Hello, I am working on a project in which I want to project a projection back into a slide projector using a concave mirror. The lens of the projector has a focal point of 100mm (and an aperture of 2.8) and I want to have approximately 1 meter space between the mirror and the projector. Unfortunately I am not sure what the curvature or focal point of the mirror should be. I tried to calculate this using the lens formula: (From slide projector to projection, focused 2 meters away) 1/o + 1/i = 1/f f = 100mm i = 2000mm (to the mirror and back is 2 meters) o = 105mm (distance between the slide and the lens) (from the mirror back to the slide projector) 1/o + 1/i = 1/f f = ? i = 1000mm (image needs to be projected 1 meter back) o = 1000mm ('object' is 1 meter behind the mirror) That makes f = 500mm, meaning that the focal point of the mirror should be half of the distance between the mirror and the projector. I am not sure if I am doing this correctly though... does anyone have any suggestions? Thank you very much!