The formula for calculating the needed focal length used in this calculator is based upon standard "Case 1" optical formulas in which the object is more than one focal length from the lens. The formula is:

Focal Length = (Distance to Screen in Feet *12)/(((Image Size in Feet *12)/object Size in Inches)+ 1)

Suppose you are planning on projecting 35mm slides of your art onto a 10' x 14' screen. The distance from the lens mounted on your slide projector to the projection screen will be about 30 feet. The aperture on a 35mm slide is 1.41" x .944". In order to calculate what lens you need you you need to use you would enter 1.41, 14, and 30 into the calculator and then press "Solve". The result will be 2.996 inches. You would then press "Clear Form" and enter .94, 10, 30 into the calculator. When you press "Solve" the result will be 2.798 inches". The closest standard slide projection lens would be a three inch lens.