Make Me a 100" Lens

[Hermes07]

Well... technically my goal is a 95.5" lens but that doesn't sound as catchy.

After a lot of research and some very helpful answers on the forum I've concluded that's it's basically impossible to get hold of a lens longer than 70" / 1800mm, other than stumbling over one by chance in the wreckage of a crashed spyplane prototype or an abandoned nuclear testing bunker. My goal is a 2400mm lens with at least 24 degrees of coverage, I would appreciate a wide-ish aperture (f/16-22) but I'll settle for something narrower if need be.

My question then is this - if you needed a 2400mm lens, what would you do to go about building/finding/adapting one? Would you get the best results by finding an existing lens and adding a negative/tele group to the back of it? Would you crack open lenses already in your possession and swap elements over? would you try building something entirely from scratch?

I'm planning on trying all of the above, starting when my shipment of elements from SS finally turns up, but I'd be grateful of any pointers. I don't have an unlimited budget but I'm willing to spend time and money on this - I also have a range of wide-aperture lenses all the way up to 1200mm and I wouldn't hesitate to "borrow" bits of any of them to make this a reality if that's what it takes.

[Math]

4 cheap M42 2x tele converters added to a small but long lens? :D

[Nathan Potter]

Essentially you could use/modify a reflecting telescope. Field of view might be a problem though, but there are wide field designs. Check into it. There are some astronomical types here who might help.

Nate Potter, Austin TX.

[Dan Fromm]

Look around on this http://alag3.mfa.kfki.hu/astro/giantlenses/index.htm site, which has some links to manufacturers. Ask about long focal length refractors on astronomy sites.

If you've got the money, honey, the manufacturers have the time.

[Jim Galli]

OK, I'll bite. Why do you need a 95.5" lens?

We use 117 inch lenses on tracking telescopes, f8, but they only cover half frame 35mm movie film, and a crane is required to lift them. BTW, you can only focus on the ground for about 5 minutes in the morning in perfect conditions and perhaps 8 to 10 minutes on a perfect afternoon just about 15 minutes before sunset. Prime time.

[ic-racer]

You need a 48X telescope. However the clincher is that the magnification specs for the telescopes are calculated with various eyepiece magnifications. If you take away the eyepiece to put on your camera adapter, you magnification is much less. For example a typical consumer scope that with specs of "100mm 50X power" with its eyepiece, may only be a 600mm focal length lens when you take off the eyepiece and attach a camera.

Another option is to use a 0.5 diopter lens (2000mm focal length) or 0.25 diopter (4000mm focal length) B&W makes both of those.

[sanchi heuser]

There was a thread from the italian cube project:
http://www.largeformatphotography.info/forum/showthread.php?t=71684
and a manufacturer of lenses was mentioned:
http://www.adrianololli.com/
$$$$:D

[Hermes07]

Ok... so I guess I'm in a 'telescopes only' region now:)

Dan, I have browsed that Giant lenses database often - I own a couple on the second page, as well as 1 or 2 more that would have a place on there if I submitted them. The trouble is that the lenses on there are mostly achromats and I have been reliably informed (ahem) that I'd be wasting my time trying to get any sort of sharp, wide field out of those.

I don't think I'd really need any sort of record-setting lens in terms of weight or diameter though. I have a 300mm lens, a 600mm lens and a 1200mm lens, all with 6"-ish diameters. All are roughly the same size & weight and are usable with a strong front standard and long bellows - a 2400mm lens with the same sort of dimensions doesn't seem so far-fetched. I'm sure from an optical design point of view it wouldn't have been any harder to design a 2400mm f/22 than a 1200mm f/11 if the demand had been there. This is what has got me hoping that there is a relatively simply change I can make to one of the lenses I already have to extend the focal length.

Jim, I need it to cover a 2 metre-high negative... of course :) I'm trying to achieve 2400mm specifically just because I'm fairly obsessive. As I mentioned, I already use lenses which are roughly 150mm f/1.2, 300mm f/2.5, 600mm f/5, 1200mm, f/10, e.t.c. so 2400mm f/20 would be the next logical step. From a practical point of view having my lenses spaced like this in terms of focal length and aperture does help as my field of view options and depth of field calcs stay pretty consistent between formats.

The telescope suggestions are appreciated. It looks like this is something I'll have to get to know in more depth.

[Steve Barber]

Maybe a ZEISS Apo Sonnar T* 4/1700 and a 2x Mutar? It weighs 564 pounds, but it would cover a 6x6 frame.

[Dan Fromm]

But Zeiss, some of the Russians, and some of the Chinese makers offer what they call apochromats. Asking is cheap ...

