A DIY 177mm f/8 "Sort of" Cooke triplet for 4x5 format from off-the shelf lenses.

[Nodda Duma]

This is posted on LFF as well. Not sure if I should post here or in apug's large format subforum, but I flipped a coin and it came up tails, so...

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.

There's a limit to what you can do with off-the-shelf optics so don't expect the same quality as a true Cooke. But that aside...


This covers a 4x5 format. Reference the layout for lens orientation (showing f/16 aperture setting).



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....umber=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....tnumber=LD4735

Airgap between lens 2 and lens 3: 4.0

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

 

[pdeeh]

where are the airgaps measured from and to on the lenses? The "apex" of the lenses' curve?

 

[Nodda Duma]

where are the airgaps measured from and to on the lenses? The "apex" of the lenses' curve?

yes from the lens vertex. Airgap in the context of optical prescriptions always refers to "along the optical axis"

 

[pdeeh]

ta.
Not that I'm intending to make this (I don't have the skills or the money) but I'm always interested to learn.

 

[Mustafa Umut Sarac]

Thank you.

Glass costs 153 euros and add post and taxes from Europe. Total bare cost is around 220 dollars.

 

[Mustafa Umut Sarac]

Thank you.

Glass costs 153 euros and add post and taxes from Europe. Total bare cost is around 220 dollars.

 

[Nodda Duma]

Update:

I bought a mini-lathe and mini-mill, so as first machining project I decided to turn the lens barrel and put this lens together.

Drawing with dimensions that I worked from.

I started with a 2.25" dia Ultem round stock that I happened to have.

Part of outer diameter turned, beginning to bore out the inner diameter.

Front half of barrel done. The smaller I.D. is the aperture stop/seat for the negative lens. The last step is for the positive lens. I then parted off the barrel, turned it around and chucked it back up, trued the runout and turned the rear lens mounts.

Barrel done.

Ordered lenses from Thorlabs. They showed up yesterday along with the snack box they always send with an order

 

[Nodda Duma]

Temporarily mounted the lenses for a dry test. They fit! First test was imaging a light bulb at 15 ft onto my notebook.

So far so good, but I wanted to see what objects at longer distances looked like. So I fabricated a camera obscura from my wife's latest discarded soda box.

Image:

Close-up

Not bad. Interestingly, focal length looks like 165mm forward, and 200mm reversed. I'll review the lens design..maybe reversed the image quality is good enough that I'll be able to have a "2-stop zoom".

So I still haven't decided what steps to take next:

a) remove the lenses and mill the slot for waterhouse stops (and the stops themselves, finish blacking the inside, and do final lens mounting, or
b) fabricate a lensboard and mount the lens to my plate camera to take pictures at wide open.

Slow and steady, or go for broke?

Decisions, decisions...

 

[Michael Guzzi]

Nice work! What material did you use for the barrel?

RE what to do next, I would first test it as-is on film.

And then, if IQ turns out good enough I would machine a more permanent barrel out of brass with provisions to waterhouse stops and all

But that's because I have access to more robust machine tools...

I have a question: would it be possible to make something similar, but in the classic Petzval design?

 

[Nodda Duma]

Barrel is Delrin. That's pretty robust stuff. The opto-mechanical engineers I work with wrap my optics in it quite often for more demanding applications than this. It's a better choice than brass, to be honest.

Catalog optics are not conducive for making imaging objectives: they are just not the right shape or glasstypes. Typical use is for simple laser and telecommunications devices. It's no joke that I tried earlier in my career to do this and failed, and was only successful after a decade's worth of additional design experience. This little self-imposed challenge is kinda the optical design equivalent to making some complex mechanical device out of bar stock and sheet metal without access to a machine shop, except more complex.

Part of designing the lens is knowing what design type has a chance of succeeding and what doesn't. Petzval's are all about glasstypes, lens shape, and lens speed.. . It requires a greater selection of glasstypes than available from the catalog vendors, and a different selection than what's available. A Petzval designed from catalog lenses isn't possible.

 

[Michael Guzzi]

So Petzval's are out of the question

Out of curiosity, how does the cost of Ultem compare to other materials useable for this application? Can`t find much info about it.

 

[Nodda Duma]

Do a search on ebay. I just took a quick look and saw some 2 1/2" Dia x 12" long roundstock that would work fine for $24. That's half the cost of brass, about that of aluminum and stainless steel, and about 1.5x that of carbon steel.

It's widely used for machinable plastic applications. Very easy to machine and capable of holding tight tolerances.


But honestly that just happened to be what I have on-hand in the right diameter. I could have easily grabbed some steel stock, aluminum, or brass (although aluminum would have to anodized black for stray light control).

 

[Nodda Duma]

One correction... I typed ultem, but I actually used delrin. Brain fart. Similar properties, but delrin is much cheaper.. closer to cost of steel. Used both at work for lens barrels.

I'll correct in the post above.

 

[Richard Man]

Wish I am more handy, would love to try this. I DID purchase a Grenaror 150mm though, which is a modern Cooke Triplet. Super tiny and does have that Cooke look...

 

[Richard Man]

p.s. Thorlabs is our customers at my $dayjobs$!! Different division though.

 

[Nodda Duma]

First light!

I 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.

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.

 

[fdonadio]

Live free or die. Which state is that?

 

[Nodda Duma]

New Hampshire

 

[fdonadio]

New Hampshire

Great! One of the 13 original colonies, the one farthest to the north.

Winter should be great over there.

 

[Nodda Duma]

Great! One of the 13 original colonies, the one farthest to the north.

Winter should be great over there.

We moved here from the Mojave desert and love it. The fall colors are beautiful, too!

 

[bdial]

We moved here from the Mojave desert and love it. The fall colors are beautiful, too!

There are some things I still miss about living in the Mojave, (mostly having to do with weather in about February) but overall I agree.

 

[fdonadio]

Lots of places to go and the list never ceases to grow: Mojave Desert Just added!