or a lens of negligible thickness, in air, the distances are related by the thin lens formula1S1+1S2=1f{\displaystyle {\frac {1}{S_{1}}}+{\frac {1}{S_{2}}}={\frac {1}{f}}}   .This can also be put into the "Newtonian" form:x1x2=f2,{\displaystyle x_{1}x_{2}=f^{2},\!} [2]where x1=S1−f{\displaystyle x_{1}=S_{1}-f} and x2=S2−f{\displaystyle x_{2}=S_{2}-f}.What this means is that, if an object is placed at a distance S1 along the axis in front of a positive lens of focal length f, a screen placed at a distance S2 behind the lens will have a sharp image of the object projected onto it, as long as S1 > f (if the lens-to-screen distance S2 is varied slightly, the image will become less sharp). This is the principle behind photography and the human eye. The image in this case is known as a real image.Note that if S1 < f, S2 becomes negative, the image is apparently positioned