versurv Posted February 7, 2018 Share Posted February 7, 2018 I ask this question because I'm working on a project and fear reinventing the wheel or making an inferior method of something that can be done easier and the knowledge publicly available. This is my setup:https://I.imgur.com/6iwGXOl.jpg Center of the lens on the right side does some pincushion distortion before the beam reaches the other lens. I need to be able to change where the center of distortion over time mechanically.Right now I plan to use stepper motors. But I think there are similar methods although used for a different purpose in both the photography world and video projector world.For photography there are tilt-shift lenses.For video projectors there is lens shift.With lens shift I get perfect, undistorted video projected up to 100% above, below, left or right from center of the lens. How are both of these done? Is it similar to my current setup, or use something else such as tilting fresnel lenses?Should I stick with my current setup for offsetting the center of distortion or are there better ideas? I'll admit I've never had chance to look inside either tilt shift camera lenses or video projection lenses with lens shift feature. Both are too expensive for me to attempt to disassemble. Nor have I had luck finding info on someone else doing it or explaining what's inside.My only guess has been that either all some lens elements are physically shifted along the DMD or LCD but I haven't been able to understand how that won't cause distortion as center of lens changes for other lenses and DMD/LCD.The only place I've found any info on this has been a youtube video for a DIY video projector where someone used a (fresnel) lens tilting, not shifting. The projector in that video is the only one I've seen where someone attempts to use fresnel lens tilting to do keystone correction optically rather than in software.I guess at extreme projector to screen angle lens tilting could also help with uniform focusing, but I haven't seen it implemented as such a feature in one.Maybe the reason for both of the above is introducing chromatic aberration? Link to comment Share on other sites More sharing options...
timsabre Posted February 7, 2018 Share Posted February 7, 2018 On the video projector lenses I have seen, lens shift is just what it says - the actual lens assembly is moved by motors. The entire lens assembly moves so that is why it doesn't cause distortion - the amount of movement is extremely small as well. Link to comment Share on other sites More sharing options...
versurv Posted February 7, 2018 Author Share Posted February 7, 2018 There was a vertical and horizontal lens shift in one hitachi projector I have. It works by rotating a screw. I just checked again and the entire lens is definitely not moving in this one, zoom and focus does cause the lens in the end to move though. Same with a Optoma projector I have although its a knob rather than a screw.I have a motorized panasonic theatre projector where the lens shift is motorized but don't have access to it right now to check if the final lens element moves up or down or not.But as you say there are some that do.Even if that was the only way lens shift was done I can't wrap my head around the idea of how the image remains undistorted as the lens assembly shifts relative to the beam coming from the DMD/LCD. For one, the beam isn't parallel, and secondly the center of the lens changes reltive to the center of the beam. The beams aren't perpendicular to the lens curvature anymore so shouldn't that distort the frame?Thanks! Link to comment Share on other sites More sharing options...
henny Posted February 7, 2018 Share Posted February 7, 2018 http://www.avsforum.com/forum/68-digital-projectors-under-3-000-usd-msrp/564079-how-does-lens-shift-work.html#post5953807 Link to comment Share on other sites More sharing options...
mac.calder Posted February 8, 2018 Share Posted February 8, 2018 Even if that was the only way lens shift was done I can't wrap my head around the idea of how the image remains undistorted as the lens assembly shifts relative to the beam coming from the DMD/LCD. For one, the beam isn't parallel, and secondly the center of the lens changes reltive to the center of the beam. The beams aren't perpendicular to the lens curvature anymore so shouldn't that distort the frame?Thanks! At the extreams of lens shift, on most projectors, it does result in quite visible distortion. That is why when blending or stacking images people get quite pedantic with their positioning - to avoid shift and the slight distortion. Some projectors get around visible distortion by reducing the amount of shift you can get in one axis when you near the extreams on the other axis. Link to comment Share on other sites More sharing options...
alistermorton Posted February 8, 2018 Share Posted February 8, 2018 Bear in mind that in photography one of the uses of t/s lenses is to quite aggressively affect the depth of field, so distorting the focus quite a lot. Link to comment Share on other sites More sharing options...
Jivemaster Posted February 8, 2018 Share Posted February 8, 2018 If you calculate a lens that has a sharp objective circle of 100mmDIA and the source is only 50mm square then you can move the lens 25mm in any direction and the projected image will move a lot. OK that's an extreme example! If the source is 70mm diagonal and the sharp objective circle is 72mm DIA then there will be 1mm of movement in any direction which may well amount to several metres of projected image movement. https://en.wikipedia.org/wiki/View_camera has some OK illustrations. Link to comment Share on other sites More sharing options...
versurv Posted February 18, 2018 Author Share Posted February 18, 2018 but will the shifted image be undistorted? Link to comment Share on other sites More sharing options...
Shez Posted February 18, 2018 Share Posted February 18, 2018 Camera tilt shift lenses intentionally distort the image- that's what they're designed to do. Projector lens shift doesn't (shouldn't) distort the image. Link to comment Share on other sites More sharing options...
canoemoose Posted February 19, 2018 Share Posted February 19, 2018 Not really - projector lens shift and camera lens shift achieve the same thing, just with the light going the other way. There's no inherent distortion produced - in fact the idea is to reduce perspective distortion in both cases. Camera lens tilt angles the focal plane relative to the normal axis of the lens, and I don't think this should cause any additional perspective distortion although I have just broken my brain trying to think about it... Link to comment Share on other sites More sharing options...
Shez Posted February 19, 2018 Share Posted February 19, 2018 Rather depends on your definition of distortion. I use the term here to mean change - desirable change but still change. Projector lens shift moves the image around but doesn't change (distort) it. Camera tilt shift changes the perspective which is a distortion of the original image. Link to comment Share on other sites More sharing options...
canoemoose Posted February 21, 2018 Share Posted February 21, 2018 Camera lens shift also moves the image around but doesn't distort it - it's the otherwise-necessary pointing up of the camera that would produce the perspective distortion, in the same way that tilting a projector produces a distorted (keystoned) image. Optically, lens shift on a projector and lens shift on a camera are the same. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.