Projection system and DOF??

[somewhatamazing]

Hi!

I don't even know I this is plausible or who to ask, but maybe some of you have some advice..

I have a system that utilizes a zoom camera lens as a projection lens. Meaning lens is mounted on a system that has a illuminated object ("picture") at the rear end of the camera lens and it projects the image on to a system of mirrors (to make the system smaller) and then to a screen. All that is done to project the image multiple times larger than the original picture. Object is at fixed position.

What I would like to know: can I relay on the fact that projected image will be sharp (mostly in focus) because the camera lens used at specific parameters has a certain depth of focus if the projection screen can have multiple positions (~20cm from first to last screen)?? The idea is that nothing is being changes in the camera settings, the position of the screen changes and the projected image needs to be considerably sharp. Thought if I focus the projection system to the middle point of the screen positions (at 10cm) if the depth of field could cover the fact that all screens show sharp image or this is not how such system would work?

I could not understand if "depth of field" is even applicable term in this case..

I added a badly drawn paint picture to make this more clear. Pleas ask in any other info is needed or if you have any tips where to look for an answer.

Tnx!

 

[bratkinson]

Going back to my Kodak Carousel slide projector days, the zoom lens I usually mounted on it always projected the image as close to what was on the slide as was practical given the tolerances of the slide and projector. After zooming in or out, a minor re-focus was often necessary. That was the flat slide image projected through a lens to a screen.

Early overhead opaque projectors (40+ years ago) were quite large and heavy. One would slide the sheet of paper onto a holder and 'raise it up' into position which would lay it flat against a piece of glass. One would adjust the projector lens in/out to focus on a screen. If you wanted it larger on the screen, move the projector backwards.

Later transparency projectors for 8x10 sheets had a 'wide angle' lens pointing down towards the transparency that was laid flat on a glass screen. The 'lens' assembly with a 45 degree mirror behind (above) the wide angle lens and another front lens aimed towards the screen. Moving the entire assembly was how focusing was accomplished.

Now then...if I understand correctly, you are looking to construct a 'reverse' digital camera where the image to be displayed would be at the location of the sensor and a 'regular' digital camera lens (in the 'normal' position attached to the camera) used to 'enlarge' the image coming out of the front of the lens, then, like an old 'mirror lens', a system of mirrors would enlarge that image and project it out the front of the device. As the camera lenses and mounts are unique to the manufacturer, the biggest issue in my mind is placing the image to be projected at exactly the appropriate distance from the rear element as that camera 'expects'. Too far away, and you'd project 'black plastic' around the outer edges of the image. Too close, and you get the 'cropped image' effect in reverse. The rear lens wouldn't 'see' the full image to be projected.

As long as the original image is a flat image, such as a photograph or slide, it should be potentially possible. If the original image is in focus, then the projected image should also be in focus once the lens-to-original image distance is correct. However, trying to project the image of a 3 dimensional object, then, I would expect DOF nightmares as the lens is less than an inch from the subject. The DOF would be so thin as to be useless.

In short, unless you are an optics engineer and know all the calculations necessary for multiple lenses and their coatings/air gap parameters, I think I would simply purchase a digital projector and use a computer to display the desired images as large as one wishes. It would save countless hours of trying and trying in my estimation.

Edit...I almost forgot...how do you intend to light the image to be projected? The light has to come through the original (slide) image to project anything out the front of the lens. Scanners that scan paper images as well as slides and negatives have a light in the lid that shines through the slides/negative and that's what hits the scanner sensor.

 

[somewhatamazing]

The case is that I'm left to continue a work of an optical engineer that left and he does not have many notes about this bit of the project. So I'm mostly in the dark trying to figure out how was this part of the project meant to be done..

You understood correctly what the set up is but maybe misundestood what I want. The object is flat 2D "object/picture" that goes through complicated optical path (I get all that), then the light forms sharp 2D "object/picture" at a pupil (projection image in my amazing drawing that should sit at camera sensor spot), then there is a M42 lens mount, and then camera lens needs to be added. Image will be projected to a certain screen (at specific distance). As I said screens can be at various distances and to which screen image will be projected is computer driven process. What I need is to choose the camera lens to use. As far as I know it must have f/2.4. I'm concerned about how to make it in focus om my 3D screen array (~20cm in depth). So I thought maybe depth of focus could save me from blurriness. Can that be true?

I know that regular projector works if I put my object in between F and 2F, then magnified upside down image will bi projected >2F. But as far as I understand cameras focal lenght is measured from the sensor to convergent point lense system. I know the geometry of the system (where the obect is, space till the camera body etc), I could determine what focal lenght I should choose to put the object between F and 2F, so it comes out as magnified upside down image. Also is thre a way to know where that magical convergent point is when choosing a lense?

Tnx a lot anyways :]

 

[dennybeall]

Perhaps it would help to say in simple terms what you're looking to accomplish without explaining what you've done so far?

 

[somewhatamazing]

Ok then in short: what I would like to ask is if it is plausible and what lens (parameters - focus distance etc.- not brand) should be used to get projector that has a depth of field around 20cm (projected ar 2 m distance)? Or how would I go about (what I need to know)calculating these lens parameters?