This is simply a misunderstanding of what focus mask actually is and how to best use it... very understandable given the inaccurate marketing-oriented name "focus mask".
It is (like all similar indicators on any camera, also called "focus peaking" or simply "peaking") a micro-contrast detector. If the subject doesn't have contrast it won't be highlighted by the mask, even if it is in sharp focus. If the subject has lots of micro-contrast then it will sometimes be highlighted by the mask even if it not completely sharp.
Reasons the subject may not have micro-contrast (focus mask can't tell the difference between these reasons):
- it is out of focus
- blur for non-focus reasons*
- subject is dark because it's dark subject matter
- subject is dark because it's under exposed
- subject has little or no texture (e.g. smooth surface)
*soft lens, low quality filter in front of lens, subject motion blur, camera shake, fog
So Focus Mask does not show, in an absolute sense, what is in focus. But it's still
incredibly useful.
Here are ways you can use the indication in focus mask with extreme precision and accuracy:
- compare similar subject matter in a single frame
- compare the same subject matter between two different frames (A:B testing)
I call the latter the "Differential Focus Mask" technique.
If the roof was not parallel to the frame in this image, you would be able to see, with great accuracy and precision, where the plane of focus intersected the roof.
You could also lower the focus mask threshold and compare a series of focus points in focus mask to see which frame shows the greatest/largest/most-prounced mask on the foreground (even if it is less, in the absolute sense, than the amount shown on the more textured background). This A:B testing is especially useful if you have a physical focusing system in which you can define and easily return to each point. For instance on the hyper precise helical of the
Arca Swiss R series you can focus at 10,11,12,13 and then see that the third image was the sharpest for your desired subject and then return (with precision and ease) to 12.
It's also very helpful when doing any Scheimpflug movements are being applied, such as tilt or swing.
If you have more questions, or are not having a good experience with focus mask even with the above experience I'd strongly suggest working with your dealer to explore your technique/approach. Applied correctly focus mask is an incredibly powerful tool.