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?
If you're going to have lenses opposing each other, use same diopter on each (you have done the opposite) like in a rapid rectilinear.
Get a book showing the design of antique lenses. Lacking that Russ Young's thesis on soft focus lenses shows quite a few lens designs. Google it.
It doesn't matter which way around the lenses are or which one goes first. Using Gullstrand's Equation I calculate if you hold a 0.5 dioptre lens 11.9225cm in front of a -0.5 dioptre lens the resultant focal length is 335.50010 cm.; which is what you want. A more compact assembly is possible if you use powers greater than +/- 0.5D.
Maris, do you have the decimal off by one place for that separation distance? I get 119 cm for that value.
Jason, assuming that is correct, the lenses in question are going to produce something of a soda-straw effect when combined, i.e., not a very large image circle on the wall.
Yep, confusion of millimetres and centimetres. I looked at a +3D and a -3D combination and note at a separation of about 3.3cm the net focal length is about 11 feet; a much more compact assembly.
Originally Posted by Denverdad
your help with my project has been greatly valued, i can't thank you enough. when you are measuring this distance, what reference points on the lenses are you using to calculate the measurements? The front of the lens? Or the optical center? If it is the optical center, than how does one determine the optical center?
Hmm. Film-to-subject distance 22 feet. Lens-to-subject distance 11 feet. Focal length should be 5.5 feet.
I may have lost the ability to read, or perhaps no one else has noticed that the OP specified an 11 foot focal length and offered a correction.
Adjusting this sort of two element lens by precision measurement of spacing is impractical. The nominal dioptre powers are close but not exact. In practice the spacing is adjusted until the image is as sharp as possible for the existing subject and image distances.
Originally Posted by jasonclements
The "optical centre" of a lens element is a difficult concept. Real lenses of real thickness have two principal planes each defined by an array of principal points. The two principal points/planes exist because light can be sent through a lens from either direction. One could think that the intersection of the optical axis and a principal plane is an "optical centre" but then there are two "optical centres". This esoteric stuff is irrelevant for a lens focussed by adjustment.