Using focal blades analyze to determine scan sharpnes

Recently purchased a betterscanning holder for the V850. This would be my first experience with a continuous height variability holder. In attempts to find the " Sweet Spot " for the scanner I've noticed that to my eye it becomes difficult to tell which scan is sharper as height is dialed in. None of this will probably make the least bit of difference visually but I was still intrigued.

Focal blades analyze command looks at the image and determines what it thinks is appropriate sharpness ( if I'm not mistaken ). So wouldn't the sharpest scan be the scan which focal blade applies the least amount of sharpness to?

To my eye I've scanned with zero turns and looked at focal blades values for sharpen under the manual tab. The value is always higher because the image is obviously more blurred. As I dial in the height the values focal blade reports become smaller. When the differences become imperceptible to the eye focal blade still reports small, repeatable and logical values.

Is this credible? It appears to 100% infallible when working with large differences in height and that the eye can see easily.

I'm not sure if this is old news as I wasn't sure how to search for it.

Sounds about right. Keep in mind that the measurements rely on the pixel values of the scan. Due to electronic noise, there will be a slight, non-zero scan-to-scan variation in the measurements. Minimize the values and you'll be good to go.

It's not surprising that the "in-focus" measurements correspond to what you perceive as good focus. Your eye and the brain's image processing is actually pretty good at doing this sort of thing.

Thanks for the response. The eye is quite good save for when the scanning OCD hits. Thought maybe this method could be used to eliminate doubt when it's present.

I don't have that Focal Blades thing. What I do is scan the same area repeatedly, stepping the variable height in equal increments (typ. 5 would be OK provided they bracket the best setting) and saving each time as jpeg with quality 90. The biggest file signals the sharpest image.
Better yet, plot file size versus height. You get a hump-shaped curve (eyeball a curve through the points). Finding the middle of the hump as a whole is more precise than trying to locate the top. This qualitative method actually rests on specific math, the details of which are superfluous.

The negative you use to make the calibration scan makes a difference too. My preference is a real scene, not a scratched negative or something like that. I have one that has lots of man-made stuff in the scene, including buildings, a billboard, street lights, and street sign. These things have sharp edges and letters/numbers that make it easier to determine if something is "sharp."