel in preparation to turn the rear lens seat and back of the aperture. That's for tomorrow.

[Peter De Smidt]

This is fun to follow. Thanks for posting!

[stawastawa]

That is a skinny little tab you have to hold the flint.
What is the tool to make the little groove just to the left of the flint? what is it's purpose? I don't see it in the picture so maybe it wasn't needed after all.

[Nodda Duma]

The skinny little tab the flint seats on is the aperture stop. I'll edge-bond the negative lens in. The positives will be face-bonded.

The little groove will be the slot where the waterhouse stops will go. I'll mill that when I'm done turning.

Drawing's not quite to scale. I'm no draftsman :)

[stawastawa]

ah water house stops! I was still thinking the aperture assembly was being used.
n

[Nodda Duma]

Yeah I figured waterhouse stops are a bit less ambitious for the first lathe / mill work I've done in several years.


Also ordered a thread turning tool. Gonna try machining internal helical light baffling (continuous thread on ID's between lenses) instead of just painting flat black.

[Ken Lee]

I have the feeling that this is the start of something really big.

Today, only a lathe. Tomorrow...

http://www.kennethleegallery.com/images/forum/BigFactory.jpg

[Nodda Duma]

Done on the lathe

https://uploads.tapatalk-cdn.com/20170406/547bc530505d5f02b31b06c9e874055b.jpg

https://uploads.tapatalk-cdn.com/20170406/81ed2fa29d8f1c7823dd52bfe5acb282.jpg

[Steven Tribe]

Also ordered a thread turning tool..........

Now this is getting interesting!!

Link, please?

[Nodda Duma]

Just look up "thread turning tool" on ebay (without the quotes). It's a lathe tool.

[Jody_S]

Just look up "thread turning tool" on ebay (without the quotes). It's a lathe tool.

Next you'll be converting that thing to CNC so you can start production...

[Nodda Duma]

Lenses arrived from Thorlabs, along with the snack pack they always include. :)

https://uploads.tapatalk-cdn.com/20170407/efb73284e2a5c50cd31fcd1b42eadce3.jpg

[Nodda Duma]

https://uploads.tapatalk-cdn.com/20170407/641400733d9c226d7bb1c220068a7681.jpg

I'll edge blacken the lenses.

[Nodda Duma]

I just dry fit the lenses to see what it'd (tack bonded in place with NOA 61). Imaged a light bulb 15 ft away onto my notebook and it forms a nice sharp image. Iphone picture compression doesn't quite do it justice.

https://uploads.tapatalk-cdn.com/20170407/0761313de8c235cd8d6148c3b6ab980d.jpg

https://uploads.tapatalk-cdn.com/20170407/dc27a1242d54fdc1c3de4e94033e45ae.jpg

[Nodda Duma]

Not satisfied with that, I made a camera obscura of sorts out of a soda box (which my wife drinks, ripping the boxes open like a grizzly bear) to measure focal length and view image from more distant objects.

https://uploads.tapatalk-cdn.com/20170407/6566724ed3e613c973f487f49e03126b.jpg

https://uploads.tapatalk-cdn.com/20170407/bdb4e1b636feab27042b92a2004836ff.jpg

[Nodda Duma]

Close-up of the focused image.

https://uploads.tapatalk-cdn.com/20170407/7e8cea6e178ba8c9a2e9b8bf1a37ab8c.jpg

[Tim Meisburger]

Looking good! It will be really interesting to see a portrait, when you have it complete.

[Nodda Duma]

Thanks.

Flange focal length (distance from the outside mounting flange to image plane) measures ~ 165mm. Reversed, focal length measures 200. Very rough measurements of course. Maybe I'll have a 2-step zoom ;)

Next I'll either pull the optics to machine the slot for the waterhouse stops, or make a lensboard to mount it to my camera. Slow and steady, or go for broke? Decisions, decisions...

[stawastawa]

sounds like you are going slow and steady then?

[Nodda Duma]

No I think I'll put it on a camera first...just have not had the time to work on it

[Nodda Duma]

First light!

