In the sign convention used in optical design a concave mirror has negative radius of curvature so. Thats just cropping your photo before you actually take it photographer Derek Boyd points out. Measure the nodal point of your lens.
2 Next you will make various measurements focusing on nearby objects.
So if I choose a focal length of 25mm which is about 1 then a working distance of about 1140mm 45 is required. As you know by now the nodal point is the optical center of a lens. Lens focal length tells us the angle of view how much of the scene will be capturedand the magnification how large individual elements will be. Go out in the open and face the lens towards distant tree or building.