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focus depth of video projector

Discussion in 'Hardware' started by wickedthoughtzzz, Jan 9, 2012.

  1. wickedthoughtzzz

    wickedthoughtzzz New Member

    I am new to video mapping and have a sculpture I am throwing video on to. It has a depth of about 3 feet. I am using a short throw video projector so that I can get a large image from only 4 to 5 feet away. My problem is that I can't get the image to be in focus (depth of field?) through all 3 feet of the depth of the sculpture. If it is in focus in the front, it is out of focus in the back and vice versa.

    What attribute of a projector determies how deep the depth of field of focus is? I realize that if I move the projector further away and use a mid or long throw projector, that would be one solution, but it is not a solution that works for me. All else being equal, what would make a short throw projector have the deepest focus range compared to other projectors?

    THANKS FOR ANY HELP!!!!!!!!!!!!!!!!!
     
  2. alfaleader

    alfaleader Member

    Your depth is not something you can change. It's one of the properties of a lens you can't change. There is only 1 place the image will be sharp (on the focal length).

    http://en.wikipedia.org/wiki/Focal_length

    But you are right in the fact that long trow projectors are a bit sharper at depth (the angle is lower).
     
  3. wickedthoughtzzz

    wickedthoughtzzz New Member

    With photography you can change the depth of field through the use of aperature......can you not do this with video projectors somehow? Do no projectors have controllable aperature? If I bought a projector with a large F stop lens, would it not make the focus be within a wider range like it does in photography?

    Amon Tobin's latest tour had tons of depth to his video mapping, yet every single plane of every single box was perfectly in focus. I can't imagine every single plane of every 4 sided box having its own projector.....

    I believe the aperature of the projector's lens is the attribute I am looking for, but want to make sure it works the same way with video projectors as it does with still cameras.
     
  4. alfaleader

    alfaleader Member

    Yes, a smaller diafragma has a larger depth focusing. But the problem is your diafragma should be between the light source and the lens, I don't know that's possible with a projector.

    There are also 2 very small extra factors (but most likely to small to make a difference). The chromic aberration (other colors have a different focal length) and sferical aberration (if your lens is to large your lens border has a different focal length then the middle).

    Maybe Amon Tobin used projectors that were much further away and with long trow lenses. How more your beams are parallel (smaller corner) how larger your depth focusing will be (parallel projection).

    btw: I'm no expert at projection techniques. I'm only using my physics knowledge to explain these things, but theory is not the same as practice!
     
    Last edited: Jan 9, 2012
  5. MJ

    MJ I've given money to VJForums

  6. deepvisual

    deepvisual visually challenged

    short throw projectors are not designed for this purpose.
    either use a regular projector from further away or use two, one for the foreground, one for the back.
     
  7. PCProject

    PCProject Moderator

    No it doesnt.
     
  8. devonmiles

    devonmiles Midi: the language of God

    heres some behind the scenes enlightement on the ISAM show http://www.derivative.ca/Events/2011/AmonTobinVSquared/
    as you can see, the stage design is specific in a way that the depth of the planes isnt too different, while the projector ( its only one) is pretty far away, probably FOH or somehwere in the middle over the audience. then the beam hits the planes at an angle of something like 45 degress and not straight from the front, which to a certain extend compensates the depths difference too.
     
    Last edited: Jan 10, 2012
  9. deepvisual

    deepvisual visually challenged

    come then.. prove it.
    my experience is the opposite. The lenses are pretty much the same, light just passes in opposite directions.
    you get narrow depth of field with long throw lenses and deep DOF with wide angle.

    thing is, on the scale the OP is operating on, you need massive DOF, which isn't going to happen.
     
  10. many2

    many2 Active Member

    I'm with Deepvisual on that one. In my experience, a camera and a projector are really really similar, except the light goes in a different direction.

    Last time I did a DOF test was many years ago with a Christie S+20k and a 0.8:1 lens. I had way more DOF than I thought I would get : if I remember correctly, the image was clear enough from the audience point of view over a depth of over 30 feet. I suppose the overall projection distance must have been around 45 feet or something like that. From a close viewing positing it wasn't really in focus all that distance, but who cares since no one was supposed to see it from there as it was meant for a large stage.

    Your results may vary : the lenses for the Christie Roadster range are really high quality (particularly the latest generation) and they alone cost more than most projectors VJs are using usually. It also was a fixed lens (the equivalent of a photographic prime lens) so this again is an advantage over most projectors embedded lenses, which are zooms.

    That being said, I do not see any kind of diafragma on projectors, beside the iris shutter that you find on higher end machines. I guess there is no point in reducing brightness to get extra DOF since most projectors are made to project on a plane.

    Finally, on high end projectors, you can also play with the focal plane. There is an adjustment in the projector itself, just behind the lens, where you can move the focus. I suppose you're not supposed to play with that if you follow manufacturers recommendations ;)
     
  11. deepvisual

    deepvisual visually challenged

    because of what they are made for, projecting, projector lenses are what photographers call 'fast' ie wide aperture - to let a lot of light through. you can make the aperture smaller by adding a donut to the back of the lens, which in turn increases the DOF, but that will cut your light output dramatically.
     
  12. PCProject

    PCProject Moderator

    I got the impression that the OP was thinking that projector lenses are equipped with an iris/F-stop facility. This is why I said they are not the same.
    I wouldnt disagree that fundamentally a projector and camera lens are the same deal but in this case (unless the OP posts differently), I think I answered the question he was actually asking!

     
    Last edited: Jan 11, 2012
  13. vjneef

    vjneef -------

    I think you could try and put a diaphragma in front of the lens. just put something in front of it that leaves a small hole and see if it works.
     
  14. vjneef

    vjneef -------

    and for the wide versus small angle discussion:

    http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
     
  15. many2

    many2 Active Member

    thanks vjneef for the link
     

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