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Discussion Starter · #1 ·
I have a 0.8x wide-angle lens for a camera which I can use in front of my projector to make the picture larger. If I hold the lens in front of the projector and pivot it up and down the picture changes geometry like with keystone correction or lens-shift. Is this how it's done in projectors with lens-shift? The upper and lower edges of the picture are very out of focus if I increase the angles too much, how is that counteracted in projectors with lens-shift, is the focus-lens in front of the shifted lens perhaps? :)


Lots of questions, lately. I'm thinking of making a hinged aluminium bracket to hold the lens in front of the projector. I work in a helicopter hangar and can make anything (well, almost) out of aluminium or steel. This would enable me to move the picture lower on the screen and still retain the full resolution by using lens-shift. It would also be very easy to incorporate a horisontal lens-shift function too.

http://home.c2i.net/ahustvedt/images/lens01.jpg
This is what the lens looks like. It has a 55mm threaded flange and comes with a

55-43mm adapter that fits on the camera. My projector has no threaded flange.




Tor Arne
 

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look into view camera technology...Sinar has a great book on the subject: view camera lenses that allow shift are designed to have a large area of coverage over which the optical properties are retained. the simple analogy is to imagine the process backwards: you have the lens reproducing the image of the screen on the projector focal plane; it will also reproduce a much larger area, so that if the screen isn't on the optical axis, but within the area covered, you just have to postion the pj focal plane in the right spot. there will also be issues with focus over the range of the screen and possible geometric distortion.


in general: shifts (x-y movements in a plane) are distinguised from swings and tilts (pivots of the lens axes or the focal plane with respect to the optical axis.) if you just do a shift of relatively small magnitude within the capacity of the lens, you will retain focus over the image area and preserve geometry. if you do a swing or tilt the lens, things get trickier; that is how you would correct for keystone, but in order to compensate for the accompanying loss of focus over the entire image, you also have to swing/tilt the focal plane of the projector. if you do make something to allow for lens tilts and swings, it is easier to predict if the axes of tilt/swing pass through the lens center.


sounds like a fun project; go to a pro camera store and check out a view camera.
 

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Discussion Starter · #3 ·
The loss of focus will also occur if one chooses to use digital keystone correction, any angling of the projector will result in an uneven focus. If one focuses the horisontal middle of the screen the top and bottom will be slightly out of focus.


I will have to zoom in (out?) to retain the picture size so I don't have to move the projector closer to the screen.


I don't have anyhing to lose, I already have the projector and the lens, so I'll give it a shot. I tried holding the lens in front of the projector and while it was dificult to hold it steady, the picture did seem to change symetrically without curving the edges. I'm no optics expert, the only experience I have with optics are my glasses, some magnifying glasses at work and some projectors. :)


I might make something tomorrow evening if I don't have to work overtime.



Tor Arne
 

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the focus thing is tricky with tilts and swings, but it can be corrected. there have been several view camera mechanisms devised to simplify the process, using various mechanical methods to hinge the components. Sinar Pro is the very best, and while quite complicated, does specifically address the de-coupling of swing/tilt and focus changes for quicker operator use. look up the Scheimflug principle for details. what has to happen is that the ratio of image distance to source distance has to be retained for focus over the area of the image. if you move the top of the lens closer to the screen, the distance ratio will change, so you have to tilt the focal plane slightly as well to compensate. this is how large format studio photography is able to get such sharp focus over subjects in receding planes over a large portion of the image.
 

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boy you found that reference pretty fast; looks good. what did you search on?
 

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Discussion Starter · #8 ·
Bah...


I've worked 14,5-hour days the last two days so I haven't had time to make anything. I don't really think it'll be worthwhile anyway but at least it's something fun to do. :)



Tor Arne
 
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