Camera lenses are converging lenses, meaning they have positive power. A simple convex lens will do this job, however, the image is flawed. I am talking chromatic aberration whereby each color will come to a focus at a different distance downstream of the lens.

Now a negative lens diverges and a positive lens converges. Additionally they display opposite chromatic aberrations. The lens maker corrects by combining strong positive lens elements with weak negatives. The result is an achromatic (without color error), well nearly but not perfection.

The focal length is a measurement taken when the lens is imaging a far distant target. This will be the shortest back-focus. If the lens and its mounting are mismatched, the infinity focus (the shortest back-focus) may not reach film or digital sensor. To correct, we must somehow shorten the distance lens-to-sensor/film or lengthen the back focus.

The only way to lengthen the back focus is to impose a weak negative (diverging) lens. This procedure actually lengthens the focal length a smidgen. Now a single element negative will do this trick, however, such a lash-up adds to the chromatic aberration.

The countermeasure is a doublet, one with negative power and the other with positive power. The powers are adjusted so the combination has negative power. This is a common trick used in telescopes, called a “Barlow”. This classic supplemental lens array is exactly what is needed to lengthen out a back-focus error.

Incidentally, an optical flat, either before, in-between, or behind the lens elements of a camera lens will microscopically lengthen the focal length so what you propose could be accomplished provided the optical flat was quite thick. Sorry this original post was in error. On with my duce cap, in the corner for two hours.