In 1849 he founded in Wetzlar a company called "Optisches Institut" for the production of lenses and microscopes. Kellner had invented a new achromatic combination of lenses for an eyepiece, published in his treatise Das orthoskopische Ocular, eine neu erfundene achromatische Linsencombination, that was able to produce an image with correct perspective and without the distortions that were usual for other optical instruments of the time. His invention is still useful and known as the Kellner eyepiece.
After his early death in Wetzlar, his widow led the company, which had twelve employees at that time.
The company expanded quickly. Its newly developed binocular microscope was a market success.