Thread: imax groundglass dof adapter?

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  1. #1 imax groundglass dof adapter? 
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    Hi

    Jack of no trade, master of none returning wanting to find/create a imax sized dof adapter. Nothing special just want to play around. If anything comes to mind that I could potentially use please comment below! thanks :)
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  2. #2  
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    Quote Originally Posted by Daniel Porto View Post
    Hi

    Jack of no trade, master of none returning wanting to find/create a imax sized dof adapter. Nothing special just want to play around. If anything comes to mind that I could potentially use please comment below! thanks :)
    This should do http://www.yedlin.net/NerdyFilmTechS...nceptions.html
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  3. #3  
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    Daniel.


    A few people have played with the notion, using really old-school plate camera hardware. A smaller practical choice of lenses and their speed becomes a limiting factor in using larger format groundglass adaptors.

    Two methods have have been demonstrated, one through groundglass, the other reflective from a screen. The groundglass arrangement was used to confer an image to a notionally Super16mm in an early SI2K camera. It was given the name, Coatwolf camera and was used for an actual indie motion picture production "Bellflower" for a specific aesthetic. It appears to have been constructed using parts of a large format plate camera and a groundglass screen. I think there may have been a version made with a moving groundglass to de-resolve the groundglass texture.






    Another young enthusiast, constructed a reflective device which used a DSLR camrera to aquire an image off an enclosed screen.




    All the drawbacks of groundglass based image relay remain :-

    Brightness falloff into corners,
    Narrowed range of available iris settings, a ceiling of f5.6 beyond which the groundglass texture will become visible or create a weird artifact.
    Two focus systems to manage, the relay focus being supercritical and easy to forget.
    Two irises to manage, the relay iris and objective lens iris lens settings interacting and requiring care for best result.
    Awkward and bulky appliance.

    I was achieving sharpness results to the limit of the SI2K (2K) and Sony EX1 (HD) cameras with a modified Letus35 which some sod stole from me during a house burg. Bizarrely, they did not touch a "big" URSA. which was sitting on the kitchen table.

    Dennis Woods" Cinvevate adaptor and a special relay lens he built achieved identical results when carefully managed.

    I would anticipate 4K resolution using plate camera pieces using a groundglass with a 5 micron finish with a very slight backpolish, but probably no better. You could use a genuine boss screen which is waxbased if they are still procurable. That will yield a sharper relayed image at the expense of a ghosting artifact around bright obects and more apparent brightness falloff into corners.

    A 3 micron finish of a groundglass will yield a similar resolution to the boss screen and be a little more robust in warm to hot conditions in which a boss screen can be expected to melt and its crystalline texture become patchy and possibly ruined.

    You would be taking the imax sized image out of the centre of the plate image and hopefully inside of the natural brghtness falloff into the corners of the groundglass plate.

    You may well have made groundglass relay devices before or used them and likely know of the thread on dvinfo/net which I think still holds the record for most posts on a topic. In case you have not, here is the web address below. It was an exciting time whilst it lasted but became quickly dead with RED and the Canon 7D DSLR. Interestingly focal reducers got a mention but were deemed then to be difficult and specific to individual camera lens focal lengths to be practicable, then along came Brian Caldwell and Metabones Speedboosters.


    https://www.dvinfo.net/forum/alterna...m-adapter.html
    Last edited by Robert Hart; 05-24-2020 at 09:22 AM. Reason: fix links which I wrecked
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  4. #4  
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    thanks for the responses! going with a 4x5 camera setup, recording the groundglass with my blackmagic micro cinema camera. will post my progress in this post :)
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  5. #5  
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    I've done this with an 8x10 camera. Your biggest challenge is the ground glass. Noticeable grain, extreme light fall off. If you like that, then great. There are solutions to this however.

    here's the thread about my project -

    http://www.reduser.net/forum/showthr...ini&highlight=

    Nick
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  6. #6  
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    Daniel.


    I forgot to mention that you will receive an inverted image to the camera sensor. For focus and framing purposes, you would best have a separate monitor and you may need to mount that upside-down for correct image orientation or mount the camera upside-down instead. For intuitive simplicity of operating the camera, I would recommend it is fitted right-way up.

    The more sophisticated home made adaptors used various prism or surface-caoted mirror systems to erect the image optically and to shorten the actual lengths of the devices by folding the image path.

    To pass a plate camera image without a prism or mirror erecting path causing a vignette, that piece would have to be huge. I would not recommend doing this but use the Coatwolf apporach of a direct or non-erecting relay and suffer the longer appliance length.

