Hi Bill,

Interesting data, and very good data I have to say. It's somewhat of a misnomer to say right and left slopes, because what you are really measuring is the "slope" in front of the focus point and behind the focus point.

I think of it like this. Imagine the light cone from the objective coming down the tube toward the back, converging to a point at the focus (well, obviously some fuzziness there...). What FocusMax is doing is essentially measuring the apex angle, or slope, of that cone. First the slope of the cone in front of the focus (if you have FMx set to start inside the focus) and then the slope of the cone outside of focus. In a perfect world of perfect measurement, these two slope values will be identical.

Your right and left slopes are practically identical - don't complain!! Theoretically, if the two surfaces of a filter are parallel, then the shape (angle; slope) of that converging cone will not change. If the thickness of the filters differ, then the focal point will be offset when compared one from another. This is why one pays so much for "parfocal" filters.

So John's right - unless the front and back surface of the filters are not made properly (and thus wedge-shaped), there will only be just a numerical statistical difference between the slopes for the various filters. I think you could average them all together and be happy.