## Building a lens

I am building a lens for a camera obscura that I am making that has an 11 foot focal length, focusing on an image 11 feet away.

I bought two glass lenses at 75mm, one with a dioptre of +.50 and one with a dioptre of -.50.
Once in possession of the lenses, I was attempting to configure them to make the focal length I was in need of.

To do this, I glued each of the lenses in the center ring of a roll of painter's tape so that the edges of the lens was snug to the sides of the ring. They both were placed in a guesstimated center of the ring, leaving 1/8"-3/8" of the ring on either side of lens. These two rings were then placed in a clear, plastic tube that I blacked out with tape. They were just snug enough to remain in place, but loose enough to remove and adjust the distance between.

I initially placed the two glass lenses within the tube with the convex sides opposing each other, and adjusted the distance between the two lenses to experiment with focal length..I came up with bright, but blurry results.

I then realized that it might make a difference which lens (one being positive, and one being negative) was receiving the image and which was projecting the image. So I attempted to switch their positions and adjust the difference between them in this configuration, still blurry.

Looking at diagrams, I decided that the lenses being back to back with the convex sides juxtaposed would be the best option. Prior to this, the plastic tube the lenses were sliding around in extended into the camera 8 inches, creating an 8 inch tube that the image would travel through after passing through the lenses. To adjust this, I took away all the tape on the plastic tube up until the sandwiched lenses, allowing the last thing the image to pass through, being the lenses. However, this didn't improve much.

So this brings me to my questions:

Should the two lenses' convex sides be opposing each other? or facing each other? or should the lenses be facing the same direction?
Would I have better results with the lenses farther apart? Or closer together?
Does the length of the tube (that the lenses are sliding around in), if it extends past the lenses into the camera, affect the image?
Does it matter the placement of the negative or positive lens?