FWIW, the 2500/12.5 ApoSaphir covers a little over 1.3 m at infinity and is quite hard to find. Boyer is out of business but the Apo Saphir prescription is on www.dioptrique.info. All you need is a pot of money and a merchant lens maker.

[johnielvis]

I'd try to make a supplement to what you have---if you use a 2x tele extender/diverting lens, you'll double your coverage---I have a HUGE 2x lens that looks like it was used for doubling the size of projection gobos---it has a 6" diverging lens "front" element on it--I have tried it with other lenses and it does work as a "tele extender", but only for the shorter focal lenghs---the longer ones are overpowered by the strong negative lens---if I use the whole thing, then the field of view gets vignetted because the other side is like 3" in diameter......here' s pic of the monster....note--you'll need a BIG diameter diverging lens---otherwise you're focal length WILL increase, but the covering power will be vignetted---you'll end up with a longer lens with the same covering power as the shorter lens..surplus shed had some big negative lenses there I saw....big diameter...make a +/- combo with them for a teleconverter

[johnielvis]

WHOA!!!!

I just did a little more experimentation---that big lens cell...when used with my fuji w 250...together---with the large thing behind the 250...increases the focal length (flange) to about 1400 mm....looks like the angle of view has not changed...this gives a possible coverage cirtcle of 2240mm given a 400mm image circle for that lens (similar triiangles/cones)!!!!

I think I GOT it!!!..I gotta get this mounted in a dark room and see what the coverage circle actually is, but that 250 is about the longest lens that works with it...WOW...

now ANOTHER experimentation project.....I'll never have time to take pics....

[Hermes07]

But Zeiss, some of the Russians, and some of the Chinese makers offer what they call apochromats. Asking is cheap ...

FWIW, the 2500/12.5 ApoSaphir covers a little over 1.3 m at infinity and is quite hard to find. Boyer is out of business but the Apo Saphir prescription is on www.dioptrique.info. All you need is a pot of money and a merchant lens maker.

Dan, you beauty! That site is brilliant, exactly what I have been looking for.

I have been planning on trying to make a lens of the APO Saphir type using a couple of matched, modern achromats with a negative element in between. What's interesting to see is that unlike the Heliar / Dynar, the negative element in the APO Saphir is spaced centrally and is almost perfectly bi-concave. Guessing that this is in order to optimise for 1:1 and minimal distortion. Would I be correct in thinking the negative element is flint glass also?

My hope is that all I would need to do is scale the design up to 2400mm, find the achromats that most closely match the lenses called for by the formula, and then have the negative element custom ground and coated to suit. The latter would cost quite a bit for a one-off, but still a lot less than having the entire optic built from scratch. Sound possible?

RE: The apochromats, I was under the impression that they were still basically single groups and wouldn't be much better than the achromats in terms of angle of sharp coverage. Have I got the wrong end of the stick?

[Hermes07]

I'd try to make a supplement to what you have---if you use a 2x tele extender/diverting lens, you'll double your coverage---I have a HUGE 2x lens that looks like it was used for doubling the size of projection gobos---it has a 6" diverging lens "front" element on it--I have tried it with other lenses and it does work as a "tele extender", but only for the shorter focal lenghs---the longer ones are overpowered by the strong negative lens---if I use the whole thing, then the field of view gets vignetted because the other side is like 3" in diameter......here' s pic of the monster....note--you'll need a BIG diameter diverging lens---otherwise you're focal length WILL increase, but the covering power will be vignetted---you'll end up with a longer lens with the same covering power as the shorter lens..surplus shed had some big negative lenses there I saw....big diameter...make a +/- combo with them for a teleconverter


WHOA!!!!

I just did a little more experimentation---that big lens cell...when used with my fuji w 250...together---with the large thing behind the 250...increases the focal length (flange) to about 1400 mm....looks like the angle of view has not changed...this gives a possible coverage cirtcle of 2240mm given a 400mm image circle for that lens (similar triiangles/cones)!!!!

I think I GOT it!!!..I gotta get this mounted in a dark room and see what the coverage circle actually is, but that 250 is about the longest lens that works with it...WOW...

now ANOTHER experimentation project.....I'll never have time to take pics....


My head hurts.. same angle of view? as in the 2240mm circle created shows exactly the same field of view as the unmodified 400mm circle?

I am planning on trying something similar soon. I have just got hold of the tele section of a wray 36" f/6.3 lens that has been cut off. Approx 110mm diameter. The positive group in the front of the lens focuses an image at around 360mm on its own. I'm guessing that rigging the negative group up to the back of a 600mm or a 750mm lens might get me somewhere close too 2400mm.

