I'm confused as to how central obscuration effects the PSF. Low pass filters make sense since they are inherently oriented with dealing with spatial frequency, resulting in blur in the spatial domain. But central obscuration confuses me in the fact that it is inherently a spatial property, obscuring what you see, but intuitively not certain frequencies outside of the obscurants shadow.
In order to help me understand this concept I looked at this pdf : http://www.beckoptronic.com/media/22126/Beck_TN_Central_Obscurations.pdf which helped me see what the PSF would look like in such systems. However now I'm very confused as to why I'm only seen a tiny dot in the PSF given a 20% central obscuration. In the real world, when I put my finger in front of my eye, I can see clearly that it is blocking my view.
If my finger instead was a mirror that only showed the light It was not blocking I would still similarly assume that I was missing part of the image, as I assume it would appear warped, or it would show the central part of the image being blocked illustrated here
Yet this is the real PSF of a 20% centrally obscured optic:
Why aren't we seeing PSFs with a lot bigger black dots in the middle of them for these cameras when these obscurants are blocking 20% of the apeture?