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95mm ED Triplet salvaged from broken camera lens

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#1 foxshark

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Posted 25 October 2021 - 06:59 AM

Judging by my post history here, you could conclude that I am a tinkerer and like to try things just to see how they work. This project would fall under that context of education and discovery, not an attempt to make a bargain SV/TV/AT/whatever EDT.

 

My optics knowledge comes first from the photography world where more elements seems to mean a better lens, and ED/Super-ED is just a marketing thing. Sure Nikon has gold trim around lenses that have the ED nameplate, but even my old kit 18-70mm had that, so it can't possibly be as fancy as the arbitrary Canon L moniker with the red trim, right? Common knowledge also states that manufacturers are very specific about the optical formula to the point that if a long lens has a drop in filter, you need a filter in there even if you don't want one, as that bit of glass is part of the optical formula. 

 

But what if some of those elements could be repurposed? Lens designs like the double-Gauss are obviously not reducible in design, but telephoto lenses seem to be a combination of [an image forming big lens] and [a bunch of other lenses that shorten and modify the light path]. I asked my friendly local camera repair shop to keep an eye out for me for any "big zoom lenses that are irreparable for me to take apart". Totally reasonable, right?? Well as luck would have it ...

 

 

IMG_8584.jpg

 

Twenty dollarydoos later, I was the proud owner of a Sigma 150-600 Contemporary that all the king's horses and all the kings men wouldn't be able to put back together again.

 

IMG_8597.jpg

 

Here is the lens diagram with the twenty elements; I have noted the big triplet group on the left and a diverging lens group that acts as the extender and I assume a flattener as well. The middle elements are responsible for focusing and image stabilization. I went into the project open minded, but was still surprised by the figures of the triplet - appears to be 280mm ƒ/3 by measurement. I assume field curvature will be rather extreme by astronomy standards.

 

 

My initial use was going all the way back to the roots - as a camera obscura. I took the front triplet and pointed the lens at a window in a darkened room and looked at the image projected on the wall 11 inches / 280mm from the lens elements, and then with the extender group held between. A trip to my favorite astronomy and optical supply shop called Home Depot gave me my optical test bed of some wood and threaded rods to hold and adjust spacing between the elements. Collimation will be optimistic at best.

 

 

IMG_8649.jpg

 

I set a Celestron SMA 25mm at the image plane and took a peek at Jupiter and could make out four tiny moons. Flare from stray light and collimation errors were so over the top that it looked like a J.J. Abrams movie, but it was fun still to see an otherwise sharp pinpricks of light.

 

Next up, integrating the extender elements group then maybe even building proper cells that would allow for real collimation attempts, if justified.


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#2 mazdak

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Posted 25 October 2021 - 08:36 AM

Really interesting project



#3 foxshark

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Posted 25 October 2021 - 09:09 AM

Thanks! There is a large clamshell chip that extends about 15mm into the first element, so by no means will it ever be perfect. I suppose you could put in an aperture mask to get a ~65mm ƒ/4.3, but that seems like it would defeat the purpose. A little black paint it will likely be.



#4 KBHornblower

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Posted 25 October 2021 - 11:30 AM

I would not necessarily expect that front doublet by itself to be a good telescope objective.  After all, it is designed to work with the rest of the elements in the zoom system.  If you get good images with it properly collimated, that will be a pleasant surprise.


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#5 foxshark

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Posted 25 October 2021 - 12:27 PM

it is designed to work with the rest of the elements in the zoom system.  […] will be a pleasant surprise.

Oh certainly, I think that’s what they call the null hypothesis if my old statistics memory serves correctly. My assumptions had always been that telephoto lens designs were un-reducible just like symmetric normal focal length lenses, but that’s where my curiosity got me wondering, at least for visual use. It would be, as you said, a pleasant surprise. 
 

Now that being said … there is no way I would suggest tearing into a good, functional lens. Even if (and that’s a big if) this results in an objective suitable for visual use, that’s a far cry from the pixel perfect high resolution scrutiny that the original lens with all 20 elements was designed for. And even if you could hand-wave all of that away… it’s a fast enough focal ratio that collimation would likely need constant tweaking as they do on fast Newtonian setups. 



#6 Echo29

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Posted 25 October 2021 - 05:14 PM

Hm i dont know if this would work but it is an interesting idea maybe if you can get some other dslr lenses remove their optical groups and dosome testing put them their constructions characteristics in OSLO and play around you might be able to build a telescope 



#7 MitchAlsup

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Posted 26 October 2021 - 02:55 PM

Generally camera lenses are designed to have an optimal focal distance way inside of infiniity--and there are no astronomical objects with a focal distance this close. This may result in some spherical.

