Christmas is over, and I need to re-capitalize the Model A project. I have several lenses I've harvested from the Lens Bunker for that purpose. This is a Gundlach Instantaneous Symmetrical 17X20 lens. Or for the Brits, 20X17!! The Gundlach Instantaneous Symmetrical is a convertible, so that means the rear alone is about 50" focus and covers a small city block. Combined it is 26" f8. It isn't physically unmanageable since it works at f8. It only measures a little over 4 inches across. It's some kind of an Aplanat, since I can see cement on the far edges up front. The aperture is smooth, and the glass is lovely. As you go up in size from 8X10, each next size is about +10 times rarer than the preceeding one. ie. 10,000 8X10 lenses 1,000 1oX12 lenses, 100 11X14 lenses, 10 14X17 lenses : 1 17X20. My numbers are conservative. Don't ask for sources............this is the internet! $615.00 for this.