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Old 17/10/08, 10:58   #1 (permalink)
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Default Home made 900mm lens

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Whilst this is not strictly Leica the following may give some of you an idea as to how to make a budget priced super long focus lens of a focal length not readily available. Hope the moderators permit its inclusion here. Such a lens could be adapted for Visoflex use

Tony Lovell and I are bothe members of Peterborough Photographic Society. Back in 2003 we were discussing the possibility of making a super long focus lens ie one in excess of 800mm. focal length. After exploring the Surplus Shed website Surplus Shed we saw some interesting ex govt. optics which seemed to offer some good potential. Tony subsequently bought a surplus flight simulator projector lens - an f3.2 11 inch diameter triplet of 36 inch focal length ie 914mm. I think it cost about £250 which at the time was considerably cheaper than any equivalent camera lens. In fact the only other lenses anywhere near this focal length were/are probably the 800mm Nikkors and Canons and 1000mm Pentax optics ... plus the Leica Apo-Telyt-R modular 280-800mm system which appears to have been discontinued ... and the Leitz Telyt-S 800/6.3 lens discontinued in 1997.

When the lens eventually arrived it weighed approx 20 pounds but there was obviously something not quite right with it. There were some very obvious Newton's rings visible which were indicative of possible balsam separation. However, when Tony took the lens apart he discovered that none of the elements were cemented but strangely, the separate elements were coated in oil. The oil was removed using alcohol and the elements reassembled into the original module.. Here is a picture of one of the 11 inch elements during the cleaning process.

http://i197.photobucket.com/albums/a...nt_with_A3.jpg

Tony works as a structural engineer and is thus an accomplished CAD designer. This is his initial lens mount drawing designed specifically for use with his Pentax 67 and large format 5 x 4 cameras. Note the attention to very adequate baffling throughout the lens barrel to reduce flare. Also note the very large lens hood to further reduce flare. The lens mount/barrel design parameters were established by trial and error by supporting the original lens module in front of a Pentax 67 camera whilst trying to focus on a distant church tower.

http://i197.photobucket.com/albums/a...anufacture.jpg

The drawing was submitted to local peterborough engineers MIDAS TECHNOLOGIES (GB) LTD a specialist aluminium fabrication company. This was the result including the natty clips for attaching the lens hood.

http://i197.photobucket.com/albums/a...ingbaffles.jpg

cont'd ...
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Old 17/10/08, 11:00   #2 (permalink)
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Default Re: Home made 900mm lens

Tony then had to "flare-proof" the interior of the aluminium barrel . A stipple finish coating was achieved using Araldite epoxy resin sprinkled with grit. Here you see both the detachable lens hood and the lens barrel.

http://i197.photobucket.com/albums/a...tedandlens.jpg

The stipple finish then received a coat of matt black spray barbeque paint

http://i197.photobucket.com/albums/a...thelastsec.jpg

Next the exterior was given a coat of white paint and the 3 lens element module mounted inside the barrel. A Pentax 67 variable length extension tube was mounted on the rear of the lens for focusing purposes. This photo shows the Pentax 67 medium format camera mounted on the lens. Note the use of 2 tripods. The bulk of the resultant 59 pounds lens weight ( 46pounds plus 13 pounds for the hood) is supported by a fluid head on an ex BBC TV outside broadcast camera tripod. The second tripod supporting the camera body is essential to stabilize the whole set-up and neutralize the inevitable magnification of camera shake/shutter vibration inherent with such a long focus lens and medium format focal plane shutter. When Tony transports the behemoth plus 2 tripods to and from his Land Rover he uses a sack truck.
http://i197.photobucket.com/albums/a...tytripod-2.jpg

An iris diaphragm was also incorporated into the original design but the biggest obtainable was only wide enough to mount at the rear end of the lens barrel. Unfortunately it failed to work in that position apart from just vignetting the image. Here is a photo of same but it was subsequently abandoned. In its place Tony uses home-made Waterhouse stops cut out from fogged Xray film plates which are inserted between the hood and the front lens element . Waterhouse stops are fixed circular hole diaphragms as used in ancient Victorian large format plate film cameras.

http://i197.photobucket.com/albums/a...taxmount-1.jpg

con'd ....
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Old 17/10/08, 11:02   #3 (permalink)
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Default Re: Home made 900mm lens

Tony took the camera and lens to Alexandra Palace for its first test where he took photographs of Canary Wharf and the Dome which are both approx 8.5 miles distant. As with all super long focus and telephoto lenses it can be difficult to obtain decent images of subjects several miles away because of atmospheric haze/dust . However, the results are pretty good especially considering this was Tony's first attempt at making such a lens . In the original copy of the Canary Wharf photo it is possible to read the HSBC logo on the top of the building.

http://i197.photobucket.com/albums/a...mAlexandra.jpg

http://i197.photobucket.com/albums/a...ace85miles.jpg

Another super long focus lens project is currently on the back burner. When finished it will be a 1600mm f10 lens which should be a bit more manageable because the front lens element is only 6.25 inches in diameter.