    You would have to do the math to work out the power of relay lens you would require. In my AGUS35 arrangement for a standard 35mm motion picture image frame, the path was from groundglass image plane :-

    Folded or direct optical axis path to relay lens of approx 125mm.
    4+ power achromatic dioptre as relay lens attached or located close to the front element of the camera lens.
    Camera zoom to approximately 50mm to 70mm to frame the groundglass image correctly.

    For best adjustablity of the relay focus, the construction should aim for the groundglass to be in sharp focus in the mid-range of the camera's focal movement.

    With groundglasses, a trace of the aerial image will pass through. The finer the texture like a 3 micron grind, more will pass through. It will confer an apparent improved sharpness at penalty of aggravated brightness falloff into corners. There may also be a "ghosting" artifact with brightly lit objects like bleached tree branches in front of a dark background. With a coarser than 7 micron grind, the ghosting and aerial image artifacts disappear but at pebnalty of poorer sharpness and loss of light getting through the groundglass. It is all a matter of compromises and finding a balance, often a personal preference for a particular "look".

    If you can devise a way to move (vibrate) your groudglass at least 0.5mm in a circular motion then this will work greatly to removing visiible groundglass texture in the image and confer an apparent improvement in sharpness. this will be okay for normal shutter speeds of about 1/50th sec. I managed about 1/100th sec before freezing the groundglass texture which then jumps about in steps.

    Normal operating practice with the groupdglass relay system :-

    Switch camera on.
    Open camera lens iris wide. Adjust focus to a blur or point at something uniformly nondefined like a wall or sky.
    Close adaptor lens iris to f16 to show groundglass texture.
    Focus on the groundglass for best sharpness of texture.
    Point camera at a subject.
    Adjust adaptor lens iris to f5.6 or wider.
    Close camera lens iris for best exposure. Use NDs as needed to maintain camers lens exposure in the zone of f4-f5.6.

    With a 1/3" videocamera ( PD150 and ZIP ), the groundglass frame I aquired at zoom 50mm was about 22mm wide. Relay lens 4+ achromatic dioptre on the AGUS35. You can use 7+ for a non-erecting shorter path but then you risk corners going out of focus.

    With a 1/2" videocamera ( EX1 ), the groundglass frame I aquired at zoom 42mm was about 32mm wide or as wide as I could get it without picking up a prism vignette on the modified Letus Extreme which had a relay power of about 4+.

    With your Imax frame which I do not know, on the BMP4K. I imagine you may need a lens of about 90mm to 120mm focal length to cover the wider sensor frame. However your Imax frame itself is larger so it may not have to be as long a focal length. I am really guessing wildly here. Your image path may need to be longer to avoid corner softness in the relay. To achieve that you may need to consider a 3+ achromatic dioptre and set the camera lens slightly further back than 7".

    For test purposes I would use a zoom lens so that you can determine the ideal focal length for your prime lens. Zoom lenses are tricky though. Unless corredctly collimted to the camera, with zoom movements, the focus may have to chage and that does weird stuff to the relay path groundglass to lens distance. What looks good on the zoom lens may be way off with a prime lens.

    Like a tube-based nightvision intensifier, the groundglass will magnify any deficiency in the image from the lens you put in front of it. A lens which is soft will appear even softer when the image is groundglass relayed. Only the best lens on front will do. With large plate camera format, this may not be as critical as with the 35mm stills or cinecamera lenses. If the lens mage contains a pinpoint detqail which impinges on one set of facets in a groundglass pit or peak, subjectively a spill of that highlight implicates about nine similar sets of facets give or take a few. On a 5 micron grind texture, that becomes a highlight plus flare of about 15 mictrons seen by the camera. I f the adaptor lens conveys a pinpoint highlight softly and impinges upon say nine sets of facets of a 5 micron grind, then potentially there are 81 sets give or take of facet set implicated in the flare.

    Now there will be better brains and theorists who can explain this correctly with the approprate technical words and definitions.
    i
    Good luck wth your endeavours.

    To deresolve the groundglass texture you need to zoom back as far as you can to get the widest image without cordern brightness falloff. That scales the individual "grains" of tghe groundglass texture smaller in the image frane.

    With the BM camera, there being no OPLF, you are likely to experience a shimmer or colour artifact similar to what you may see when a highly textured subject like forest foliage is in the image. There is not much to be done about that except to use a wider aperture to defocus the texture slightly behind your intended subject. This will be most apparent when using wide or ultrawide lenses on the front of the adaptor. I am not sure there are affordable ultrawide lenses for plate cameras so the artifact may be a self-solving problem unless a texture like a forest or a scree slope IS your intended shot.


    Last edited by Robert Hart; 05-25-2020 at 10:21 AM. Reason: added video link
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  7. #7  
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    Thanks everyone! Nice work nick
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