View Full Version : simple len building question for 11x14 and 12x20
06-04-2009, 11:57 AM
Because I am an idiot when it comes to optics, I can not seem to figure out the appropiate focal lenght and diameter needed to cover 11x14 and 12x20. My plan is to build a very simple one element lens, using a plano convex element. I have a plastic lens from surplus shed that is 600mm that covers the 12x20, yet this lens is no longer available. Would someone be able to let me know fl and diameter for covering these to formats. I have lens building info via alan greene, I just need to order the right sized element. Any help would be very much appreciated.
06-04-2009, 12:15 PM
For a plano-convex lens my experiments showed coverage of an image circle of 1/2 the focal length. YMMV, depending on the f-stop you want to use and how much peripheral blur you want. A PCX lens is pretty poor performance with a flat film plane.
You may want to try a meniscus, convex side to the film and the stop in front of the lens - called a "Wollaston Landscape Lens". With a suitably small stop you can get coverage about equal to the focal length.
For a lens you might try a 49/52/55mm +2 close-up lens - it's a 500mm meniscus. You can also try two +1 lenses around a central stop - this will correct the barrel (stop in front) or pincushion (stop in back) distortion you get with a meniscus.
f14 is about the optimum stop, however if you want an image circle of ~500mm you won't be able to put the stop at the optimum location with only a 50mm diameter. The location of the stop is about 15% of the focal length distant from the concave side of the lens. With a 55mm close-up lens and the stop 75mm in front of the lens the coverage circle before vignetting would be 14", not really enough for 11x14. However, you should have coverage if you move the stop to about 50mm in front of the lens.
06-04-2009, 10:00 PM
I tried the landscape lens from his book for an 8x10. I designed the lens to be about 1.5 the diagonal of the format to get decent coverage. Wide open it was about f7 (focal length of the lens system/lens dia....not sure whether this is strictly correct). The coverage was adequate at f7 with lots of pleasing (to my eye) light fall off. I also used it with an f64 stop (calculated from greene's book). The F64 made a fairly pleasing contact print. I have the images and the camera on my flickr page at fotofish. I purchased my lens elements from Anchor Surplus. They have a huge selection.