In addition camera lenses are not designed for have as little coma as possible.

 

On the other hand, for casual use, they might get the job done.


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#8 luxo II

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Posted 26 October 2021 - 06:42 PM

Virtually everyone has a play with discarded lenses - at some point. Enjoy, and learn from the experience. Sometimes you can find a use for them that works, well enough.


Edited by luxo II, 26 October 2021 - 06:44 PM.

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#9 foxshark

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Posted 26 October 2021 - 07:55 PM

Virtually everyone has a play with discarded lenses - at some point.

And everyone grows out of it, too. Out of an abundance of clarification, at no point was/am I suggesting that a broken camera lens be a rival to a proper telescope, or that I’ve somehow discovered “one weird trick that telescope manufacturers don’t want you to know”.  That being said, there is enough light pollution where I live that you might as well find some way to entertain yourself with optics, because finding dim fuzzies won’t be it! When I do make it out to the dark sites, I just want a star chart, a red flashlight, and a light bucket of a scope.



#10 luxo II

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Posted 27 October 2021 - 05:20 AM

Don’t get me wrong - it’s possible to be very creative even with just one low-quality element, eg.

https://www.business...ll-image-2018-6


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#11 foxshark

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Posted 27 October 2021 - 06:04 AM

Oh wow, now that is pretty fantastic and creative



#12 Argonautt

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Posted 27 October 2021 - 01:16 PM

Interesting project.

 

Bearing in mind the construction of an afocal zoom lens, which this is

 

image.png

 

 

I think you might consider using both sets of elements behind the aperture stop. But then this quickly becomes, 'why not use most of the elements', when from experience I know the first, positive-power focusing group in a telephoto zoom lens can often be used as a diopter or its own objective.

 

 

Also, if you can, you might want to find out if your lens is a  mechanically or optically compensated zoom, and what groups may move when changing the focal length. This will tell you about the relationship between element groups and how best to eke out performance. For example, the last relay group may actually move, like it does in this Minolta AF 100-200mm f4.5 below:

 

image.png


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#13 foxshark

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Posted 27 October 2021 - 01:56 PM

Also, if you can, you might want to find out if your lens is a  mechanically or optically compensated zoom, and what groups may move when changing the focal length. 

Thank you for the insight!

 

I took notes, photos, and even a video (I think?) of the elements moving in the lens as I adjusted zoom and focus before I started to disassemble it. From what I could tell, the 1st group obviously moves forward as you go from 150mm to 600, but the middle elements do not move. The rear element set (the other one I boxed) moves rearward as the first group goes forward. At 600mm the front and rear groups are at maximum distance from each other. Focusing moves some of the middle elements, and other middle elements are bound to the optical stabilization system.


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#14 RichA

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Posted 29 October 2021 - 05:47 PM

Judging by my post history here, you could conclude that I am a tinkerer and like to try things just to see how they work. This project would fall under that context of education and discovery, not an attempt to make a bargain SV/TV/AT/whatever EDT.

 

My optics knowledge comes first from the photography world where more elements seems to mean a better lens, and ED/Super-ED is just a marketing thing. Sure Nikon has gold trim around lenses that have the ED nameplate, but even my old kit 18-70mm had that, so it can't possibly be as fancy as the arbitrary Canon L moniker with the red trim, right? Common knowledge also states that manufacturers are very specific about the optical formula to the point that if a long lens has a drop in filter, you need a filter in there even if you don't want one, as that bit of glass is part of the optical formula. 

 

But what if some of those elements could be repurposed? Lens designs like the double-Gauss are obviously not reducible in design, but telephoto lenses seem to be a combination of [an image forming big lens] and [a bunch of other lenses that shorten and modify the light path]. I asked my friendly local camera repair shop to keep an eye out for me for any "big zoom lenses that are irreparable for me to take apart". Totally reasonable, right?? Well as luck would have it ...

 

 

attachicon.gifIMG_8584.jpg

 

Twenty dollarydoos later, I was the proud owner of a Sigma 150-600 Contemporary that all the king's horses and all the kings men wouldn't be able to put back together again.

 

attachicon.gifIMG_8597.jpg

 

Here is the lens diagram with the twenty elements; I have noted the big triplet group on the left and a diverging lens group that acts as the extender and I assume a flattener as well. The middle elements are responsible for focusing and image stabilization. I went into the project open minded, but was still surprised by the figures of the triplet - appears to be 280mm ƒ/3 by measurement. I assume field curvature will be rather extreme by astronomy standards.

