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Old January 14th, 2007, 06:53 AM   #1
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Purely Optical Solution

I know it's possible (JVC's doing it in their 16mm adapter), but I have no idea how it would work. Any one know? I'm not about to try and make a purely optical adapter, I'm just very curious how the concept works--since it seems impossible.
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Old January 14th, 2007, 07:23 AM   #2
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I was spending a few months trying to create an optical design to increase the relative apperture and thus decrease Fstop rating. A few lenses in the front of the camcorder optics to increase the size of the opening pupil in comparison to the focal length (1/Fstop = opening pupil/focal length).
In my point of view, the optical adapters must be some kind of variance of that in order to get shallow DOF.

On the other hand, to build an optical adapter with deep DOF and 16mm lens field of view is quite easy.
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Old January 14th, 2007, 09:40 AM   #3
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JVC`s adaptor is basically a static DOF adaptor that doesn`t even flip the image. Sure it might have a relay without iris and less light loss.

T
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Old January 14th, 2007, 11:05 AM   #4
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But does it use some intermediary screen?
If I understood Matthew's question correctly, he was asking if a direct optical solution without any projecting element is possible.
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Old January 14th, 2007, 02:23 PM   #5
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It is my understanding (maybe my misunderstanding) that the JVC is purely optical--no focusing screen at all. It allows for 16mm PL mount lenses, retains their depth of focus and optical characteristics, loses next to no light, and is purely optical from what I understand.

Of course, it also only works with 16mm, not 35mm, doesn't flip, and is over $4,000. But I can't figure out how it works. Sure you can just put the lens on without a focusing screen, but then you don't get the shallow depth of focus or even the ability to focus.
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Old January 14th, 2007, 03:54 PM   #6
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Well, with a complicated lens assembly, it could "convert"

Let's say you got a 16mm PL mount lens: 25mm F2.0

Converted to 1/3" this should be roughly 14mm. If you retain your field of view, your maximum aperture opening will be approximately twice as high (F1.0). This is because opening pupil stays equal, effective focal length halves.
Then you would indeed have a far more shallow DOF.

Does it optically work? I don't know. I have tried exactly that with no success. But I am very willing to share my gathered knowledge with anyone who wants to continue trying, as long as it stays public domain.
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Old January 14th, 2007, 04:04 PM   #7
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I believe the new JVC's flip the image "in-cam" so any lens adaptor can be used. They are trying to target the indie crowd and this is one of their new features.
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Old January 14th, 2007, 04:57 PM   #8
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I may be confused by the initial question, but here's what it sounds like:

Does the JVC 16mm Lens adapter give the same FOV, DOF and T-stop as a 16mm lens on a 16mm film camera?

The answer is no. It gives the same FOV, DOF and T-stop as that particular lens...on a 1/3" camera. When people talk about "converting" a lens from 1/3" to 2/3" or 35mm (academy), they're talking about equivalent FOV. A 25mm lens is a 25mm lens, period. It may look very telephoto on a 1/3" camera, but it will look wide angle on a 35mm film camera. BUT, the focal length does not change! The Field of View (FOV) does change, drastically.

The 16mm adapter for the HD-100 is a mechanical adapter to make the mount type as well as the flange-focal distance compatible between 16mm lenses and the HD-100 body. The reason for this is to expand the number of possible lenses you could use with the system, especially to include extremely fast film-style primes. I won't go into the difference between film-style lenses and ENG-style lenses, there's a lot of info about that.

In terms of DOF, the quest to get 35mm-like DOF on 1/3" cameras requires understanding of what factors affect DOF. Image size (1/3", 35mm, s16, etc), focal length, aperture, and focal distance from lens to subject. Now,with a 50mm lens at T/4.0 18" from the subject on 35mm, the DOF will be pretty damn shallow. Take that same lens, and change nothing except the camera...to the HD-100 (1/3" chips). All of a sudden, the image appears to be "zoomed WAY in." If you take a still from the 1/3" camera, and a frame from the 35mm (cropped to the same composition as the still frame from the 1/3") you'll have images which look very similar. They will have the exact same DOF. So what's the problem? On a 1/3" chip, the 50mm lens has a very very narrow FOV. So the natural solution is to switch to a wider lens. But switching to a wider lens also changes one of the factors of DOF.