The lens cost ££hundreds to make but anything "off the shelf" would have cost ££thousands.

Tony is still experimenting with the lens and plans to adapt it for use with his Nikon D700. He is also planning to write a more comprehensive article and submit it for publication. These photos are his copyright so please treat as such. He has given me permission to reproduce them here.

Here is a moon photograph taken with the lens. A Pentax teleconverter may have been used with the lens for this photograph.

http://i197.photobucket.com/albums/a...e2xDec20-1.jpg

I don't think Tony has used the lens after a rainstorm yet. When he does the results should be better because there will be less atmospheric dust to penetrate. Note the difference in contrast in the above photographs between the far distant and intermediate distant buildings. The London atmospheric dust concentrations must vary greatly. Tony also tried the lens from the top of the Queensgate multi storey car park in Peterborough. When focusing down Bourges Boulevard the lens resolved sharp images of vehicle tax discs 200 metres distant. The lens is surprisingly sharp considering it has only three lens elements. And it is flare free thanks to the comprehensive baffling and internal barrel coating - and the relatively few lens elements compared to other types of design.

Tony and I are both fans of Andreas Feininger and his pioneering long focus lens photographs of Manhattan. In fact it was after reading "Feininger on Photography" (originally published in 1949 but still a wonderful read) and seeing illustrations of Feininger's home made long focus lenses that this project was born. Reading Sidney Ray's book "Applied Photographic Optics" also helped particularly chapter 15.3 regarding flare reduction and designing an effective lens hood.

How would you like to use something like this with a Visoflex?

Cheers

dunk
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Old 17/10/08, 11:05   #4 (permalink)
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Old 17/10/08, 14:08   #5 (permalink)
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Default Re: Home made 900mm lens

Excellent!
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Old 17/10/08, 17:07   #6 (permalink)
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Default Re: Home made 900mm lens

It's always nice to read about home-made projects, because that's always great adventures begin!
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Old 18/10/08, 23:41   #7 (permalink)
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What a great project, congratulations.
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Old 12/11/10, 19:56   #8 (permalink)
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Default Re: Home made 900mm lens

Terrific work. Thanks for sharing!

Would I be correct in guessing that the images shown are monochrome due to excessive CA displayed with color work?
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Old 12/11/10, 19:58   #9 (permalink)
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Default Re: Home made 900mm lens

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Originally Posted by cheekygeek View Post
Terrific work. Thanks for sharing!

Would I be correct in guessing that the images shown are monochrome due to excessive CA displayed with color work?
The monochrome images were taken using B&W film stock.

Welcome to the forum.

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Old 12/11/10, 20:01   #10 (permalink)
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Default Re: Home made 900mm lens

Welcome, Cheekygeek. You are clearly an archaeologist!

Regards,

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Old 12/11/10, 20:05   #11 (permalink)
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Thank you for the welcome!

I apologize if you found my question insulting, as that was not my intention. I come at this from more of an astronomical perspective, and I believe that CA was a common problem with simple triplets, especially from the "old days".

You've essentially built a pretty fast telescope with impressive light grasp. I'd love to substitute a 2" eyepiece for the camera body and see what Milky Way star fields would look like under a dark sky!

I was just curious as to whether you have tried color with it and had any color images to share. Thanks once again for sharing your project and information!
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Old 12/11/10, 21:42   #12 (permalink)
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Default Re: Home made 900mm lens

Fascinating project!

When you say 900mm focal length, do you mean with 35mm film (or D700)? It will obviously be less with the Pentax.
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Old 12/11/10, 22:16   #13 (permalink)
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Default Re: Home made 900mm lens

Correct me if I'm wrong, but focal length is focal length which is focal length.
It is a 900mm focal length lens. Now what that means in 35mm-equivalence is a different question.

Normal on a 6x7 camera is approx. 90mm f.l.. So this would be 10x normal.
Normal on a 35mm camera is approx. 50mm. So 900mm would be 18x normal.

