I would guess that for people that are not familiar with MTF plots that a simple basic concept may escape them.
This concept is that while we typically think of the top left corner as being 100% contrast, with contrast dropping to zero in the bottom right corner, the reality is that the top left corner can represent any starting contrast.
Let's say for example that the top left corner represents a starting contrast of 20%. Let us also assume that the observer has a scotopic contrast sensitivity threshold of maybe 10% for large detail, and 5% for small detail.
This would mean that once the line dips to or below the observer's contrast sensitivity detail, the observer would no longer be able to differentiate that detail from its background.
The plot I attached is for a 30% obstructed instrument and a perfect aperture of the same size (though no such instrument exists).
This plot has been "stretched" to show kind of show a starting contrast in the upper left corner of 20%. This would mean that the .5 on the Z axis would represent 10% contrast and the purple line representing the observer's scotopic contrast sensitivity threshold starts here.
As can be seen, by about .4 of the maximum spatial frequency, a detail with 20% contrast on the target will have lost enough contrast that is at the contrast sensitivity threshold of the observer, and the observer will not be able to pick this detail out from the background.
The "perfect" aperture though does not loose enough contrast to hit the observer's contrast sensitivity threshold until it crosses the .62. line.
Note that also, at no point does the slight recovery in the obstructed instrument ever cross back above the observer's contrast sensitivity threshold.
I like Figure #74 on this link:https://telescope-op...romatic_psf.htm as a good example where the author has included the "Bright low contrast detail cutoff" to show how this comes into play with different size apertures. A bigger aperture might have more contrast loss at a given frequency, but since that frequency might be higher (smaller detail) in the bigger scope, then even though that scope losses more contrast at that frequency, the observer would still have a better chance of seeing that detail in the larger scope.
And this vitally important point. The MTF plot is expressed for the "perfect" detail, which is a straight sinusoidal line. This will represent the best possible kind of feature for resolution (Cassini Division, while not straight, is in relative terms, ans some length, and this is why it is so easy to resolve in even very small telescopes). In other words, this chart (in my opinion) overstates the actually size of the detail the observer might detect. An irregular feature like a very low contrast small oval on Jupiter would be much harder than a straight line becuase while it might be as wide, it has rounded ends, making it harder to pick out from the background.