So the long answer is no, there is no straight "optical" adapter that magically turns a 1/3" chip into a 35mm sized chip.*****

The Mini35, M2, Letus, etc... all use the same principal to achieve the shallow DOF look. They all present the camera with an image resolved on a ground glass that's approximately the size of a 35mm frame. If you photograph that image on the ground glass, and compare it to a frame from a 35mm camera using the same lens, same T-stop, and same subject distance, you will have essentially the same FOV AND DOF. That's the trick - focusing the lens on "film" that's the same size as 35mm film, and then shooting that image. The reason that the HD-100 has a flip setting is to make it work easier with such adapters, or lenses whose nodal point is different than the ENG-style lenses.


*****Well, kinda. Theoretically, you could make a lens with a T-stop SO small that it has the same DOF as a 35mm lens with equivalent FOV. But current technology ain't there yet, and may never be.
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Old January 14th, 2007, 07:17 PM   #9
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No GG 35mm Adapter solution... Possible.

Quote:
Originally Posted by Jaron Berman
So the long answer is no, there is no straight "optical" adapter that magically turns a 1/3" chip into a 35mm sized chip.*****
I think the definitive Dof Adaptor may use a fiber optic taper attached to the sensor. Have a look...:

http://www.proxitronic.de/prod/fo/ekoppeln.htm
http://www.proxitronic.de/
http://home.comcast.net/~mwaltuck/Tapermag/index.html
http://www.us.schott.com/fiberoptics...ts/tapers.html
http://www.grand-illusions.com/acatalog/info_58.html
http://www.plastecs.com/fiber%20optics.htm
http://www.o-eland.com/Instrumentation/faceplate.htm

In the last link you can find inverters for image flipping if necessary.
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Old January 14th, 2007, 07:30 PM   #10
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Carlos,
That's just what I was thinking. It would explain why the adapter only handles 16mm lenses, as a 35mm sensor-sized to 1/3" taper might be too bulky to integrate or difficult to manufacture. However from what I've read of these things they don't go much above 70% light efficiency. That's worse than typical DOF adapters on the market today...
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Old January 14th, 2007, 08:22 PM   #11
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Essentially you're replacing the relay lens with a fiber optic pipe. Same principal as mini35, m2, etc. The image is still formed on a plane in front of the sensor, and photographed through a lens assembly to the sensor. 6 of one, half a dozen of another. Two different approaches to the same solution. Technically, they're both "straight optical solutions." True, this version has no "GG" but the ends of fiber optics ARE GG's.
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Old January 15th, 2007, 04:19 AM   #12
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its na funny thing but I had suggested using fibre optic tapers and image inverters some six months ago but a number of people said that they were not suitable because there would be to much light loss, but it seems that fibre optrics have already been coupled with ccd's to deal with things such as low light image inversion, reduction and enlargment, for a while now.


told ya so
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Old January 15th, 2007, 04:21 AM   #13
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I replied to all this too soon, 70% light loss , sheesh . maybe I didn't told ya so..... teehee
rob
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Old January 15th, 2007, 08:01 AM   #14
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Quote:
Originally Posted by Robert Gradisen
I replied to all this too soon, 70% light loss , sheesh . maybe I didn't told ya so..... teehee
rob
Robert, you´re wrong, it´s 70% light efficiency, no 70% light loss.
When you use an adapter with GG, the light goes throught the GG, the macro lenses, and the camera lenses (some lenses in the complete objetive). Each lens could have a 2-5% light loss. If you use the Fiber Optic system, you don´t need all these elements. I think the image could be even brighter, and there are no problems with image distortion, chromatic aberrations, vigneting, grain, motor noise... I think this is the cleanest solution for a 35mm adapter.

Jaron, I think the image is projected directly over the sensor, not over a GG, but I can be wrong.
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Old January 15th, 2007, 10:07 AM   #15
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We are already seeing adapters for less than $1000 that see negligable resolution loss, have very little light loss, and are improving a great deal with respect to ease of use and reliability. Even given those attributes, we are continuing with product developement to further refine our own implementation.

It would seem RED, which will allow the use of full format lenses with their inherent DOF properties, would be the logical next step up with respect to total camera/adapter cost.
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