## V Curves, how good is good?

Tried to find this with a search but if it is there, I cannot find it.

I have been using FocusMax since it was announced at the old "Imaging the Sky" conference years ago and generally like it.

I have always wondered why the slopes have six digits after the decimal. I would have to assume that this is a pretty high degree of what a machinist would call "false precision". That is, some of the digits to the right just don't matter much.

The question is, just how many digits should one actually pay attention to? I would assume this would also depend on F ratio to a degree as the focal band is narrower with faster systems.

Mainly I would like to be able to compare V curves taken with different filters. They are always different to a small degree, even with supposedly parfocal filters and even with many data points, clean curves, low PIDs, etc.

So at what point (decimal-wise) does a different final curve indicate a truly different curve and different focal situation?

For example, for situations I KNOW are different:

No Filter (not clear, none):

RightSlope =0.051096
LeftSlope =-0.051099

Red Filter:

RightSlope =0.051747
LeftSlope =-0.051742

So here, with clearly different situations, where clearly different curves would be needed, the
differences show up at the fourth decimal.

How about the 5th decimal, is that significant as well?

Thanks