Acquired a Burke & James press camera which accommodates a lens board sufficiently large for the lens. This allowed me to capture an image on one of my homemade glass plates coated with a homemade emulsion. This particular exposure was 3 minutes (it was getting dark) wide open at f/8.

167387

I'm pretty sure my focus is slightly off due to the difference in distance from lens to the emulsion plane for a glass plate (0.050" thick) and for sheet film (~0.0075"). I focused the words of the license plate while viewing ground glass with a loupe to be centered in the depth of focus, but the license plate appears to be at the outer edge of the depth of focus in the image. The can blocking the plate number is about 6" wide and is completely within the DOF. Still, it allows some fair judgement of image quality -- which is close to prediction -- and bokeh, which is difficult to analyze in Zemax.

Next test will be for object distance much further. Or a portrait. I think this would be a good portrait lens.

[Peter De Smidt]

Great job!

[Jim Michael]

There is an online machine shop emachineshop.com which provides a design tool to use in designing parts and generating quotes. Perhaps that would be an option.

[Nodda Duma]

Someone asked the question about replacing the center lens with a custom one:

If you can get a custom lens made, use Ohara S-FTL10 or Schott K10 for the negative lens with the following parameters:

R1 = -65.25
R2 = 90.80
Center Thickness is 2.0mm.
Diameter is 36mm.

Grind a flat onto R2 for mounting. The clear aperture should be 32mm diameter. Sag will be 1.4208mm.

It is a concave-concave lens, but not equi-concave. R1 faces towards the front of the assembly and the orientation is important.

Other changes to the prescription:
Set the airgap (on the optical axis, see previous posts) between L1 and this lens to 9.5mm
I moved the stop to between this lens (L2) and L3. Bokeh looks better off-axis. It's more swirly than starburst like above with the stop between Lens #2 and #3.
The airgap between L2 and the stop is 2.25mm

Airgap from stop to L3 is 3mm.

Tolerances on airgaps are +/- 0.1mm, but hit the prescription values as close as possible.

This allows you to make a 177 f/5.6 lens. It will be soft wide open but will sharpen up on-axis quite a bit at f/8 and will be sharp across the field at f/16 to f/22. Distortion is < 0.5%

Here is the new prescription with a custom center (negative) lens:


Surf...R (mm)......T(mm)...Glass....Diameter(mm)
1.......64.38.........8.22......N-BK7...50.8......(Thorlabs LA1384)
2.......Infinity.......9.50......air.........50.8
3......-65.25.........2.00......K10.......36.0
4.......90.80.........3.00......air.........32.0
5.......Infinity.......2.25......air.........(f/5.6, D = 28.621)
6.......Infinity.......9.69......N-BK7....50.8.....(Thorlabs LA1050)
7......-51.50........158.15...air

f/5.6: stop diameter = 28.621
f/8: stop diameter = 19.84
f/11: stop diameter = 14.3742
f/16: stop diameter = 9.8604
f/22: stop diameter = 7.1646
f/32: stop diameter = 4.923
f/45: stop diameter = 3.5

This will give you a fine moderate speed 4x5 Cooke triplet with nice bokeh and soft portraiture if you shoot wide open, or a pleasing old-style swirly effect on landscapes that becomes less pronounced as you stop it down.

You could also mount the lens on a medium format camera to achieve a portrait focal length. I wouldn't shoot it with 35mm... it will be quite soft until you stop down to f/16 or f/22.

-Jason

[C. D. Keth]

Well, this is very exciting!
Any tips on barrel material?

PVC pipe works pretty well. You can cut C shapes that spring fit inside the main barrel to hold elements in place and act as spacers between them. It's also cheap.

[leviking]

I've sourced the custom element in Schott K10. In small quantities it's $110 per but there is a minimum order. Send me a message if you're interested. I am not selling this for a profit, just want to get costs down so I can afford a minimum order and figured there might be others who want to build this lens.

