The two star pairs are from a program called "Aberrator" and represent how a pair of double stars seperated by two arc seconds would look in a perfect 155mm aperture (top) and and a 30% obstructed 200m aperture.
Now from center to center, the distance is two arc seconds.
Now, imagine that this pattern was formed by two points, exactly oppsite one another at the edge of a 2 arc second wide black line running from top to bottom.
Notice that the Airy Disk in the 155mm apeture is larger, but less energy is going into the ring.
The middle is the 200/30 apeture. You can see that the Airy Disk themselves are slighly smaller, but more energy is going into the rings. On your line between them, the line would appear "darker in the top telescope because there is less energy overlapping it than in the bottom.
But this only holds up so far.
When the larger telescope produces a diffaction disk and first ring that fits within the diameter of the smaller scope, the bigger scope will have better contrast tranfer (assuming no other optical issues).
So, if you put a 310mm aperture in the place of the 200mm, even wiht the central obstruction, the biggest apeture will transfer the most contrast.
Bottom pair is a 310mm apeture with 30% obstruction.
Again, this is only diffration, not optical quality.
But it is clear to see that the 310mm apeture will easily have better contrast transfer. ALmost no light is falling on your line...
You can very easiy see that your black line is no longer "Black" in the middle picture. It has lost a lot of contrast. Think back to the MTF chart and look at the deepest drop or "Elbow" and you are looking at the size detail on the MTF plot that is represented in this picture!
The top image though shows that the line is no longer "Black" either. If you look closely, you can see that the contrast is not perfect. The space on the line is now very dark gray, but not black.
In the 310mm apeture though, the line appears black, but I assure you that it is not. Remeber, there is some light out there in the other airy disk rings, and if this were really the edge of a white line, it would have an infinite number of overlapping airy disks.
You would see the line as gray in all three apetures, but it would be darkest gray in the bottom apeture, medium in the top apeture, and grayest in the center apture.
The bottom apeture will have done the best job of transferring contrast, the top will do the second best, and the center will come in last for detail this size. For different size details the MFT chart shows that much smaller than this and the effects of the obstruction become meaningless (past the elbow, or for detail that is perhaps less than .5 arc seconds wide in this case).
And this.. Here, I am assuming a black line on a white background. If the line starts with 50% contrast and you loose 50% contrast, it will only show up as having 25% contrast at the focal plane. And it gets more compliacated if you use irregular shape and various colors.
it is just the physics of diffraction at work. Diffraction of the apeture is absoute and 100% concrete scientific fact, and every time you look at the Airy Disk in a telescope, you are witnessing it first hand. The smaller the apeture, the bigger the disk. It is an absolute consequence of diffraction of the aperture itself.