Thinking of it another way, if a 35mm camera replaced the 6x7, it would be like taking the center crop out of the 6x7's image. But the focal length and the image itself would be unchanged. Replace that 35mm camera with a Pentax APS-C DSLR and you're taking an even smaller crop out of the middle of the image.

Now I'm wondering if I'm even explaining it right...
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Old 12/11/10, 22:23   #14 (permalink)
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Originally Posted by elansprint72 View Post
Fascinating project!

When you say 900mm focal length, do you mean with 35mm film (or D700)? It will obviously be less with the Pentax.
Focal length does not change with format. 900 == 900.
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Old 12/11/10, 23:21   #15 (permalink)
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Take no notice of me (I'm an Engineer)! Of course, the focal length does not change, what an idiot!

What I'm trying to ask is.... I think.... will the image produced be "full-frame" on the 6x7 camera or the 35mm camera? Am I making sense? Even my 1947 copy of the Ilford Manual of Photography is not helping me out here!

Cheeky, I think you are probably explaining it right but I'm a poor pupil!
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Old 12/11/10, 23:38   #16 (permalink)
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Given that the diameter of the front element was given as 11in, we can safely assume that the image it produces is amply larger than both the 6x7cm and the 24x36 format. Hence, you can place the smaller or the larger bit of film behind the lens. The magnification will remain the same but the crop will change with the size of the film piece. Does this help any?
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Old 12/11/10, 23:39   #17 (permalink)
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Default Re: Home made 900mm lens

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Originally Posted by elansprint72 View Post
Take no notice of me (I'm an Engineer)! Of course, the focal length does not change, what an idiot!

What I'm trying to ask is.... I think.... will the image produced be "full-frame" on the 6x7 camera or the 35mm camera? Am I making sense? Even my 1947 copy of the Ilford Manual of Photography is not helping me out here!

Cheeky, I think you are probably explaining it right but I'm a poor pupil!
The lens will cover more than 6x7 as it was actually designed for a flight simulator.

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Old 13/11/10, 00:57   #18 (permalink)
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I'm wondering about the oil you found coating the inner surfaces of the lens elements.

If the lens was designed as an air-spaced triplet it's very surprising that the spaces between the elements would be small enough for Newton's rings to appear. OTOH strain from thermal expansion could be a huge problem in a cemented triplet of this huge diameter in a projector. So maybe the oil was intended to fill the space between the elements (cf. oil-immersion microscope lenses)?
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Old 13/11/10, 09:25   #19 (permalink)
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I'm wondering about the oil you found coating the inner surfaces of the lens elements.

If the lens was designed as an air-spaced triplet it's very surprising that the spaces between the elements would be small enough for Newton's rings to appear. OTOH strain from thermal expansion could be a huge problem in a cemented triplet of this huge diameter in a projector. So maybe the oil was intended to fill the space between the elements (cf. oil-immersion microscope lenses)?
I was not present when the oil was discovered; the new lens mount was designed and built by Tony Lovell with a bit of help from my optics books. When the lens was received by Tony there were obvious 'rings' visible inside and he decided to dismantle the original mount to investigate ... whence the oil coating was discovered and removed. I'll have to ask Tony if there could have been a possibility that originally the lens may have had oil filled spaces. And yes, a projection lens of this size would become very hot. I do know a flight simulator designer but he is probably mainly involved with the software side rather than the hardware; I'll try and contact him to enquire if he is aware of any oil filled lenses used in the projectors.

Cheers

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Old 13/11/10, 10:49   #20 (permalink)
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Default Re: Home made 900mm lens

There have been some comments on another forum about this lens which seem to 'poo poo' the project and say that anyone can attach a telescope to a camera and obtain similar results. However, those denigrators are missing the point that this lens has a much wider aperture than any budget priced refracting telescope and that it was primarily designed for medium and large format film photography ... as distinct from 35mm film and digital . And someone else suggested that using a catadioptric lens would be much easier. Perhaps they have overlooked that mirror lenses of this aperture would be prohibitively expensive and that they would be limited to one aperture and also result in the distracting 'doughnuts' syndrome. Unlike a cat. lens, this lens is capable of being modified for use with an iris diaphragm. Someone else suggested the images are not that sharp but they are overlooking the fact that atmospheric pollution plays havoc with ultra-long focus lenses ... plus the fact that the images are compressed before posting.

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Last edited by dkpeterborough; 13/11/10 at 11:38.
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