[Jim Noel]

It appears it is time for me to clean up my old Sears mini lathe. Thankfully, they are still made under another name so parts and accessories are available. I Think I'll order a screw cutter today.

[m00dawg]

Original thread mentions 4x5. I would guess due to the off the shelf nature of the lenses, larger sizes (like 5x7) might be difficult/impossible? Curious because J's sample photo is BEAUTIFUL and it seems like this lens would be a lovely coupling to his dry plates if I can get a working ambrotyping process for those but was planning to do those in 5x7. And/or just wondering if it were possible.

PVC was mentioned which seems cheap and sensible but I was also pondering if some of the necessary support pieces could be 3D printed to perhaps make the process easier. If I get that far, I'll certainly keep folks in the loop on that.

[afotandolaciudad]

Hello.

How would a Rapid Rectilinear design compare to this Cooke Triplet?
I'm planning to build a very simple compact long focus RR. I don't have lenses longer than 300mm. I'm wondering how would be to order two identical achromatic doublets and put it on both sides of a copal #3. This size is for focusing aid (and because I have this shutter available at home).
The barrel would be easier to prepare and fewer degree of freedom mean less possibilities of go wrong.
How would a lens like that perform at f/22 to f/45?
Mi thoughts begin with two of this doublets (D54 f786):

https://a.aliexpress.com/_uj4p5X

Any thoughts?

____________

I forgot to ask...
How would this diy lens compare with my symmar 210mm converted to 370mm?

[Nodda Duma]

You can get it to work, but it won’t be a rapid rectilinear as off the shelf achromat are the wrong lens shape to correct spherical. You will be able to correct distortion, lateral color and, with proper distancing from the stop, coma.

[reddesert]

Hello.

How would a Rapid Rectilinear design compare to this Cooke Triplet?
I'm planning to build a very simple compact long focus RR. I don't have lenses longer than 300mm. I'm wondering how would be to order two identical achromatic doublets and put it on both sides of a copal #3. This size is for focusing aid (and because I have this shutter available at home).
The barrel would be easier to prepare and fewer degree of freedom mean less possibilities of go wrong.
How would a lens like that perform at f/22 to f/45?
Mi thoughts begin with two of this doublets (D54 f786):

https://a.aliexpress.com/_uj4p5X

Any thoughts?

____________

I forgot to ask...
How would this diy lens compare with my symmar 210mm converted to 370mm?

Assuming the numbers on those lenses mean diameter 54mm, focal length 786mm, then if you use two of the doublets separated by 50 mm you will get roughly a 405mm focal length lens. This is from the formula for a combination of thin lenses separated by distance d:
1/f_total = 1/f1 + 1/f2 - d/(f1*f2)
see for example https://courses.lumenlearning.com/boundless-physics/chapter/lenses/ - it is idealized but close enough for lenses like these.

Symmetrical lens systems make some of the aberrations very small, the ones Nodda Duma mentioned. If you use small stops that could help reduce some of the uncorrected aberrations. I don't have any real experience with the 370mm single group of the Symmar, but have often read that the single groups are not very well corrected for color. So the symmetrical pair-of-achromats lens might have an advantage if avoiding lateral color is important.

[afotandolaciudad]

Thank you both for your replies.

I forgot to mention that I'm going to use the lens on 4X5. That means I'll only use the central part of the coverage. Considering that RR are used as normal lenses and my RR approximation would be about 400mm, that rules out the periphery. Spherical aberration is more important in short focals and also decreases stopping down.

Focus shift would be a problem if I focus wide open I guess. Symmar rear cell too suffer focus shift that I think is related to spherical. When you stop down the lens spherical aberration minimizes and focus shift appears. I thought maybe it's better to use a copal #1 instead of #3. For the barrel I could use pvc pipe gluing natural sheet cork bands to slightly expand the diameter and facilitate threading on the shutter. This is something I've already done in the past (using cork like this).

With all this in consideration I think this pseudo RR can work pretty well when stopped down to f/22-32. In any case, the monetary loss will have been very small.