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---------------- Sensor Analysis Primer - Energy Spectra and Filtering

--------------------------------------------------------- By Bill Claff


In this article we will investigate how can reveal itself in energy spectra.
Filtering can be applied to all pixels but is sometimes only applied to PDAF pixels.

Low-pass Filtering

Let's reexamine the horizontal energy spectrum for the Nikon Z6:

Looking closely at the horizontal we see that it drops from left-to-right which we recognize this as a low-pass filter.
With some research it turns out that this filtering is only present on the PDAF rows which are 12 rows apart; and in fact only every other pixel, the Blue ones:

More Low-pass Filtering

Here is the energy spectrum for the Canon EOS 1D X Mark III:

What causes this valley? It is caused by different low-pass filters being applied to each of the Color Filter Array (CFA) channels.
For example, here's just one channel:

Now we see the familiar descending low-pass filter.

High-pass Filtering

High-pass filtering is uncommon in dedicated cameras when noise reduction is disabled. But here's an example from a mobile phone, the Apple iPhone XS Max:

As with the low-pass filtering we quickly recognize that this is different high-pass filtering on each channel of the CFA.


Filtering is uncommon and unless uncommonly strong unlikely to show in normal photography.
Strong low-pass filtering would present as a slight blurring or loss of resolution while high-pass filtering would sharpen an image.
An investigation across multiple cameras implies that low-pass filtering often results from the handling of PDAF pixels.