[johnielvis]

DUDE...I just got it mounted on my studio camera...I RAN OUT Of MOVEMENTS


the only problem is the rear element was taped on the back---it gives FULL coverage but magnifies any deficiencies---I didn't get the elements close enough to get the 1.4 meters but I got about 45" of bellows at a building about 150 feet away or so....

I got the lens axis approx 2 feet from the bottom of the gg and could still see the full aperture....BUT..it looked like coma from the point lights at the building....however...slight stopping down and it seemed to dissappear quickly....

I got it on another board now that can center it better....gotta go to work though now....yeah...this has promise...at least something to play with as a start, you know....the focal length seems to depend on the distance between the "teleconverter" and the lens---I tried my 360 apo symmar but I could not get them close enough together to get a focus...I have a 330 lens I should try...that should be about the limits and give me a VERY long lens....

for longer lenses you 'll need a very slight negative element...very very slight---best is use like a large coverage wide angle lens with the "extender".....that will utilize the full angle at it's best...and you can use a more available negative lens...it's tunable too...the more you vary the distance between the extender the more the effective focal length changes.....

maybe try looking for these things where they sell professional playhouse lighting systems--gobo projectors--seems to me they may still be making these things, and they do still make very large gobo projectors/lenses for them

[Dan Fromm]

Dan, you beauty! That site is brilliant, exactly what I have been looking for.

I have been planning on trying to make a lens of the APO Saphir type using a couple of matched, modern achromats with a negative element in between. What's interesting to see is that unlike the Heliar / Dynar, the negative element in the APO Saphir is spaced centrally and is almost perfectly bi-concave. Guessing that this is in order to optimise for 1:1 and minimal distortion. Would I be correct in thinking the negative element is flint glass also?

My hope is that all I would need to do is scale the design up to 2400mm, find the achromats that most closely match the lenses called for by the formula, and then have the negative element custom ground and coated to suit. The latter would cost quite a bit for a one-off, but still a lot less than having the entire optic built from scratch. Sound possible?

RE: The apochromats, I was under the impression that they were still basically single groups and wouldn't be much better than the achromats in terms of angle of sharp coverage. Have I got the wrong end of the stick?You don't want to match a layout. You want to make a lens that matches a prescription.

The prescriptions Eric published give elements' thicknesses and radii, inter-element spacing, and the glasses refractive indices and abbe numbers. Most of them are scaled to make a 100 mm lens. Multiply all dimensions by (desired focal length in mm/100), buy the glasses specified, and start grinding.

After you've learned to read prescriptions in the format in which Eric publishes them, you'll see that the Apo Saphir is perfectly symmetrical.

[Hermes07]

You don't want to match a layout. You want to make a lens that matches a prescription.

The prescriptions Eric published give elements' thicknesses and radii, inter-element spacing, and the glasses refractive indices and abbe numbers. Most of them are scaled to make a 100 mm lens. Multiply all dimensions by (desired focal length in mm/100), buy the glasses specified, and start grinding.

After you've learned to read prescriptions in the format in which Eric publishes them, you'll see that the Apo Saphir is perfectly symmetrical.

Dan, I was looking at the site on a smartphone and I'll confess that I didn't go too far into the numbers - just looked at the pretty pictures :)

In case anyone is interested, here is my attempt at the 100mm APO Sahphir:-

http://imageshack.us/f/849/aposaphir100mmf10.jpg/

It's astounding that even picking the closest glass types I could find out of hundreds, that small discrepancy that remains means the focal length is off and I had to reduce the spacing a fraction to improve the resolution (could easily be that I've entered something wrong too - I'm by no means great with the software).

I will try scaling it up to 2400mm in a minute but even if it works, the optical glasses used to run the simulation are listed as obsolete. This is why I'm thinking it might make more sense (price-wise) to start with the best modern achromats I can find and then try to design the negative element around them to make a semi-faithful version, rather than resurrect the original design exactly as it was. It wouldn't be as good or as precisely engineered but it would be better than nothing and would cost only a few hundred to grind and coat a single element than £10,000+ to manufacture and assemble an entire lens.

[Hermes07]

DUDE...I just got it mounted on my studio camera...I RAN OUT Of MOVEMENTS


the only problem is the rear element was taped on the back---it gives FULL coverage but magnifies any deficiencies---I didn't get the elements close enough to get the 1.4 meters but I got about 45" of bellows at a building about 150 feet away or so....

I got the lens axis approx 2 feet from the bottom of the gg and could still see the full aperture....BUT..it looked like coma from the point lights at the building....however...slight stopping down and it seemed to dissappear quickly....

