A possible explanation - the "sampling window"
Steen Bay, on DPR, has suggested a possible basis for the anomaly. He asked whether, if the actual "effective acceptance dimensions" of the sensels were not tthe same in the two directions (even though the pitch was for all practical purposes identical), this could cause a difference in perceived resolution along the two axes.
I hark to my experience in digital telephony. There, when we think of sampling an audio waveform at a certain interval and then reconstructing it at the distant end, the "ideal" model has us taking an "instantaneous" sample - noting the voltage of the waveform only at the sample instants.
If we do that, then straightforward reconstruction restores the original waveform precisely (with no disturbance in its frequency content). (This assumes that the original waveform had all its frequency components below the Nyquist limit.)
In actual practice (especially in early practice), we actually measure the average voltage of the waveform over a modest finite interval centered on the desired sampling instant(s). One reason is that, were the time width of the "sampling gate" infinitesimal, the energy extracted from the waveform would be infinitesimal as well, and could not be read (or digitized)!
When we use a finite-width sampling window, the reconstituted waveform is not identical to the original waveform, but is rather is that waveform as if passed through a filter with a certain frequency response.
In fact, we then compensate for that by passing the reconstructed waveform through a filter with the complementary frequency response, thus restoring the frequency content of the original waveform - restoring the waveform.
In our situation, the "width" of the effective receptive area of the sensel is closely parallel to the width of the sampling window I mention above. Changes in that, then, will affect the frequency content of the image - the spatial frequency response of the system. If we are thinking in terms of the MTF of the system, (and I mean where the MTF is plotted as a function of spatial frequency), that curve would differ depending on the width of the sampling window.
Our visual perception of the resolution limit comes down essentially to our deciding at what spatial frequency the MTF of the image falls below some non-quantified threshold - essentially, where the contrast is so reduced that we say "we can't see the individual bars".
Thus, if the MTF curve differed for the two directions as a result of the different "sample window widths", we would judge the resolution to be different.
Now, do we know how the "effective receptive dimensions" of the sensel (taking into account the working of the microlenses and such) compares in the two directions?
Just a thought.
Thanks to Steen Bay (and others) for introducing this possible consideration.
Best regards,
Doug