 

 

My initial use was going all the way back to the roots - as a camera obscura. I took the front triplet and pointed the lens at a window in a darkened room and looked at the image projected on the wall 11 inches / 280mm from the lens elements, and then with the extender group held between. A trip to my favorite astronomy and optical supply shop called Home Depot gave me my optical test bed of some wood and threaded rods to hold and adjust spacing between the elements. Collimation will be optimistic at best.

 

 

attachicon.gifIMG_8649.jpg

 

I set a Celestron SMA 25mm at the image plane and took a peek at Jupiter and could make out four tiny moons. Flare from stray light and collimation errors were so over the top that it looked like a J.J. Abrams movie, but it was fun still to see an otherwise sharp pinpricks of light.

 

Next up, integrating the extender elements group then maybe even building proper cells that would allow for real collimation attempts, if justified.

I've torn apart a lot of camera and other lenses and it's a miracle when one yields anything even marginally usable for a telescope objective.  However, long, slower, older lenses (400mm or more) of high quality would probably allow for a doublet or triplet group that must might work.  Problem is, much of the optical work is also done by smaller lenses in the train so you don't know what you'll get till you try it.  The only camera lenses known to absolutely work as telescope objectives are apo process lenses which usually have f9 focal ratios.


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#15 Benschop

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Posted 30 October 2021 - 10:55 AM

Neat topic. Camera lenses are great sources of fun for optical education to inquisitive minds.

 

Active Surplus on Queen West was a great source of such supplies back in it's day

 

I've torn apart a lot of camera and other lenses and it's a miracle when one yields anything even marginally usable for a telescope objective.  However, long, slower, older lenses (400mm or more) of high quality would probably allow for a doublet or triplet group that must might work.  Problem is, much of the optical work is also done by smaller lenses in the train so you don't know what you'll get till you try it.  The only camera lenses known to absolutely work as telescope objectives are apo process lenses which usually have f9 focal ratios.



#16 ccaissie

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Posted 30 October 2021 - 11:11 AM

Try the visual test again, build a stable holder for the eyepiece (cardboard tube/ tape/ whatever) and give it a good star test on a real star or distant glint. 

Up the power and see what ya got.  Note the aberration present and maybe you could compensate it a bit with some trickery.  Some eyepeices/barlows contribute aberrations that might complement what you have.

 

This testing should jive with your experimental spirit!


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#17 RichA

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Posted 30 October 2021 - 11:11 AM

Neat topic. Camera lenses are great sources of fun for optical education to inquisitive minds.

 

Active Surplus on Queen West was a great source of such supplies back in it's day

Yes, I once got 14 tank periscopes off them really cheap and each had a 32mm 2 inch Erfle eyepiece in them.


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#18 Bob4BVM

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Posted 30 October 2021 - 02:13 PM

Yes, I once got 14 tank periscopes off them really cheap and each had a 32mm 2 inch Erfle eyepiece in them.

Ouch !  Where was I back then !  Still have any ?



#19 Bob4BVM

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Posted 30 October 2021 - 02:30 PM

Virtually everyone has a play with discarded lenses - at some point. Enjoy, and learn from the experience. Sometimes you can find a use for them that works, well enough.

Sure enough, and btw, i haven't "grown out of it" any more than i  have grown out of any other part of the atm hobby.

 

I have a few WF finders i still use made from ~50mm cam lenses with EP  (& sometimes a diagonal) attached.

 

Also i have used many internal elements in my DIY-EP experiments, in a 'try and see' fashion. Maybe one in ten lens-stack combinations tested produces good & sometimes great results, good enough to be worth turning a proper EP housing for.

 

Then there are a few lenses. like this 3-dollar thrift store 70/400 tele that with an hour's work can yield a nice little refractor with the addition of a bit of tubing and an EP holder:

CS

Bob

 

IMG_4170s.jpg IMG_4171.JPG


Edited by Bob4BVM, 30 October 2021 - 02:33 PM.

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#20 foxshark

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Posted 30 October 2021 - 09:27 PM

…build a stable holder for the eyepiece (cardboard tube/ tape/ whatever) and give it a good star test on a real star or distant glint. 

Absolutely! That is what I am working on now - the photo in the first post was a “sanity check” type test to even see if it was worth trying. I took the lens housing apart even further to see how to extract the lens cell to make a proper collimating cell. Well, proper as far as plywood goes. 


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#21 RichA

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Posted 31 October 2021 - 04:32 PM

Ouch !  Where was I back then !  Still have any ?

No, that dates from the 1990's.  I sold most of the eyepieces to observers.  They were three achromat lens groups.  I was just amazing that the bottom of the barrel that came out of the periscope was exactly 2 inch fit to 2 inch diagonals.




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