I got it on another board now that can center it better....gotta go to work though now....yeah...this has promise...at least something to play with as a start, you know....the focal length seems to depend on the distance between the "teleconverter" and the lens---I tried my 360 apo symmar but I could not get them close enough together to get a focus...I have a 330 lens I should try...that should be about the limits and give me a VERY long lens....

for longer lenses you 'll need a very slight negative element...very very slight---best is use like a large coverage wide angle lens with the "extender".....that will utilize the full angle at it's best...and you can use a more available negative lens...it's tunable too...the more you vary the distance between the extender the more the effective focal length changes.....

maybe try looking for these things where they sell professional playhouse lighting systems--gobo projectors--seems to me they may still be making these things, and they do still make very large gobo projectors/lenses for them

Interesting Stuff John. From the little I've played about with things like this in the past, I've also thought that a large, very weak negative lens could have a lot of promise.

Not knowing it's focal length, I'm definitely going to try this negative group I have with every lens I can find to see if any of them are close enough opposites to achieve the effect you've seen. Appreciate the suggestion about the projection lenses too.

[Dan Fromm]

[QUOTE=Hermes07;777799It's astounding that even picking the closest glass types I could find out of hundreds, that small discrepancy that remains means the focal length is off and I had to reduce the spacing a fraction to improve the resolution (could easily be that I've entered something wrong too - I'm by no means great with the software).

I will try scaling it up to 2400mm in a minute but even if it works, the optical glasses used to run the simulation are listed as obsolete. This is why I'm thinking it might make more sense (price-wise) to start with the best modern achromats I can find and then try to design the negative element around them to make a semi-faithful version, rather than resurrect the original design exactly as it was. It wouldn't be as good or as precisely engineered but it would be better than nothing and would cost only a few hundred to grind and coat a single element than £10,000+ to manufacture and assemble an entire lens.[/QUOTE]You can do it cheaply or well.

Re glass not meeting the specifications in the prescription, this has always been the case. Eric tells me that Boyer tested each lot of glass when received for refractive index and Abbe number and recalculated designs to take get best results with the glass on hand. He says no glass matches the catalog perfectly.

Don't limit yourself to "heliar" types. Dialyte types have their charm and so does the humble Cooke triplet.

And don't limit yourself to Eric's collection of patents. He's interested mainly in old old lenses, hasn't worked hard at collecting modern patents. I'll betcha that dialyte type Apo-Nikkors and Apo-Ronars have been patented, also that TTH patented some of their triplet process lenses.

[Ash]

Biggest lens I have is a single element, has about 60" focal length and is f/20 I think.

[Hermes07]

You can do it cheaply or well.

Re glass not meeting the specifications in the prescription, this has always been the case. Eric tells me that Boyer tested each lot of glass when received for refractive index and Abbe number and recalculated designs to take get best results with the glass on hand. He says no glass matches the catalog perfectly.

Don't limit yourself to "heliar" types. Dialyte types have their charm and so does the humble Cooke triplet.

And don't limit yourself to Eric's collection of patents. He's interested mainly in old old lenses, hasn't worked hard at collecting modern patents. I'll betcha that dialyte type Apo-Nikkors and Apo-Ronars have been patented, also that TTH patented some of their triplet process lenses.

Dan, needless to say if I had the funds I'd walk into Schneider or Rodenstock, dump a wheelbarrow of cash on their doorstep and tell them not to sleep until I had my lens but at the moment I don't think anything commercially built from scratch (even on a smaller scale) is going to be do-able.

Hence I've focused on lenses with the least possible number of elements that I may stand a chance of making / modifying myself with a bit of fiddling and a grand or two to spend on parts. I'm not enlarging so I don't need a lot of resolution and I'll be using it at 1:1 at least so the demands on coverage are not huge.

A symmetrical lens seemed the obvious candidate as it takes out three aberrations and works well at 1:1. A design with a negative group also seemed logical as the focal length needed is so weak that getting there with positives only would be difficult. That's how I landed on the triplet and then discovered that it was actually much easier to get large-diameter achromats than simple meniscus lenses. That's left me with an APO-Saphir sort of design and I'm pretty confident (possibly blissful ignorance) that if I could find out the exact composition of the two achromats I'd be using, I could have a negative element for the centre ground that would correct them well enough to use for my limited application, if not perfectly.

Would love to have a dialyte at this focal length and I would definitely try to find some modern formulas If I went this way but as there are more groups to work with, I can't help thinking I'd be making things harder than they already were for myself.

[Hermes07]

By the way, when the design is scaled up to 2400mm, the two achromats required end up at 1000mm on the dot - I'm going to take that as a sign from god :)